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LARY 


MINERALOGY 


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THE  MACMILLAN  COMPANY 

NEW  ^  ORK    •    BOSTON   -    CHICAGO 
DALLAS    •    SAN    FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON   •    BOMBAY   •    CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


MINERALOGY 

AN  INTRODUCTION  TO 

THE  THEORETICAL  AND  PRACTICAL 

STUDY  OF  MINERALS 


BY 


ALEXANDER   HAMILTON   PHILLIPS,  D.Sc. 

PROFESSOR    OF    MINERALOGY   IN   PRINCETON 
UNIVERSITY 


THE   MACMILLAN  COMT  \NY 
1912 

All  rights  reserved 


EARTH 
SCIENCES 

UBRARY 


COPYRIGHT,  1912, 
BY  THE  MACMILLAN  COMPANY. 


Set  up  and  electrotyped.     Published  October,  1912. 


NortDoob  Ortss 

J.  8.  Cashing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


.     ERRATA 

Page  7,  line  14,  for  78.2°  read  -  78.2°. 

Page  9,  line  2,  for  anode  read  cathode. 

Page  32,  line  9,  for  9.815  read  0.815. 

Page  63,  Fig.  107,  read  Plus  Right  Tetartohedron. 

Page  83,  line  6,  for  —  read  —  • 
ca  oa 

Line  10,  for  .6494  read  .6404. 

Page  89,  line  20,  for  didigonal  read  digonal. 

Page  95,  lines  28  and  29,  read  Mimetite,  Pb5Cl(As04)3,  and 
Vanadinite,  Pb5Cl(VO4)3. 

n  p 

Page  112,  line  7,  for  -  read  — 
c  a 


Page  205,  line  19,  for  cos  V  =     /£— -  read  cos  V  = 


Page  456,  line  34,  for  ZrO  read  Zr02. 

Line  35,  for  .6493  read  .6403. 

Page  512,  line  11,  for  Pb5Cl(PO4)3  read  Pb5Cl(V04)3. 

Page  535,  line  17,  for  4  Na2S04 .  Na2C03  read  9Na2SO4.2Na2 
CO3 .  KC1. 


2857^1 


PREFACE 

THE  object  of  this  book  is  to  bring  together  for  the  beginner,  in 
concise  form  and  under  one  cover,  the  facts  and  basic  principles  of 
the  several  branches  of  mineralogy,  unadulterated  by  an  excess  -of 
data.  It  is,  therefore,  not  a  book  of  reference,  but  it  takes  the  stu- 
dent along  the  various  branches  of  the  subject  to  a  point  from  which, 
if  he  wishes  to  continue,  he  will  be  in  a  position  to  appreciate  and  to 
use  the  advanced  literature  and  books  on  the  subject. 

The  system  of  Dana  has  been  followed  to  a  great  extent,  as  that  is 
the  book  of  reference  which  is  largely  used  by  the  American  stu- 
dent, though  other  sources  have  been  freely  consulted  in  the  com- 
pilation of  this  volume.  In  Part  I,  Crystallography,  the  thirty-two 
types  have  been  described  for  completeness,  even  though  there  are 
no  minerals  crystallizing  in  some  of  them.  The  names  used  are 
those  of  Miers,  as  they  embody  the  symmetry  of  the  type  and  thus 
require  very  little  memory  on  the  part  of  the  student.  Dana's 
names  are  given  in  each  case  under  the  term  "  class." 

The  old  method  of  deriving  the  hemihedrons,  etc.,  from  the  holo- 
hedrons  has  been  retained  and  may  meet  with  criticism,  but  it  is  a 
simple  method  of  determining  what  forms  are  possible  to  combine 
on  crystals  of  lower  symmetry.  I  have  always  found  it  a  very  mate- 
rial aid  to  the  student,  leaving  no  false  impressions. 

Only  graphical  methods  of  solving  the  problems,  after  the  meas- 
urement of  a  crystal,  are  given,  and  the  mathematical  solutions  are 
left  to  the  more  advanced  courses. 

In  Part  II,  a  knowledge  of  general  chemistry  is  presupposed. 
Here  some  two  hundred  and  twenty -five  mineral  species  are  included 
in  the  general  descriptions,  embracing  the  common  rock-forming  and 
ore  minerals.  Their  crystallization,  optical  properties,  decomposi- 
tion products,  genesis,  occurrence,  uses,  and  synthesis  are  included 
in  the  short  description  of  most  of  the  species. 

The  determinative  tables  and  chemical  tests  used  in  the  blowpipe 
table  for  the  identification  of  the  elements  are  included  in  Part  III. 
This  table  includes  all  minerals  with  the  exception  of  some  very 
rare  species  found  only  in  one  locality,  and  in  many  cases  includes 
even  these.  It  therefore  serves  the  purpose  of  placing  before  the 


vi  PREFACE 

student  a  nearly  complete  list  of  the  mineral  species  with  their 
chemical  formula,  hardness,  color,  crystallization,  and  specific  grav- 
ity. The  table  has  been  arranged  after  years  of  experience  in 
teaching  blowpipe  analysis,  and  only  those  tests  are  employed  and 
described  which  are  quite  easily  manipulated,  and  wherever  possible 
the  dry  or  blowpipe  tests  are  given  the  preference.  In  its  scheme  it 
is  modeled  after  Brush  and  Cornwall's  determinative  table. 

The  table  for  the  determination  of  the  common  minerals  by  use 
of  their  physical  properties  includes  about  one  hundred  and  fifty 
species,  and  in  connection  with  the  short  descriptions  of  these  species 
given  in  Part  II,  makes  their  identification  a  simple  matter. 

Table  I  includes  about  fifty  species  of  the  rock-forming  min- 
erals arranged  for  their  identification  in  rock  sections  under  the 
microscope. 

The  illustrations,  with  the  exception  of  two,  have  been  drawn  by 
the  author,  and  the  photographs  reproduced  are  of  specimens  in  the 
collection  of  Princeton  University. 

For  advice  and  assistance  in  this  work  my  sincere  thanks  are  due 
to  many  of  my  colleagues,  but  particularly  to  my  esteemed  friend 
and  professor,  Henry  B.  Cornwall. 

ALEXANDER   HAMILTON  PHILLIPS. 

PRINCETON,  NEW  JERSEY, 
September,  1912. 


CONTENTS 

PART   I 
CRYSTALLOGRAPHY 

CHAPTER  PAGB 

I.    INTRODUCTION 1 

II.     DRAWING  OF  CRYSTALS 31 

III.  ISOMETRIC  SYSTEM 47 

IV.  TETRAGONAL  SYSTEM 65 

V.     HEXAGONAL  SYSTEM 84 

VI.     ORTHORHOMBIC,  MONOCLINIC,  AND  TRICLINIC  SYSTEMS         .  113 

VII.     RELATION  OF  INDIVIDUAL  CRYSTALS 134 

VIII.     ON  THE  MEASUREMENT  OF  CRYSTALS  AND  THE  USE  OF  THE 

GONIOMETER    .        .        .        .        .        .        .        .        .  149 

IX.     OPTICAL  PROPERTIES  OF  CRYSTALS 160 

PART   II 

DESCRIPTIVE  MINERALOGY 

I.    THE  RELATION  OF  MINERALS  TO  THE  ELEMENTS          .        .219 

II.     THE  ORIGIN  OF  MINERALS 237 

III.  PHYSICAL  PROPERTIES        ........  256 

IV.  THE  NATIVE  ELEMENTS 281 

V.     SULPHIDES,  ARSENIDES,  ANTIMONIDES 294 

VI.     SULPHO  COMPOUNDS 320 

VII.     HALOID  COMPOUNDS 327 

VIII.       OXIDES,  INCLUDING    THE    ALUMINITES,    FERRITES,  AND   CHRO- 

MITES 337 

IX.     CARBONATES 379 

X.     SILICATES  AND  TITANATES        .  403 
XI.    COLUMBATES,    PHOSPHATES,    VANADATES,    INCLUDING    THE 

NITRATES,  BORATES,  AND  URANATES  .....  507 

XII.     SULPHATES,  CHROMATES,  TUNGSTATES,  AND  MOLYBDATES   .  527 


viii  CONTENTS 

PART   III 
DETERMINATIVE   MINERALOGY 

CHAPTER  PAGE 

I.    DESCRIPTION  OF  THE  INSTRUMENTS,  REAGENTS,  AND  CHEMI- 
CAL TESTS  USED  IN  THE  BLOWPIPE  TABLE  FOR  THE  IDEN- 
TIFICATION OF  THE  MINERAL  SPECIES         ....     546 
II.     TABLE  FOR  THE   DETERMINATION  OF   THE   MORE   COMMON 

MINERALS  BY  THE  USE  OF  THEIR  PHYSICAL  PROPERTIES     595 

III.  TABLE  FOR  THE  DETERMINATION  OF  THE  -PRINCIPAL  ROCK- 

FORMING  MINERALS  IN  SECTIONS 609 

IV.  TABLE  FOR  THE  DETERMINATIONS  OF  MINERALS  BY  THEIR 

CHEMICAL  TESTS    .        .  617 


MINEEALOGY 


MINERALOGY 

CHAPTER  I 
CRYSTALLOGRAPHY 

THE  solid,  liquid,  and  gaseous  states  of  matter  depend  upon 
temperature  and  pressure.  It  is  possible  to  cause  a  solid  to  pass 
to  the  liquid  state  by  raising  the  temperature.  At  0°  C.  water 
passes  from  the  solid  ice  to  the  liquid  water;  0°  C.  is  the  fusing 
point,  or  the  temperature  at  which  the  solid  passes  over  to  the 
liquid  water.  If  the  temperature  is  increased  until  100°  C.  is 
reached,  water  passes  to  the  gaseous  state  —  steam.  The  tem- 
perature at  which  the  vapor  passes  off  freely  and  where  there  is  no 
further  rise  in  the  temperature  of  the  body  of  the  liquid  is  known 
as  the  boiling  point.  The  fusing  point  and  the  boiling  point  are 
fixed  temperatures  for  pure  chemical  compounds.  Upon  decreas- 
ing the  temperature  and  increasing  the  pressure  sufficiently  all 
substances  become  solid.  If  the  solids  formed  by  a  slow  transi- 
tion from  liquids  or  gases  are  examined,  it  will  be  found  that  the 
larger  number  are  bounded,  in  part  at  least,  by  smooth  plane 
faces.  When  arsenious  oxide  is  heated  it  volatilizes;  the  vapors 
upon  contact  with  a  cold  surface  condense,  forming  a  white  coat- 
ing ;  on  examination  with  a  lens,  this  white  coat  is  found  to  be 
composed  of  small  particles  bounded  by  eight  triangular  faces, 
Fig.  1.  Each  individual  is  a  crystal  of  arsenious  oxide.  All 
polyhedra  formed  by  substances  when  passing  to  the  solid  state 
are  crystals. 

It  has  been  the  conception  of  scientists,  since  the  time  of  Dalton, 
that  the  fundamental  unit  of  matter  was  the  atom;  that  the  num- 
ber of  kinds  of  atoms  is  limited,  and  that  each  kind  possesses  dis- 
tinct properties  separating  it  from  all  others,  thus  forming  a 
simple  substance,  an  element.  While  the  number  of  different 
kinds  of  atoms  is  small,  all  objects  and  compounds  of  nature  are 
possible  by  the  combination  of  this  small  number  of  elements. 

The  number  of  atoms  joining  to  form  the  unit  or  molecule  of  a 
compound  can,  by  the  law  of  Avogadro,  be  determined  for  all 


MINERALOGY 

volatile  and  dissolved  substances.  The  number  of  hydrogen  atoms 
combining  with  one  atom  of  oxygen  to  form  one  molecule  of  water, 
is  two;  yielding  the  chemical  expression  H20  for  water.  The 
chemical  molecule  is  the  smallest  particle  of  a  compound  which 


.    FIG.  1.  —  Octahedral  Crystals  of  Arsenious  Oxide,  As2O3. 

can  exist  and  still  retain  the  physical  properties  of  the  compound. 
In  the  attempt  to  divide  the  chemical  molecule  of  water,  the 
atoms  being  indivisible,  the  one  atom  of  oxygen  must  be  sepa- 
rated from  the  two  atoms  of  hydrogen,  resulting  in  two  substances, 
oxygen  and  hydro*gen,  neither  of  which  possesses  the  properties  of 
water. 

There  is  no  method  of  demonstrating  whether  the  solid  or  crys- 
talline molecule  and  the  chemical  molecule  are  identical.  Since  a 
number  of  atoms  combine  to  form  a  chemical  unit,  it  is  also  probable 
that  a  number  of  chemical  units  combine  to  form  a  crystalline  unit 
or  molecule.  The  study  of  the  structure  of  crystals  indicates  that 
there  is  such  a  combination,  but  as  to  the  exact  relation  of  the 
one  to  the  other,  all  that  can  be  said  is  that  the  crystalline  unit 
may  be  expressed  by  a  multiple  of  the  chemical  molecule ;  in  case 
of  ice,  n(H20).  The  light  of  recent  research  would  indicate  that 
n  is  a  small  number,  and  the  crystal  unit  is  not  the  complex  group- 
ing of  a  large  number  of  chemical  molecules  it  was  formerly  thought 
to  be. 


CRYSTALLOGRAPHY 


The  forces  surrounding  the  molecule  in  a  solid  may  radiate 
equally  in  every  direction  as  if  from  a  center,  such  a  group  of 
molecules  would  form  an  amorphous  solid ;  or  again,  the  lines  of 
force  may  vary  with  the  direction,  when  the  solid  would  be  crys- 
talline. The  physical  properties,  as  elasticity,  hardness,  trans- 
mission of  light,  conductivity  of  heat,  will  be  the  same  for  all 
directions  in  amorphous  solids.  Glass,  which  is  an  amorphous 
substance,  will  expand  equally  in  all  directions  upon  heating. 
If  a  sphere  of  glass  is  heated  and  measured  at  various  temperatures 
it  will  at  each  measuring  be  a  true  sphere ;  while  all  its  diameters 
have  increased  in  length  with  the  rise  in  temperature,  they  all 
have  increased  by  exactly  the  same  amount.  Amorphous  solids 
possess  no  regular 
outward  form 
bounded  by  plane 
faces,  but  are  ir- 
regular, globular, 
or  rounded  masses, 
Fig.  2.  The  physi- 
cal properties  of 
crystalline  solids 
are  the  same  along 
parallel  directions, 
but  not  necessarily 
so  along  directions 
that  are  not  par- 
allel. The  direc- 
tional variation  of  the  physical  properties  is  a  primary  character 
of  crystals,  and  all  compounds  to  be  crystalline  must  possess  it ; 
as  will  be  shown  later,  this  is  caused  by  the  regular  arrangement  of 
the  molecules,  assumed  as  the  substance  passes  from  the  liquid  or 
gaseous  state  to  the  solid. 

Crystals  are  generally  bounded  by  plane  faces,  but  the  smallest 
fragment  of  a  crystal  will  possess  this  directional  quality  of  its 
properties,  and  by  it  may  be  identified  as  crystalline.  The 
tendency  to  form  crystals,  or  the  crystalline  force,  varies  with  the 
substance,  being  very  strong  in  quartz  or  calcite,  which  are  almost 
never  found  but  in  the  crystalline  condition;  while  in  others,  as 
chrysocolla  and  turquoise,1  upon  which  crystal  faces  have  never  been 


FIG.  2.  —  Hyalite  from  Waltsch,  Bohemia  :    An 
Amorphous  Mineral. 


Turquoise  has  lately  been  found  in  crystals. 


4  MINERALOGY 

observed,  but  in  which  the  physical  properties  are  directional, 
the  crystalline  force  may  be  considered  as  being  very  feeble.  The 
crystalline  force  will  vary  not  only  with  the  substance,  but  will 
differ  with  the  direction  in  the  same  substance.  When  the  mole- 
cules pass  to  the  solid  state,  from  a  condition  in  which  they  are  free 
to  move,  and  become  fixed  under  the  influences  of  the  crystalline 
force,  it  is  reasonable  to  suppose  that  in  the  direction  in  which  the 
crystalline  force  has  the  strongest  attraction  the  molecules  will  be 
packed  closer  together.  In  the  direction  in  which  the  force  is 

feeble  they  will  be  farther  apart.  The 
forces  influencing  and  surrounding  each 
molecule  are  exactly  the  same  as  the 
forces  influencing  and  surrounding 
every  other  molecule ;  there  is  no  sin- 
gular molecule.  The  distance  from 
any  molecule  to  its  nearest  neighbor  in 
the  same  direction  will  be  exactly 
equal  in  all.  In  Fig.  3,  a,  b,  c,  d  are 
7"  in  complete  crystalline  position,  while 

e  must  be  revolved  90°,  and  f  must  be 

both  revolved  and  translated  to  reach  a  crystalline  position  in 
regard  to  a,  b,  c,  and  d. 

The  whole  will  form  a  regular  molecular  network,  or  point- 
system,  in  which  each  molecule  holds  an  exact  position,  just  as 
each  individual  man  in  a  marching  regiment  must  be  in  his 
exact  position,  holding  a  fixed  relation  to  those  surrounding 
him;  and  further,  if  our  attention  is  directed  to  the  complete 
formation,  it  will  be  seen  that  as  a  consequence  of  the  orderly 
position  of  each  man,  the  whole  is  bounded  by  straight  lines.  If  it 
were  possible  to  place  one  regiment  on  top  of  another,  the  straight 
lines  would  then  become  planes,  and  the  solid  thus  formed  would 
be  bounded  by  plane  faces ;  in  a  similar  way  crystals  are  bounded 
by  plane  faces. 

The  smooth  plane  faces  bounding  a  crystal  are  its  most  striking 
external  character.  It  must  always  be  remembered  that  they  are 
only  reflections  of  the  internal  orderly  arrangement  of  the  mole- 
cules. If  the  crystalline  molecules  are  identical  with,  or  a  small 
multiple  of,  the  chemical  molecule,  then  the  crystalline  units  may 
be  considered  infinitely  small,  as  regards  any  power  we  may  pos- 
sess to  distinguish  them  when  packed  together,  forming  crystal 
faces.  All  artificially  polished  surfaces  fall  far  short  of  the  smooth- 


CRYSTALLOGRAPHY 


-*- 


ness  and  perfection  of  the  natural  polish  of  crystal  faces  and  cleav- 
age surfaces. 

If  a  crystalline  molecule  is  placed  at  each  corner  of  a  cube,  the 
distance  between  each  molecule  will  be  measured  by  the  length  of 
the  edge,  Fig.  4  a.     The  cube  may  be  considered  an  elementary  form 
or  unit  of  a  homogeneous 
point-system  which  may 
be  built  up,  as  in  Fig.  5. 
If    the    cubical    unit    is 

w 

lengthened  in  one  direc-  Q  b 

tion,  it  will  now  possess  FIG.  4. 

edges  of  two  different 
values,  Fig.  4b.  This 
unit  when  packed  to- 
gether so  as  to  fill  space 
will  produce  a  regular 
point-system  of  another 
type,  Fig.  6.  The  sec- 
ond unit  may  now  be 
broadened,  when  it  will 
possess  edges  of  three 
different  values,  Fig.  4  c, 
and  when  packed  to- 
gether will  fill  space,  pro- 
ducing a  point  system  of 
still  another  type.  There 
are  fourteen  such  ele- 
mentary units,  which 
when  packed  together 
will  fulfill  the  crystalline 
requirements  of  com- 
pletely filling  space,  and 
place  each  molecule  of 
the  system  in  such  a  position  that  its  surroundings  shall  be 
exactly  the  same  as  the  conditions  surrounding  every  other 
molecule. 

The  shape  of  the  fourteen  elementary  units  must  in  no  way  be 
considered  to  represent  the  shape  of  the  molecule,  as  the  space 
between  molecules  is  far  greater  than  the  diameter  of  the  molecule, 
and  there  is  no  method  by  means  of  which  the  shape  of  a  molecule 
can  be  determined.  These  dimensions  of  space  may  be  considered 


FIG.  5. 


FIG.  6. 


6  MINERALOGY 

as  representing  the  sphere  of  action,  or  the  sphere  of  vibration,  01 
oscillation,  of  each  molecule. 

While  the  number  of  elementary  point-systems  is  limited  to  14, 
the  number  of  complex  point-systems  must  be  much  extended  in 
order  to  reach  a  satisfactory  explanation  of  the  symmetry  of  those 
crystalline  types  in  each  system  lower  than  the  normal.  These 
lower  types  of  symmetry  may  be  produced  by  a  combination  oJ 
the  elementary  point-systems:  1.  As  if  one  were  pushed  within 
the  other,  there  will  be,  in  such  a  case,  two  sets  of  molecules,  one 
occupying  a  position  in  respect  to  the  other,  as  if  translated  along 
a  definite  direction.  2.  By  the  rotation  of  one  in  regard  to  the 
other.  3.  By  both  rotation  and  translation.  The  number  oi 
point-systems  made  possible  by  these  methods  will  reach  230, 
It  still  remains  true,  however,  that  in  regard  to  their  symmetry 
they  will  all  be  included  in  the  32  possible  types  of  crystals. 

Definition  of  a  crystal.  —  A  crystal  is  a  homogeneous  chemical 
compound  bounded  by  plane  faces,  and  its  physical  properties 
are  alike  along  parallel  directions. 

Crystal  growth.  —  If  growth  is  considered  to  be  an  increase  oi 
size  only,  then  crystals  may  be  said  to  grow.  This  crystalline 
growth  must  ,not  be  confounded  with  organic  growth,  which  is  a 
development.  The  tissues  in  an  organism  increase  in  complexity 
the  unit  in  organic  growth  is  the  cell,  which  increases  by  division, 
One  cell  producing  two,  these  in  turn  increase  in  size  by  an  assimi- 
lation of  material  within  the  cell  wall.  Organic  growth  takes 
place  from  within,  while  crystalline  growth  goes  forward  by  the 
attachment  of  crystalline  molecules  from  without  the  point- 
system,  extending  the  individual  crystal  laterally  in  every  direc- 
tion by  the  thickness  of  each  molecular  sheet  added.  One  crysta] 
therefore  cannot  be  an  embryo  of  another,  as  when  a  sufficient 
number  of  molecules  have  collected  to  form  a  unit  of  the  point- 
system,  and  are  fixed  in  the  required  position,  they  will  possess 
all  the  crystalline  characters.  Microscopic  crystals,  however 
small,  are  just  as  perfect  in  regard  to  their  chemical  and  physical 
properties  as  a  crystal  a  foot  in  diameter,  the  difference  being  a 
mere  matter  of  mass  or  bulk. 

Crystallization.  —  It  has  been  shown  that  in  the  crystalline  state 
of  matter  each  molecule  has  a  definite  and  fixed  position  relative 
to  those  surrounding  it.  In  the  liquid,  gaseous,  and  dissolved 
states,  every  molecule  is  free  to  move,  and  in  any  direction,  leav- 
ing out  of  account  the  so-called  liquid  crystals;  and  until  their 


CRYSTALLOGRAPHY  7 

discovery,   crystals    were    always    considered    to    be   necessarily 
solids. 

If  it  is  wished  to  crystallize  any  substance,  and  thus  obtain 
crystals  of  any  compound  for  study,  it  will  be  necessary  to  bring 
the  substance  into  one  of  those  conditions  of  matter  in  which  the 
molecules  are  free  to  move,  and  then  to  reverse  the  process  under 
such  conditions  that  the  transition  to  the  solid  state  will  take  place 
very  slowly.  Each  molecule  will  be  fixed  upon  the  network  with 
all  equivalent  lines  of  force  parallel,  providing  .always  that  the 
substance  is  capable  of  forming  crystals.  Several  cases  may  arise  : 
1.  The  substance  may  be  a  gas;  all  gases,  with  the  exception  of 
helium,  have  been  solidified  by  decreasing  the  temperature  and 
increasing  the  pressure.  Carbon  dioxide,  a  gas  at  ordinary  tem- 
peratures, becomes  a  liquid  at  78.2°  C..  Liquid  carbon  dioxide  is 
sold  in  the  market  in  iron  tubes.  At  ordinary  temperatures  these 
tubes  are  subjected  to  a  pressure  of  60  atmospheres.  If  a  small 
jet  of  the  carbon  dioxide  be  allowed  to  escape  in  a  beaker,  by  the 
sudden  expansion  and  vaporization  a  large  amount  of  heat  is  ab- 
sorbed and  the  temperature  of  the  remainder  caught  in  the  beaker 
falls  below  the  freezing  point,  and  snowlike  crystals  of  solid  carbon 
dioxide  are  formed.  2.  The  substance  to  be  crystallized  may  be  a 
liquid ;  leaving  out  of  consideration  supercooling,  if  the  tempera- 
ture of  a  liquid  is  decreased,  at  a  definite  temperature,  the  freez- 
ing point,  crystalline  nuclei  will  appear.  From  these  as  centers 
crystallization  will  take  place  until  all  the  liquid  has  become 
solid.  These  centers  of  crystallization  may  be  seen  on  the  surface 
of  any  pool  of  water  just  as  ice  begins  to  form.  3.  The  substance 
to  be  crystallized  is  a  solid ;  substances  in  this  class  will  fall 
under  three  divisions:  A.  Solids  which  when  heated  volatilize 
without  fusion.  If  the  metal  arsenic  is  heated,  it  volatilizes 
without  fusion ;  on  resolidification,  out  of  contact  with  oxygen  of 
the  air,  the  metal  will  be  crystalline.  Solids  formed  in  this  way 
are  known  as  sublimates.  B.  When  heated,  the  substance  fuses 
without  chemical  change.  Such  metals  as  silver,  copper,  lead, 
in  fact  most  of  the  elements,  may  be  crystallized  in  this  way. 
The  crystals  of  the  igneous  rocks,  as  the  feldspars,  olivine, 
augite,  etc.,  have  been  formed  from  a  fusion;  only  here  there 
has  been  a  segregation,  or  a  separation  of  the  various  kinds 
of  molecules  at  the  same  time.  C.  The  substance  is  either 
infusible,  or  is  decomposed  when  heated.  When  a  solid  is 
dissolved  in  a  liquid,  its  molecules  pass  off  from  the  surface  and 


8  MINERALOGY 

move  as  free  as  those  of  a  gas.  Just  as  water  evaporates  in  the 
air,  the  solid  may  be  said  to  evaporate  in  the  liquid ;  this  continues 
until  the  liquid  is  no  longer  able  to  hold  more  of  the  solid,  and 
equilibrium  between  the  liquid,  solid,  and  the  dissolved  substance 
is  established,  when  the  solution  is  said  to  be  saturated.  This 
condition  will  remain  as  long  as  the  temperature,  pressure,  and 
solubility  remain  constant.  Crystals  of  copper  sulphate  may  be 
obtained  from  a  saturated  solution  by  cooling  the  solution.  Salts 
with  few  exceptions  are  more  soluble  the  higher  the  temperature. 
Again,  crystals  may  be  obtained  from  a  saturated  solution  by  de- 
creasing the  amount  of  the  solvent ;  let  the  solution  slowly  evapo- 
rate, both  processes  will  be  combined,  as  the  slow  evaporation  will 
cool  the  solution;  counteracting  this  decrease  of  temperature  is 
the  heat  of  crystallization,  for  where  crystals  are  forming  there  heat 
is  being  liberated.  Perfect  crystals  may  be  secured  by  suspending 
a  small  crystal  on  a  thread  in  the  slowly  evaporating  saturated 
solution,  at  the  same  time  guarding  against  any  sudden  change  in 
temperature.  It  is  also  well  to  mechanically  revolve  the  growing 
crystal  to  insure  its  being  surrounded  by  solution  of  the  same  con- 
centration, when  the  deposition  will  be  uniform. 

Crystals  may  also  be  formed  from  solution  by  a  decrease  of  the 
solubility,  produced,  as  in  precipitation,  by  the  addition  of  some 
reagent  in  which  the  dissolved  salt  is  less  soluble,  or  as  in  the  salting 
out  process  by  the  addition  of  a  common  ion.  All  sulphates  are 
insoluble  in  alcohol ;  if  alcohol:  is  poured  carefully  over  the  surface 
of  the  copper  sulphate  solution  so  as  to  lie  as  a  layer  covering  the 
surface,  it  will  mix  slowly  with  the  solution  and  the  solubility  of 
the  sulphate  will  be  decreased  gradually,  producing  perfect  little 
crystals  of  copper  sulphate. 

A  large  number  of  chemical  compounds,  especially  the  more 
insoluble  salts,  may  be  prepared  in  crystalline  form  by  chemical 
precipitation.  If  to  a  neutral  solution  of  calcium  chloride  a  solu- 
tion of  sodium  carbonate  is  added,  a  white,  fiocculent,  amorphous 
precipitate  of  calcium  carbonate  is  produced  which  on  standing 
becomes  crystalline.  In  the  first  rapid  separation  the  more  un- 
stable amorphous  solid  is  formed,  which  becomes  crystalline,  not  by 
the  rearrangement  of  the  molecules  in  the  solid,  but  by  a  slow  trans- 
fer of  molecules  from  the  unstable  amorphous  solid  to  the  crystal- 
line nuclei  by  resolution,  the  crystalline  form  being  the  more  stable. 

The  methods  mentioned  are  the  more  important;  there  are 
modifications  and  combinations  of  these  which  are  applicable  to  con- 


CRYSTALLOGRAPHY  9 

crete  cases.  The  metals  which  are  easily  reduced  electrolytically 
are  deposited  on  the  anode  in  crystalline  form ;  these  are,  however, 
always  distorted  and  irregular  through  structural  anomalies.  With 
a  very  weak  current  good  crystals  of  copper,  silver,  or  lead  may  be 
obtained,  Fig.  7.  Crystallization  is  a 
method  employed  in  the  separation  and 
purification  of  chemical  compounds,  and 
especially  is  this  so  in  the  commercial 
field,  where  efficiency  and  cheapness  are 
factors  of  such  great  importance.  Gran- 
ulated sugar,  one  of  the  few  chemical 
compounds  produced  in  enormous  quan- 
tities in  almost  absolute  purity,  is  sepa- 
rated by  crystallization. 

mu          "••:••  .    ii.  j      FlG-  7.— Crystals    of   Silver 

The  purity  of  a  crystalline  compound        obtained  by  Electrolysis, 
will  depend  upon  the  rate  of  separation, 

the  viscosity  of  the  mother  liquid,  and  its  solubility.  If  perfect 
crystals  are  sought,  great  care  must  be  exercised  in  the  control  of 
the  growth  of  the  crystals,  the  deposition  of  molecules  must  go 
on  very  slowly.  If  there  is  a  sudden  decrease  in  temperature  of 
the  solution,  a  heavy  shower  of  molecules  upon  the  forming  crys- 
tals results;  they  will  increase  more  rapidly  along  the  edges  at 
the  expense  of  the  center  of  the  faces,  producing  skeleton  crystals. 
The  hollow  faces  may  ultimately  build  out,  leaving  interior  cavi- 
ties filled  with  mother  liquid.  All  foreign  matter  incorporated  in 
the  body  of  a  crystal,  whether  of  liquid,  gas,  or  solid,  is  known  as 
an  inclusion.  The  purity  of  a  crystalline  salt  is  inversely  propor- 
tional to  the  rapidity  of  formation  and  to  the  size  of  the  crystals. 
When  a  pure  salt  is  required,  it  is  best  to  let  the  crystals  form 
slowly  and  remove  them  from  the  mother  liquid  while  still  small. 

Constancy  of  angles.  —  The  size  of  a  crystal  and  the  general 
shape  will  depend  to  a  large  extent  upon  the  conditions  prevailing 
at  the  time  of  its  formation ;  the  question  may  be  asked,  if  the  size 
and  shape  of  a  crystal  is  variable,  is  there  anything  that  is  constant 
upon  which  the  science  of  crystallography  may  be  based?  Nico- 
laus  Steno,  a  Danish  geologist,  in  1669,  while  cutting  sections  of 
quartz  crystals,  noticed  that,  however  variable  the  outline  of  the 
sections  may  be,  due  to  irregularities  of  growth  and  to  the  difference 
in  size  of  faces,  whenever  the  sections  from  the  various  crystals 
were  cut  in  a  parallel  direction  the  corresponding  angles  were 
always  equal.  The  ordinary  quartz  crystal  is  terminated  by  six 


10 


MINERALOGY 


triangular  faces  of  equal  size,  Fig.  8,  or  by  three  large  faces  and  three 
smaller  faces,  as  b,  or  the  point  may  be  stretched  out  to  a  straight 
line,  as  in  c.  If  a  section  be  cut  directly  through  the  apex  of  a 


FIG.  8.  —  Distorted  Crystals  of  Quartz,  Herkimer  County,  New  York. 

quartz  crystal  at  right  angles  to  the  opposite  faces,  the  outline  of 

the  section  in  every  case  will  be  different,  but  the  corresponding 

angles,  Fig.  9,  a,  or  b,  are  always  equal.     This  law  may  be  stated 

~  a,  Q,  as  follows :  the  dihedral 

solid  angle  between  simi- 
lar faces  of  crystals  of 
the  same  substance  is 
constant,  provided  al- 
ways that  the  substance 
is  chemically  pure  and 
that  the  angle  is  measured 
at  the  same  temperature. 
Crystal  angles  are  as 
characteristic  of  chemi- 
cally pure  compounds 
as  their  chemical  or 
physical  properties ;  not 
only  may  they  be  iden- 
tified by  the  angles,  but 
they  are  an  index  to  the 
purity  of  compounds. 

The  constancy  of  the 
interfacial  angle  is  a  di- 
rect result  of  the  regular  molecular  network  or  point-system.  In 
Fig.  10  a  the  round  dots  represent  one  sheet  of  molecules  of  the  point- 


FIG.  9.  —  Parallel  Sections  of  Quartz  Crystals. 

a  b 


FIG.  10. 


CRYSTALLOGRAPHY 


11 


system ;  should  the  crystal  stop  growing  after  five  sheets  were  laid 
down  it  can  be  seen  that  the  cross  section  is  a  square,  and  the  solid 
formed,  a  cube.  If  for  any  reason  growth  is  irregular  and  molecules 
are  laid  on  faster  at  one  end,  Fig.  10  b,  the  cross  section  is  now  no 
longer  a  square,  but  a  parallelogram ;  the  solid  is  no  longer  a  cube, 
but  is  elongated  in  one  direction.  All  angles  are  right  angles  and 
cannot  vary  as  long  as  the  molecules  are  laid  on  in  this  order.  We 
cannot  imagine  the  angles  varying  from  a  right  angle,  any  more 
than  it  would  be  possible  for  a  cube  to  possess  angles  not  right  angles. 
In  measuring  and  comparing  angles  between  similar  faces,  those 
faces  are  considered  similar  which  cut  the  point-system  in  the  same 
direction  or  inclination.  The  six  faces  of  the  cube  are,  in  regard 
to  the  point-system,  interchangeable.  The  configuration  of  the 
molecules  in  the  plane  of  each  face  is  the  same,  therefore  the  physi- 
cal properties  of  each  face  will  be  the  same  and  the  angles  between 
them  will  be  the  same.  This  is  also  true  for  the  elongated  cube,  for 
the  addition  of  molecules  on  one  side  will  distort  the  form,  but 
cannot  possibly  change  the  arrangement  of  those  molecules  al- 
ready laid  down,  upon  which  the  value  of  the  interf acial  angles 
depends.  In  all  similar  faces  the  molecules  are  the  same  distance 
apart  in  any  given  direction:  they  will  lie  parallel  or  equally  in- 
clined to  the  same  lines  of  force ;  they  will  show  the  same  luster, 
polish,  and  hardness;  they 
are  equally  soluble  and  yield 
the  same  corrosion  figures ; 
they  will  expand  with  an 
increase  of "  temperature 
equally  along  parallel  di- 
rections. The  distribution 
of  the  magnetic  force  and 
electric  charge  will  be  alike ; 
in  fact,  all  physical  proper- 
ties of  whatever  description 
will  be  exactly  the  same, 
and  must  be  considered  in 
the  identification  of  similar 
faces. 

The       goniometer.  —  The        FIG.  11.  —  The  Penfield  Card  Goniometer. 

exactness  of  the   interfacial 

angle  is  so  great  that  the  accuracy  of  the  angles  of  chemically  pure 

crystals  far  surpasses  the  capabilities  of  any  instrument  we  may 


12 


MINERALOGY 


construct,  however  delicate,  to  measure  them.  The  more  time 
and  patience  used  in  both  the  construction  of  the  instrument  and 
in  measuring  the  angle,  just  so  much  nearer  is  the  result  to  the 
theoretical  angle  of  the  crystal .  The  instrument  used  for  measur- 
ing angles  of  crystals  was  invented  by  Carangeot  in  1783.  It  is  to 
Rome  de  1'Isle  that  the  science  owes  the  development  of  the  crys- 
tal model ;  he  modeled  some  500  forms.  It  was  to  facilitate  this 
work  that  Carangeot,  his  assistant,  devised  the  contact  goni- 
ometer. This  form  of  instrument  is  still  in  use  for  the  rough 
measurement  of  large  crystals  and  for  crystals  with  dull  faces. 


<D 


Figure  11  represents  the  Penfield 
model,  a  very  inexpensive,  but 
convenient  form,  useful  also  as 
a  protractor  in  laying  out  angles 
and  measurements  in  crystal 
drawings. 

All  contact  goniometers  are 
constructed  upon  the  same  prin- 
ciples, the  simplicity  of  which  is 
such  that  it  needs  no  explanation 
other  than  the  figure  given.  The 
angle  is  read  directly  from  the 
scale.  In  1809  Wollaston  con- 
structed the  reflecting  goniom- 
eter, realizing  the  need  of  a  more 
accurate  instrument  in  his  work 
on  isomorphism.  The  principle 
of  this  form  is  illustrated  in 
Fig.  12,  where  the  ray  of  light  la 
is  reflected  from  the  face  d  to  the 
eye  at  e,  when  a  reading  of  a 
scale  attached  to  the  instrument 
is  taken.  The  crystal  is  now  re- 
volved around  the  edge  a,  formed 
by  the  intersection  of  the  two 
faces,  the  angle  between  which  it 
is  wished  to  measure.  When  the 
ray  of  light  is  again  reflected  to 
the  eye  at  e,  the  crystal  will  have 

assumed  the  dotted  position;   another  reading  is  taken  and  the 
difference  between  the  two  readings  will  give  the  angle,  rar',  the 


CRYSTALLOGRAPHY 


13 


supplement  of  the  interfacial  angle  r'ac ;  for  details  of  measure- 
ment see  page  149. 

Symmetry.  —  All  nature  in  its  building  follows  rules  of  sym- 
metry; worms  possess  many  segments,  one  of  which  is  a  repeti- 
tion of  the  conditions  found  in  that  adjacent.  The  leaves  of 
plants  are  placed  on  the  stem  following  definite  rules  of  repetition ; 
and  in  vertebrates  the  right  side  is  a  mirror  image  of  the  left, 
following  a  bilateral  plan.  These  rules  in  organic  nature  are  only 
partially  adhered  to  and  are  not  followed  with  that  mathematical 
exactness  which  the  constancy  of  the  interfacial  angles  in  crystals 
demands.  Symmetry  may  be  defined  as  the  repetition  of  condi- 
tion following  definite  rules.  The  rules  of  symmetry  in  crystals 
apply  not  only  to  the  repetition  of  faces,  which  determine  the 
outward  form,  but  to  the  internal  molecular  arrangement  and  all 
the  physical  properties  as  well. 

In  crystals  the  three  rules  of  symmetry  are :  (1)  symmetry  in 
regard  to  a  plane ;  (2)  symmetry  in  regard  to  an  axis ;  (3),  sym- 
metry in  regard  to  a  center. 

A  solid  is  said  to  possess  a  plane  of  symmetry  when  that  plane 
divides  the  solid  in  two  halves,  in  such  a  manner  that  all  the  con- 
ditions on  one  side  are  faithfully 
repeated  on  the  other  side'of  the 
plane,  as  in  a  mirmr 4  image ;  or  if 
from  any  pomt  c,  Fig.  13,  on  one 
side  of  the  plane  ab,  a  perpen- 
dicular be  drawn  to  the  plane  and 
extended  in  a  straight  line  an 
equal  distance  on  the  opposite 
side  of  the  plane,  if  the  plane  is 
a  plane  of  symmetry,  there  will 
be  a  point  c  at  its  extremity,  the 
conditions  surrounding  which  will 
be  exactly  the  same  as  the  con- 
ditions surrounding  the  original 
point,  and  the  solid  is  said  to  be 
symmetrical  in  respect  to  a  plane. 
A  plane  mirror  is  the  best  ex- 
ample of  a  plane  of  symmetry,  as  on  looking  in  the  mirror  every 
object  in  front  is  apparently  repeated  in  the  mirror.  Symmetry 
in  regard  to  a  plane  is  the  symmetry  of  reflection. 

If  the  similar  parts  of  a  solid  are  repeated  more  than  once  in 


FIG.  13.  —  Plane  of  Symmetry. 


14 


MINERALOGY 


360°  by  a  revolution  of  the  solid  around  an  axis,  then  it  is  said  to 
be  symmetrical  in  respect  to  an  axis,  or  to  possess  an  axis  of  sym- 
metry. In  Fig.  14,  if  from  any  point  a,  in 
the  solid,  a  line  be  drawn  perpendicular  to 
the  axis  at  o,  when  revolved  about  the 
axis  o  as  a  center  the  point  a  will  describe 
a  circle ;  if  on  this  circle  it  meet  a  point 
a',  the  conditions  surrounding  which  are 
exactly  the  same  as  those  surrounding  the 
point  a,  then  o  is  an  axis  of  symmetry. 
The  crystal  will,  in  this  instance,  after  a 
rotation  of  180°  become  congruent,  or 
will  appear  as  if  it  had  not  been  revolved. 
Symmetry  in  respect  to  an  axis  is  the 
symmetry  of  revolution. 

An  axis  of  symmetry  is  a  digonal  axis, 
if  the  crystal  becomes  congruent  after  a 

revolution  of  180°.     Such  an  axis  is  represented  by  the  conven- 
tional sign  as  at  o,  Fig.  14.     A  didigonal 
axis  is  a  digonal  axis  at  the  intersection  of 
two  planes  of  symmetry. 

If  the  crystal  becomes  congruent  after 
a  revolution  of  120°,  the  axis  is  a  trigonal 
axis,  represented 
as  at  o,  Fig.  15. 
A  ditrigonal  axis 
is  a  trigonal  axis 
at  the  intersec- 


FIG.  14.  —  A  Digonal  Axis 
of  Symmetry :  Gypsum. 


FIG.    16.  —  Tetragonal  Axis 
of  Symmetry :  Scheelite. 


FIG.  15.  —  Trigonal  Axis  of 
Symmetry :  Tourmaline. 


tion  of  three  planes  of  symmetry. 

If  it  becomes  congruent  every  90°,  the 
axis  is  a  tetragonal 
axis  and  is  repre- 
sented   as    at    o, 
Fig.    16.     A   dite- 

tragonal  axis  is  a  tetragonal  axis  at  the  in- 
tersection of  four  planes  of  symmetry. 

If  it  becomes  congruent  every  60°  it  is  a 
hexagonal  axis  and  is  represented  as  at  o, 
Fig.  17.  A  dihexagonal  axis  is  a  hexagonal 
axis  at  the  intersection  of  six  planes  of  sym- 
metry.  The  above  four  axes  are  all  the  tite. 
possible  axes  of  direct  rotation  to  occur  in  crystals. 


CRYSTALLOGRAPHY 


15 


FIG.  18.— Axis  of  Alternating  Sym- 
metry :  Chalcopyrite. 


Alternating  axis.  —  In  Fig.  18,  if  any  point  a  is  revolved  around 
an  axis  oo',  90°,  to  a  position  a',  then  reflected  over  the  plane  bb', 
to  a  position  a",  becoming  congru- 
ent, and  so  on  four  times  in  one 
complete  revolution,  then  oo'  is  a 
tetragonal  alternating  axis.  If  the 
crystal  becomes  congruent  by  ro- 
tation and  reflection  six  times  in 
360°,  then  the  axis  is  a  hexagonal 
alternating  axis;  in  all  such  cases 
the  plane  of  reflection  is  not  a  plane 
of  symmetry  in  the  crystal.  Di- 
gonal  or  trigonal  alternating  axes 
are  not  possible. 

Center  of  symmetry. —  In  Fig.  19, 
if  from  any  point  a,  a  line  be  drawn  to  o,  the  center,  and  ex- 
tended an  equal  distance  in  the  same  direction  beyond  the  center, 

when  o  is  a  center  of  symmetry  it 
will  meet  a  point  a',  similarly  lo- 
cated. The  face  abc  will  be  re- 
peated at  a'bV,  and  all  crystals 
having  a  center  of  symmetry  will  be 
bounded  by  pairs  of  parallel  faces. 

Crystallographical  axes.  —  In  or- 
der that  crystal  faces  may  be  located 
in  space,  their  relations  mathe- 
matically calculated,  their  angles 
measured,  at  the  same  time  furnish- 
ing a  concise,  accurate,  and  con- 
venient form  of  expressing  all  their 
relations,  crystal  faces  are  referred 
to  imaginary  lines  drawn  through 
the  crystal  and  known  as  the  crystallographical  axes.  These  axes 
are,  as  in  analytical  geometry,  generally  three  (in  one  system  four) 
intersecting  at  a  common  point  within  the  crystal,  the  origin. 
The  length  and  inclination  of  the  axes  will  vary  with  the  system 
to  be  represented.  The  direction  through  the  crystal  is  always  so 
chosen  as  to  give  the  simplest  relation  possible,  which  is  deter- 
mined by  the  symmetry  that  is  present.  Where  there  are  axes 
of  symmetry  present,  the  axes  of  highest  symmetry  are  chosen  as 
crystallographical  axes.  Where  axes  of  symmetry  are  absent  the 


FIG.  19.  —  Axinite,  showing  a  Cen- 
ter of  Symmetry. 


16 


MINERALOGY 


FIG.  20.  —  CrystallographicalAxes. 


crystaliographical  axes  are  chosen  so  as  to  pass  through  the  crystal 
parallel  to  the  edges  formed  by  the  intersection  of  three  faces, 

simply  related  and  occurring  the 
more  often  on  the  individual  crys- 
tals. By  this  method  the  axes  are 
so  placed  as  to  be  parallel  to  rows  of 
^t>  molecules  in  the  point-system.  This 
will  be  understood  by  a  considera- 
tion of  Fig.  6,  page  5. 

In  referring  to  the  axes,  the  verti- 
cal axis  is  always  denoted  by  the 
letter  c,  Fig.  20.  The  axis  running 
from  right  to  left  in  the  plane  of  the  paper  is  denoted  by  the  let- 
ter b ;  the  axis  running  back  and  front  through  the  paper  is  the  a 
axis.  If  in  any  case  the  axes  are  equal  and  interchangeable,  the 
equal  axes  are  designated  by 
the  same  letter,  as  a.  If  planes 
are  passed  through  the  origin, 
so  that  each  plane  shall  con- 
tain two  axes,  Fig.  21,  there  b 
will  be  three  such  axial  planes, 
or  principal  sections,  intersect- 
ing at  a  common  point  o, 
which  will  divide  space  dis- 
tributed around  o  into  eight 
octants,  in  any  one  of  which 
it  will  be  possible  for  a  crystal  face  to  occur.  Each  octant  is 
distinguished  by  measuring  in  a  -f  or  —  direction  on  the  axes 
from  the  origin  o,  as  indicated  in  Fig.  20.  The  upper,  front  right 
octant  will  be  -f  a,  -f  b,  +  c ;  the  lower,  back,  left  octant  will 
be  —  a,  —  b,  —  c ;  the  minus  sign  is  the  only  one  written.  When 
the  angle  between  the  axes  are  not  right  angles,  they  are  distin- 
guished as  in  Fig.  21,  aob  =  a,  aoc  =  p,  boc  =  -y. 

Crystal  systems.  —  When  referred  to  their  crystaliographical 
axes,  crystals  fall  into  six  systems,  here  defined  in  terms  of  their 
axes. 

I.  Isometric.  —  Includes  all  those  crystals  which  may  be  re- 
ferred to  three  equal  and  interchangeable  axes  at  right  angles. 
All  three  axes  are  designated  by  the  letter  a. 

II.  Tetragonal.  —  Includes    all    those   crystals   which   may   be 
referred  to  three  axes,  all  at  right  angles,  two  of  which,  the  lateral 


FIG.  21. — Axial  Planes  and  Angles. 


CRYSTALLOGRAPHY 


17 


axes,  are  interchangeable  and  equal.     The  axes  are  designated, 
a  :  a  :  c. 

III.  Hexagonal.  —  Includes  all  those  crystals  which  may  be 
referred  to  four  axes,  three  of  which  are  equal  and  interchangeable, 
being  in  the  same  plane  at  an  angle  of  60°  with  each  other ;  all  are 
at  90°  to  the  fourth,  or  c  axis.    The  axes  are  designated,  ai :  a2 :  a3 :  c. 

IV.  Orthorhombic.  —  Includes  all  those  crystals  which  may  be 
referred  to  three  unequal  axes,  all  at  right  angles.     The  axes  are 
designated,  £  :  b  :  c. 

V.  Monoclinic.  —  Includes    all    those    crystals    which    may  be 
referred  to  three  axes,  all  unequal ;   two  of  these,  the  lateral  axes, 
are  at  right  angles  to  each  other.     One  of  these  is  at  right  angles 
to  the  third,  or  c,  axis ;  the  other  is  inclined.     The  axes  are  desig^ 
nated,  a  :  £  :  c. 

VI.  Triclinic.  —  Includes  all  those  crystals  which  may  be  re- 
ferred to  three  axes,  all  unequal  and  all  inclined.     They  are  desig- 
nated, &  :  b  :  c. 

Some  are  accustomed  to  add  to  these  six  systems  a  seventh  system, 
the  rhombohedral  or  trigonal  system,  referred  to  axes  parallel  to 
the  edges  of  the  rhombohedron.     The  forms  included  in  this  sys- 
tem are  very  closely  re- 
lated to  the  hexagonal 
system,  and  can  be  in- 
cluded in  that   system 
equally  as  well. 

Parameters.  — The 
distance  from  the  origin 
at  which  any  plane,  or 
face,  cuts  a  crystallo- 
graphical  axis,  Fig.  22, 
as  ob',  is  the  intercept  of 
that  plane  a'bV  on  the 
axis  b.  This  intercept 
expressed  in  terms  of 
the  unit  on  that  axis, 
and  written  as  a  co- 
efficient of  the  symbol  FIG.  22.— Axial  Intercepts, 
standing  for  or  repre- 
senting the  axis,  is  the  parameter  of  the  plane  on  the  axis.  If  ob  is 

the  unit  on  the  axis  b,  then  ?_,  in  this  case  8,  8b,  is  the  parameter 

OD 


18  MINERALOGY 

of  the  plane  a'b'c'  on  the  axis  b.  In  the  same  manner  parameters 
are  derived  for  the  axes  a  and  c.  In  general  the  parameters  of  any 
plane  xyz  would  be 

^a:  ^b:  ^c; 
oa       ob       oc 

in  this  case  12  a  :  3  b  :  6  c  are  the  parameters.  They  definitely  fix 
the  inclination  to  the  axes.  The  actual  length  of  the  intercepts 
varies  with  the  size  of  the  crystal  and  is  unimportant.  It  is  the 
relative  length,  one  to  the  other,  or  their  ratio,  which  determines 
the  inclination  of  the  faces,  and  fixes  the  interfacial  angles.  The 
plane  abc,  intersecting  all  three  axes  at  unit  lengths  from  the 
origin,  is  designated  by  a :  b  :  c,  and  is  crystallographically  identical 
with  the  plane  a'b'c'  (8  a  :  8  b  :  8  c) ;  multiplying  all  the  coefficients 
by  8  simply  moves  the  plane  out  from  the  origin  parallel  to  its 
former  position.  It  still  stands  with  the  same  inclination  to  the 
axes  and  will  intersect  all  three  planes  with  the  same  angle  as  before ; 
the  crystal  is  only  increased  in  size.  It  is  the  custom  to  simplify 
the  parameters  by  moving  any  plane  back  or  forward  on  the  axes 
until  the  intercept  on  one  axis  is  unity.  If  the  parameters  12  a: 
3  b  :  6  c  of  the  plane  xyz  are  divided  by  3,  they  become  4  a  :  b  :  2  c  ; 
the  coefficient  of  b  is  reduced  to  unity.  This  is  the  same  as  moving 
it  to  the  position  x'y'z',  cutting  the  axis  b  at  unity,  parallel  to  the 
original  position.  It  represents  the  same  crystal  in  either  posi- 
tion. When  a  plane  is  parallel  to  an  axis,  it  intercepts  that  axis  at 
infinity,  and  is  expressed  oo  a ;  when  a  set  of  parameters  contain 
two  infinities,  the  plane  is  moved  until  the  remaining  intercept  is 
unity  and  the  parameters  are  written  oo  a  :  oo  b  :  c.  This  system  of 
denoting  crystal  faces  was  one  of  the  earliest  methods  devised,  and 
is  known  as  the  parameter  system  of  Weiss ;  it  has  the  advantage  of 
simplicity  and  ^directness  in  expressing  the  relation  of  intercepts 
which  enables  one  to  see  at  once  the  relation  of  the  plane  to  the 
axes.  In  the  drawing  of  crystals  it  is  practically  necessary  to  reduce 
all  other  symbols  to  their  equivalents  in  Weiss's  system,  in  order  to 
lay  out  the  axial  intercepts ;  for  this  reason  it  is  well  to  become 
thoroughly  accustomed  to  the  notation  of  Weiss  in  the  very  begin- 
ning. 

Indices  of  Miller.  —  There  are  a  number  of  other  notations  which 
are  in  use,  the  most  important  of  which  is  Miller's  system  of  in-^ 
dices,  now  generally  used  in  all  works  on  crystallography.  The 
most  general  form,  or  the  indices  of  any  plane,  are  written  hkl; 
the  three  axes  always  maintain  their  usual  order.  The  indices 


CRYSTALLOGRAPHY  1 9 

may  be  derived  from  the  parameters  of  Weiss,  by  dividing  the 
parameters  by  their  least  common  multiple  and  reducing  the 
fraction  to  the  lowest  terms ;  now  each  coefficient  will  stand  as 
a  fraction  in  which  the  numerator  is  one.  Let  it  be  required  to 
convert  2  a  :  3  b  :  4  c  to  indices.  Dividing  by  12,  we  have  T2^  a  :  T3^  b  : 
T\c,  reducing  to  the  lowest  terms,  ^a  :  Jb  :  Jc,  the  three  denomi- 
nators are  then  written  643  (read  six,  four,  three)  as  the  indices. 
The  same  operation  may  be  expressed  thus  :  the  reciprocals  of 
the  parameters  are  written  in  the  order  of  the  axes,  cleared  of 
fractions,  reduced  to  their  simplest  form,  and  then  written  as  the 
indices.  Taking  the  same  parameters  as  before,  2 a  :  3b:  4c,  the 
reciprocals  are  J,  J,  J ;  cleared  of  fractions  by  multiplying  by  12 
and  reducing  to  simplest  form,  the  indices  643  are  obtained  as  be- 
fore. The  reverse  of  this  is  necessary  in  order  to  obtain  the  param- 
eters from  the  indices ;  it  is  almost  unnecessary  to  point  out  that 
the  indices  are  always  whole  numbers  and  cannot  be  fractions. 
When  oo  appears  in  the  parameters  its  reciprocal  0  takes  its  place 
in  the  indices.  The  minus  direction  on  the  axes  is  indicated  by 
writing  the  sign  above  the  figure,  as  123. 

Examples  of  equivalent  planes  : 

PARAMETERS  OF  WEISS  INDICES  OF  MILLEB 

b:c  111 

oo  b  :  c  102 


2a 

—  a 

ooa 

a 


—  a 


2b  :  -  3c  632 

-  b  :  oo  c  010 

b : 3c  231 

b  :  f c  634 

a:  oo  a  IlO 


Rationality  of  the  indices.  —  All  crystals  are  formed  by  a  regular 
deposition  of  sheets  of  molecules.  The  relation  of  these  sheets  to 
the  point-system  of  which  they  form  a.part  will  determine  the  faces 
and  angles  of  the  crystal,  as  well  as  the  intercepts  on  the  crystallo- 
graphical  axes.  Each  intercept  is  determined  by  a  definite  number 
of  whole  molecules,  for  it  is  impossible  to  divide  a  molecule  and  have 
it  possess  the  same  properties ;  when  divided  it  becomes  a  substance 
of  a  different  character,  belonging  possibly  to  a  different  crystal 
system.  Every  face  possible  on  a  crystal  is  determined  by  a  whole 
number  of  molecules  which  determine  the  relative  size  of  the  inter- 
cepts. The  ratio  of  the  intercepts  of  any  or  all  planes  possible 
on  a  crystal  to  any  other  plane  on  the  crystal  must  be  a  rational 


20 


MINERALOGY 


number.  Both  the  parameters  and  the  indices  can  be  expressed 
in  whole  numbers,  0,  or  oo  .  It  has  been  the  experience  in  the  past 
that,  with  few  exceptions,  these  numbers  are  small,  rarely  larger 
than  9.  In  Fig.  23,  the  sheet  of  molecules  lying  in  the  axial  plane 
cob  is  represented.  Possible  planes  intersecting  this  sheet  at  right 
angles  are  represented  by  aa,  dd,  ee,  etc.,  each  of  which  intersects 


*-er 


FIG.  23. 


the  axis  b  at  greater  distances;  let  all  these  possible  faces  be 
moved  up  towards  o  until  they  intersect  the  axis  c  at  unit  distance, 
or  the  diameter  of  one  molecule ;  they  will  now  be  represented  by 
the  dotted  lines  a'a',  d'd',  e'e',  etc.  The  ratio  of  the  intercepts  of  the 
plane  a'a'  on  the  axes  c  and  b  is  as  1 : 1 ;  of  d'd',  1 :  2 ;  of  e'e',  1 :  3 ;  of 
f'f ,  1:7;  of  g'g',  1 :  oo  .  Thus  the  parameters  are  all  whole  num- 
bers and  the  ratios  are  rational  quantities.  Theoretically  it  would 
be  possible  for  a  face  to  occur  with  an  intercept  greater  than  any 
indicated,  but  actually  they  are  very  rarely  observed. 

The  distance  between  neighboring  molecules  lying  in  the  plane 
of  any  face  will  increase  with  the  intercept,  except  when  the  plane 
becomes  parallel  to  an  axis.  The  molecules  in  the  plane  aa  are 
much  nearer  each  other  than  the  molecules  in  the  plane  ee ;  mole- 
cules have  the  tendency  to  crowd  together  as  closely  as  possible.  It 
follows  therefore  that  those  faces  will  appear  the  more  often  on  crys- 
tals in  which  the  molecules  are  the  nearest.  In  Fig.  23,  the  cube 
face  hh  and  rhombic  dodecahedron  aa  will  occur  the  more  often,  as 


CRYSTALLOGRAPHY 


21 


FIG.  24. 


the  molecules  are  the  nearest  in  the  planes.     They  will  also  possess 
small  intercepts  and  a  simpler  relation  of  their  parameters. 

Crystal  forms.  —  When  space  is  divided  by 
the  three  axial  planes  into  eight  octants  it  is 
evident  that  one  set  of  parameters  may  rep- 
resent more  than  one  face ;  in  fact  there  will  be 
eight  planes,  or  one  in  each  octant.  The  solid 
bounded  by  these  eight  faces  is  known  as  a 
crystal  form ;  the  symmetry  of  the  type  may 
not,  however,  require  all  the  eight  faces  to  be 
present.  A  crystal  form  may  be  denned  as  the 
solid  bounded  by  the  combination  of  all  those 
faces  possible  to  be  represented  by  one  set  of 
parameters  irrespective  of  sign ;  and  required  by 
the  symmetry  of  the  type.  The  combination  of  planes  may  inclose 
space  or  may  not.  In  Fig.  24,  eight  faces  are  shown,  all  of 

which  are  represented  by  the  set  of  param- 
eters, a :  b  :  c,  and  the  symmetry  of  the 
type  requires  all  eight  faces  to  be  pres- 
ent ;  Fig.  25  represents  four  of  the  same 
faces,  but  producing  an  entirely  differ- 
ent form,  as  the  symmetry  of  the  type 
requires  only  four  of  the  eight  faces  to 
be  present.  When  one  faee  only  is  in- 
tended to  be  represented  by  a  set  of 
parameters,  or  indices,  they  are  written 
without  pa- 
renthesis 111 ; 
when  the  entire 

form  is  represented,  they  are  written  (111). 
The  number  of  faces  possible  on  any 
crystal  form  may  vary  from  48,  in  the 
type  which  possesses  the  highest  sym- 
metry, to  one  face  in  the  type  which  con- 
tains no  symmetry.  As  at  least  four 
faces  are  required  to  inclose  space,  there 
are  two  classes  of  crystal  forms,  those 
that  inclose  space  and  are  termed  closed 
forms,  Fig.  26,  and  those  which  do  not  in- 
close space,  and  are  termed  open  forms,  Fig.  27 ;  theoretically  the 
open  forms  extend  to  infinity  on  the  open  sides,  unless  terminated 


FIG.  25. 


FIG.  26.  —  Unit  Pyramid  of 
Barite  ;   a  Closed  Form. 


22 


MINERALOGY 


by  a  combination  with  other  forms.     Combinations  of  open  forms 
may  inclose  space.     The  number  of  faces  occurring  on  any  crys- 
tal form  is  very  limited,  but 
the  number  of  faces  possible 
on  a  crystal  which  is  a  com- 


! 

i  . 

-f— - 
j 
i 


FIG.  27.  —  An  Open 
Form. 


FIG.  28.  —  Combination 
of  a  Closed  and  Open 
Form. 


bination  of  crystal  forms  is 
not  limited  and  in  some 
cases  may  be  very  large. 
The  forms  which  may  occur 
in  combination  on  crystals 
are  limited  to  those  possible 
to  be  derived  from  the  same 
point-system,  and  they  will 
therefore  have  the  same 
symmetry.  From  the  sym- 
metry of  the  type,  forms 
in  combination  always  bear  the  same  relation  to  each  other; 
Fig.  28  is  a  combination  of  the  closed  form  of  Fig.  26  and  the 
open  form  of  Fig.  27 ;  here  equivalent 
edges  are  cut  by  the  prism,  or  the  four 
edges  of  the  pyramid  are  replaced  by  the 
prism  faces.  When  the  replacement  is 
symmetrical,  as  in  this  case,  the  angles 
between  the  prism  and  the  pyramid  faces 
above  and  below  are  equal ;  the  pyramid 
edges  are  said  to  be  truncated  by  the  faces 
of  the  prism.  In  the  same  way,  corners  of 
forms  may  be  truncated  by  other  forms 
and  replaced  not  only  by  one  face,  but  by 
a  group  of  faces,  Fig.  29.  When  the  edge  of  one  form  is  sym- 
metrically replaced  by  two  faces,  it  is  said  to  be  beveled,  Fig.  30. 

Zones.  —  The  edge  of  a  form  may  be 
replaced  by  a  series  of  faces,  the  mutual 
intersections  of  which  are  all  parallel  to  the 
edge  replaced.  Such  a  series  of  faces  is 
termed  a  zone.  The  intersections  of  all 
faces  possible  in  any  one  zone  will  be  rep- 
resented by  possible  edges  on  the  crystal, 
all  parallel  to  each  other  and  parallel  to  an 

FIG.  30.  — The  Cube  bev-     .  .  ,.  .  „ 

eled  by  the  Tetrahexa-    imaginary  line  drawn  through  the  point  of 
hedron.  intersection  of  the  crystal  axes,  termed  the 


FIG.  29.  —  The  Cube  with 
the  Corners  replaced  by 
the  Tetragonal  Trisocta- 
hedron. 


CRYSTALLOGRAPHY 


23 


zonal  axis,  Fig.  31.  In  the  study  of  crystal  faces  it  will  be  found 
that  they  all  belong  to  a  comparatively  few  zones.  The  intersec- 
tion of  any  two  faces  on 
a  crystal  will  determine 
the  direction  of  a  pos- 
sible zonal  axis.  Faces 
belonging  to  the  same 
zone  must  be  so  related 
that  two  of  their  inter- 
cepts will  bear  a  con- 
stant relation,  and  their 
intersections  with  the 
axial  plane  in  which 
these  two  intercepts  are 
measured  will  be  paral- 


FIG.  31.  —  Crystal  of  Topaz  in  which  the  Faces 
c,  i,  u,  o,  e,  and  m  are  in  the  Same  Zone,  the  Axis 
of  which  is  aa'. 


lei   lines.      In  Fig.    32 
four  faces  belonging  to 
the  same  zone  are  rep- 
resented and  extended  to  the  axes  a  and  b ;   the  ratio  of  these  in- 
tercepts is  easily  understood  from  the  similar  triangles,  and  the 

intersections  of 
all  the  faces  with 
the  axial  plane 
aob  are  parallel 
lines.  A  zone  may 
be  interrupted  at 
any  point  by  the 
interposition  of 
other  faces  not 
belonging  to  that 
zone.  Zonal  re- 


lations help  very 
materially  in  the 
measurement  of 
crystals,  for  once  a  face  has  been  located  as  a  member  of  a  zone, 
its  parameters  when  determined  must  fulfill  the  zonal  relations. 

Fundamental  forms.  —  Among  the  faces  found  on  the  crystals 
of  any  substance,  a  face  which  cuts  all  three  axes,  and  is  simply 
related  to  all  other  faces  occurring  on  the  crystals,  is  selected ;  its 
intercept  on  each  axis  is  taken  as  the  unit  of  measurement  on  that 
axis  ;  its  parameters  would  be  a :  b  :  c ;  the  form  is  termed  the 


FIG.  32. 


24 


MINERALOGY 


unit,  or  fundamental  form,  to  which  all  other  faces  are  referred. 
The  intercepts  of  the  unit  form  on  all  interchangeable  axes  are 

equal,  their  ratio  -  =  1 ;  the  intercepts  on  axes  that  are  not  inter- 
changeable are  always  an  indeterminate  quantity,  7-  =  0.81520+  ; 

-=  1. 31359 +,  when  b  is  taken  as  unity  and  express  the  ratio  of 
b 

the  units  on  the  axes.  The  axial  ratios  of  barite  are  written  &  :  5  :  c  = 
0.81520:1:1.31359.  For  chemically  pure  substances  the  axial 
ratios  are  constant  and  are  characteristic  of  the  substance,  just  as 
much  as  any  of  its  chemical  properties.  The  axial  ratios  and  the 
value  of  the  interaxial  angles  in  the  monoclinic  and  triclinic  systems, 
which  are  also  constant  for  pure  substances,  are  termed  the 
crystalline  characters  or  elements.  The  crystalline  characters  in 
the  isometric  system  are  determined  by  all  the  axes  being  inter- 
changeable; they  are  the  same  for  all  substances  that  crystal- 

a  c      c 

lize  in   the  system;  -  =  1.     In  the  tetragonal   system,  -  =  y  = 
a  a      1 

0.1644154 +,   axial    ratio    of    rutile.     In    the    hexagonal    system 

c     c 

-  =  j-  =  9.734603 +,  apatite.     In  the  orthorhombic  system  there  are 

two  axial  ratios,  ^  and  ^,  a  :  B  :  c  =  0.81520+  :  1 : 1.31359 +,  axial 

ratios  of  barite.  In  the  mono- 
clinic  system  the  two  axial  ratios 
and  the  value  of  the  angle  p 
are  £he  crystalline  characters; 
a  :  b  :  c  =  0.658510  +  :  1 :  0.55538  +, 
P  =  63°  56'  46",  orthoclase.  In 
the  triclinic  system  there  are  three 
angles  in  addition  to  the  axial 
ratios : 

& :  b  :  c  =  0.49211 +  :  1 :  0.47970+  ; 
a  =  82°  54'  13"; 
p  =  91°  51' 53"; 
Y  =  131°  32'  19",  axinite. 
Holohedral,  holosymmetric,  or 
normal,  are  terms  denoting  a  type  of  crystals  in  each  system,  in 
which  the  symmetry  requires  all  the  faces  possible  to  be  repre- 
sented by  one  set  of  parameters  to  be  present  to  complete  the 


FIG.  33.  —  Holohedral  Form  a  :  c  :  3  a, 
(331). 


CRYSTALLOGRAPHY 


25 


form.  The  set  of  parameters  a  :  a  :  3  a  represents  three  faces  in 
each  octant,  as  here  the  axes  are  interchangeable  and  when  one  axis 
is  cut  all  must  be  cut  by  a  plane 
at  3  ;  these  three  planes  are  rep- 
resented by  (a :  a :  3  a),  (a :  3  a :  a), 
(3a:a:a),  or  24  faces  in  the 
eight  octants;  Fig.  33  repre- 
sents this  form. 

Hemihedral  form  is  the  term 
used  to  denote  those  types  in 
which  the  symmetry  requires 
only  one  half  of  the  faces  pos- 
sible to  be  represented  by  one 
set  of  parameters,  to  be  present 
to  complete  the  form.  There 
are  several  classes  of  hemihe- 
drons,  according  to  their  symmetry.  If  the  most  general  form  of 
a  system  as  (a  :  2  a  :  3  a),  represented  by  Fig.  34,  with  48  faces,  is 
taken,  there  may  be  numerous  ways  of  selecting  one  half  of  these 
48  faces;  the  symmetry  of  the  types  allows  but  three  to  form 
hemihedrons. 

I.  By  taking  all  the  faces  in  alternate  octants  and  extending  them 
until  they  inclose  space,  as  the  shaded  faces  of  Fig.  35,  which  when 


FIG.  34.  — The  Hexoctahedron, 
a  :  2  a  :  2  a. 


FIG.  35. 


FIG.  36. 


extended  will  produce  the  form  represented  in  Fig.  36.  Since  some 
holohedrons  have  a  center  of  symmetry  and  are  formed  of  pairs  of 
parallel  faces ;  this  class  of  hemihedrons  in  which  one  face  of  each 
pair  of  faces  of  the  holohedron  is  extended,  will  not  be  formed  of 


26 


MINERALOGY 


pairs  of  parallel  faces,  and  for  that  reason  they  are  known  as  the 
diagonal-faced  hemihedrons,  represented  in  the  Miller  system  of 
indices  by  ic(hkl). 

II.  By  selecting  one  half  of  the  pairs  of  faces,  taking  those  which 
intersect  in  the  axial  planes,  as  represented  in  Fig.  37';  these  when 


FIG.  37. 


FIG.  38. 


extended  will  produce  the  hemihedral  class,  Fig.  38,  known  as  the 
parallel-faced  hemihedrons,  designated  in  the  Miller  system  by 


III.  By  selecting  every  other  face  around  the  extremity  of  an  axis, 

and  those  alternating  with  it  around  the  adjacent  axis,  as  repre- 

f  sented  by  the  shaded  faces  in  Fig.  39  ;   these  when  extended  will 


FIG.  39. 


FIG.  40. 


produce  the  gyroidal,  or  plagiohedral,  class  of  hemihedrons,  repre- 
sented in  Fig.  40.     These  are  denoted  in  Miller's  system  by  r(hkl). 


CRYSTALLOGRAPHY 


27 


In  all  cases  there  are  two  forms  of  hemihedrons  possible  to  be 
derived  from  each  holohedron,  for  the  white  faces  in  each  case  could 
be  extended  to  obliterate  the  shaded  faces ;  one  of  these  is  the  +, 
the  other  the  —  hemihedron,  in  classes  I  and  II.  In  class  III  they 
are  right  and  left  forms. 

Tetartohedral  forms.  —  In  some  types  of  crystals  with  still 
lower  symmetry,  only  one  quarter  of  the  face  of  the  general  form 
may  be  required  by  the  symmetry  to  complete  the  form;  such 
forms  are  termed  tetartohedrons.  The  faces  extended  to  form  te- 
tartohedrons  must  in  each  case  modify  the  extremities  of  inter- 
changeable axes  in  the  same  manner.  If  in  Fig.  41  the  shaded  faces 
are  extended,  the  tetartohedral  form  of  Fig.  42  will  be  produced, 
having  12  faces.  This  is  the  right  positive  form,  designated 

+  R—     ^-~—  or  irk  (hkl).     If  the  three  unshaded  faces  in  the 

upper  right  octant  and  the  corresponding  faces  in  other  octants  are 
extended,  the  -f-  left  form  will  be  produced.  These  two'  forms  are 


FIG.  41.     (a:2a:3a.) 


FIG.  42.    +  R 


mirror  images  of  each  other ;  there  is  no  way  in  which  they  can  be 
revolved  into  congruent  positions.  It  is  like  a  left  glove  on  a  right 
hand;  such  forms  are  enantiomorphic.  Two  other  forms  are 
possible:  the  minus  right  produced  by  extending  the  faces  -  R, 
Fig.  41,  and  the  minus  left  produced  by  extending  -  L.  -There 
are  always  four  tetartohedral  forms  possible,  the  ±  rights  and  the 
±  lefts ;  the  rights  are  congruent  with  each  other  and  the  lefts  are 
also  congruent,  but  the  rights  are  enantiomorphic  with  the  lefts. 


28 


MINERALOGY 


Models.  —  In  the  study  of  crystals  and  the  relation  of  crystal 
forms,  models  cut  from  wood  are  indispensable.   The  student  should 

cut  the  simpler  forms  and  their 
combinations  from  cork,  as  rela- 
tions once  established  in  this  way 
are  never  forgotten.  Crystal 
models  are  cut  showing  similar 
faces  of  the  same  size,  or  equally 
developed,  thus  representing  the 
ideal  symmetry  of  crystals,  Fig. 
43.  In  nature  crystals  seldom  if 
ever  present  the  ideal  symmetry, 
as  some  faces  are  always  enlarged 
at  the  expense  of  others ;  in  this 
way  some  faces  may  be  entirely 
obliterated,  when  the  appearance 
of  a  crystal  may  be  so  changed  by  the  unequal  development,  or 
distortion,  as  to  be  difficult  of  recognition.  Distortions  take  the 
form  of  elongation  along  a  set  of  parallel  edges,  as  in  Fig.  44,  a 


FIG.  43. 


FIG.  44. — A  Distorted  Rhombic  Dodecahedron  of  Garnet. 

distorted  rhombic  dodecahedron  of  garnet  with  the  edges  parallel 
to  ab  elongated ;  again,  in  the  distortion  of  crystals,  points  are  re- 
placed by  edges,  as  in  Fig.  45  a  and  45  b,  a  regular  crystal  of  quartz 


CRYSTALLOGRAPHY 


29 


and  a  distorted  crystal  in  which  parallel  edges  c  and  d  replace  the 
points  c  and  d,  and  the  edge  ab  is  elongated. 

Habit.  —  Different  combinations  of  the  various  forms  in  which  a 
substance  may  crystallize  will  produce  crystals  of  widely  varying 


FIG.  45  a.  —  Symmetrical  Quartz  Crystal. 

shapes,  and  especially  when  combined  with  distortions.  These  com- 
binations, peculiar  to  localities  or  conditions  of  crystallization,  are 
known  as  the  habit.  Forms  will  be  found  on  crystals  from  one 

c 


FIG.  45  6.  — A  Distorted  Quartz. 


locality  which  may  not  necessarily  be  found  on  those  from  another. 
Even  though  the  forms  are  identical,  their  relative  development  will 


30 


MINERALOGY 


yield  a  crystal  of  an  entirely 
different  appearance.  Figure 
46  is  a  crystal  of  barite  from 
Felsobanya  with  a  tabular 
habit;  Fig.  47  is  a  crystal  of 
barite  from  Cumberland,  Eng- 
land, with  a  prismatic  habit,  being  elongated  parallel  to  the  b 
axis ;  the  two  crystals  present  combinations  of  the  same  forms ; 
their  different  appearance  is  due  to  inequality  of  development. 


FIG.  46.  — Tabular  Habit  of  Barite  from 
Felsobanya. 


FIG.  47. —  Elongated  Habit  of  Barite  ;    a  Combination  of 
the  Same  Forms  as  in  Fig.  46. 


CHAPTER  II 
CRYSTALLOGRAPHY 

Drawing  of  crystals.  —  The  object  to  be  attained  in  the  drawing 
of  crystals  may  be  either  to  represent  their  relation  and  habit  in 
perspective,  or  to  represent  the  relation  of  forms  on  individual  crys- 
tals. The  methods  in  use  for  this  purpose  are  those  of  general 
projection,  though  modified  in  some  cases  to  fit  the  conditions. 
The  edges  of  crystals  are  formed  in  every  case  by  the  intersection 
of  two  faces ;  in  the  drawing  they  are  represented  by  straight  lines ; 
to  find  the  position,  inclination,  and  foreshortening  of  any  edge 
is  nothing  more  than  a  problem  in  the  intersection  of  planes.  The 
position  of  each  face  is  given  by  the  crystallographical  symbols. 
Interfacial  angles  are  used  in  the  drawing,  only  in  so  far  as  they 
are  necessary  to  determine  the  axial  ratio  and  intercepts.  The 
methods  of  perspective  projection  are  modified,  not  only  to  simplify 
the  construction,  but  to  adapt  it  to  the  representation  of  crystal 
edges,  so  that  edges  parallel  on  the  crystal  will  be  parallel  in  the 
drawing,  and  of  a  length  proportional  to  their  actual  length  on  the 
crystal.  This  modification  is  simply  placing  the  eye  at  infinity ;  all 
rays  will  then  be  parallel.  All  parallel  and  zonal  directions  will  be 
preserved ;  it  will  be  necessary  to  determine  the  direction  of  only 
one  edge  of  a  zone  in  the  drawing,  all  other  edges  in  the  same  zone 
will  be  parallel.  It  is  customary  to  represent  the  ideal  symmetry 
of  a  crystal  in  the  drawing,  unless  it  is  wished  to  illustrate  some 
peculiar  development  or  habit.  There  are  two  general  methods  of 
projection,  the  orthographic  'and  clinographic  methods,  both  of 
which  place  the  eye  at  infinity. 

Orthographic  projection  is  a  plan  or  map  of  the  crystal  faces  and 
edges,  drawn  on  a  plane  perpendicular  to  the  c  axis.  It  represents 
in  crystallography  exactly  what  the  foundation  and  roof  plans  of  a 
house  do  to  a  builder.  When  the  plane  of  projection  is  perpendicu- 
lar to  the  c  axis  the  eye  will  lie  in  the  direction  of  the  c  axis,  at  an 
infinite  distance,  vertically  above  the  plane  ;  the  c  axis  will  appear 

31 


32 


MINERALOGY 


m 


as  a  point  in  the  center  of  the  drawing  at  the  intersection  of  the 
lateral  axes.  All  edges  parallel  to  c  will  also  appear  as  points. 
Planes  parallel  to  the  c  axis  will  appear  as  straight  lines.  The 
angles  between  these  planes  will  be  represented  of  true  size. 
.  Edges  parallel  to  the  plane  of  projection  will  also  be  represented 
in  the  drawing  by  lines  equal  to  the  lengths  of  the  edges ;  edges 
inclined  to  the  plane  will  be  proportional  to  their  inclination. 
Supposing  it  is  wished  to  represent  the  crystallization  of  barite, 
axial  ratio,  £  :  b  :  c  =  9.815+  :  1 :  1.313+,  and  the  following  forms: 

a=  (100)  =  a:  oob  :_ooc,          b  =  (010)  =  oo^:b^ooc, 
c  =  (001)  =  oo  a  :  oo  b  :  c,          d=  (102)  =  2  a  :  oo  b  :  c, 
m=  (110)  =  a:b  :  ooc,  p=  (111)  =  a  :  b  :  c, 

When  the  indices  alone  are  given  it  is  necessary  to  transform  them 
to  Weiss  parameters,  as  these  represent  the  proportional  inter- 
cepts on  each  axis.     As  the  two 
lateral  axes  appear  in  the  draw- 
ing in    their  true   length,  lay 
out  from  c,  Fig.  48,  the  vertical 
axis,  a  distance  cb  and  cb'  each 
equal  to  b,  the  selected  unit  of 
length;  at  right  angles  to  cb', 
as  barite  is   an   orthorhombic 
mineral,   lay    off    ca    and  ca', 
each  =  cb  X  .815 ;  through  b,b' 
draw  lines  parallel  to  aa',  which 
will  represent  the  form  (oio) ; 
through  a,  a'  draw  lines  par- 
allel to  bb'  representing  the  form  (100) ;  m  (no)  would   be  rep- 
resented by  joining  the  extremities  of  the  axes  a  and  b,  but  the 
cross  section  can  be  varied  to  suit  the  crystal  at  hand,  by  moving 
the  line  connecting  a,  b  out  to  a  parallel  position  m,  when  m,  m', 
m",  m'"  drawn  symmetrically  will  represent  the  four  faces  of  the 
unit  prism,  m  (no) ;  these  four  lines* will  also  represent  the  inter- 
section of  the  unit  pyramid,  p  (in),  with  m  (no).     The  apex  of 
the  pyramid  is  at  c  and  the  ridges  will  fall  on  the  axes  a  and  b.    The 
base  c  (ooi)  is  in  the  same  zone  as  m  (no)  and  p  (in),  therefore 
the  intersection  of  c  and  p  will  be  parallel  to  the  lines  m,  m',  rep- 
resented by  four  lines  drawn  around  c  parallel  to  m,  m',  m",  m'". 
The  size  of  the  base  can  be  varied  by  the  distance  from  c  at  which 
the  four  lines  are  drawn.     Of  the  forms  to  be  represented  there 


FIG.  48.  —  Orthographic  of  Barite. 


CRYSTALLOGRAPHY  33 

remains  the  dome,  d  (102) ;  a  construction  section  of  the  crystal, 
containing  the  a  and  c  axes,  must  be  drawn,  Fig.  48  a ;  from  b 
draw  be  at  right  angles  to  the  base.  Make  be  =  b  (the  unit  on  b) 
X  1.313,  and  ba  =  b  X  .815,  connect  c  and  a  which  will  represent 
the  ridge  of  the  unit  pyramid  p  ( 1 1 1) . 
The  base  c  cuts  this  pyramid  at  a 
point  p,  found  by  laying  off  bo  equal 
to  cc',  Fig.  48,  projecting  up  to  the 
ridge  of  the  pyramid  at  p,  then  pc 
will  be  the  base  at  the  same  height  at 
which  it  is  represented  in  Fig.  48. 
The  form  (102)  will  cut  the  £  axis  at 
a',  ba'  =  b  X  .815  X  2,  and  the  c  axis 
at  c,  be  =  b  X  1.3-13  ;  ca'  connecting 
these  two  points  will  give  the  slope  FIQ  ' 

and  angle  of  the  macrodome  (102), 

this  may  be  moved  in  parallel  to  ca',  to  cut  the  pyramid  at  any 
required  point  as  at  cd;  project  cd  to  cd'  and  lay  off  on  the  a 
axis  in  Fig.  48  from  c,  ex,  and  cy,  also  ex'  and  cy'  =  bd'  and  be', 
then  yy'  will  be  the  projections  of  the  points  at  which  the  dome 
enters  the  edge  of  the  pyramid;  through  xx'  draw  lines  parallel  to 
the  b  axis,,  which  will  be  the  intersection  of  the  dome  with  the 
base ;  then  the  triangle  xzy,  and  similarly  above  x'z'y',  will  repre- 
sent the  dome  faces. 

Uniformity  of  lettering.  —  It  is  the  custom  to  adopt  a  uniform 
system  of  lettering  crystal  forms,  at  least  those  forms  which  deter- 
mine the  crystalline  characters,  that  they  may  be  recognized'  at 
once  and  the  position  of  the  axes  fixed.  The  pinacoid  (100)  = 
a;  (010)  =  b;  (100)  =  c;  m  represents  the  unit  prism  (110); 
p,  the  unit  pyramid  (111). 

In  the  hexagonal  and  tetragonal  systems  the  prism  of  the  first 
order  is  m,  that  of  the  second  order  a,  and  the  unit  rhombohedron  is 
r.  In  the  isometric  system  a,  o,  and  d  represent  the  cube,  octahe- 
dron, and  rhombic  dodecahedron  respectively.  In  addition  to  the 
above,  individual  faces  are  indicated  by  accents ;  all  faces  in  the 
right  front  octant  are  not  accented,  faces  in  the  right  back  octant 
are  indicated  with  one  accent,  etc.;  thus  p,  p',  p",  p'"  would 
indicate  the  four  upper  faces  of  the  unit  pyramid. 

Clinographic  projection. —  The  clinographic  method  of  illustra- 
tion, in  addition  to  expressing  the  relation  of  various  forms,  also 
gives  the  impression  of  solidity  and  perspective,  which  is  not  the 


34 


MINERALOGY 


object  of  the  orthographic  method.  The  disadvantage  of  the 
method  is  that  no  angles  and  only  those  edges  which  are  parallel 
to  the  c  axis  are  given  their  true  size  in  the  drawing ;  one  method 

therefore  supplements 
the  other  and  crystals 
should  be  illustrated 
by  both  methods. 

In  the  clinographic 
method  the  crystal  is 
projected  upon  a  ver- 
tical plane  containing 
the  c  axis.  The  c  axis 
is  the  only  one  given 
its  true  length  in  pro- 
jection. In  Fig.  49 
an  octahedron  is  rep- 
resented in  ortho- 
graphic projection;  in 
order  to  pass  to  the 
clinographic  two  steps 
are  necessary  :  1st,  the 
crystal  is  revolved 
around  the  c  axis  some 
selected  angle,  usually 
18°  26',  after  this  revo- 
lution the  octahedron 
will  assume  the  posi- 
tion of  the  dotted 
lines.  The  axis  oa  will  be  moved  to  oa' ;  when  projected  upon  the 
plane  of  projection  of  which  xy  is  the  trace  it  will  appear  foreshort- 
ened as  oa" ;  in  the  same  manner  oai  will  appear  upon  the  plane 
of  projection  as  oa/ ;  the  amount  of  foreshortening  will  depend 
upon  the  angle  of  revolution.  2d,  after  -the  revolution  about  the 
c  axis,  in  order  that  the  plane  of  the  lateral  axes  shall  not  be  repre- 
sented in  the  drawing,  by  a  line,  as  xy,  the  eye,  hitherto  at  infinity 
in  a  horizontal  direction,  is  now  elevated  until  the  lines  of  vision 
form  an  angle,  selected  generally  as  9°  28',  Fig.  50,  with  the  horizontal 
plane.  It  is  as  if  the  eye,  being  at  the  same  height  above  the  floor 
as  the  table  top,  the  relative  positions  of  objects  on  the  table 
would  not  be  appreciated,  as  the  lines  of  vision  are  parallel  to  the 
table  top ;  if  the  eye  is  elevated,  the  top  will  come  immediately  into 


FIG.  49. 


CRYSTALLOGRAPHY 


35 


9    ae 


c 

FIG.  50. 


view  and  the  relative  positions  of  objects  on  the  table  is  at  once 
seen.  A  clinographical  projection  of  an  octahedron  is  represented 
in  Fig.  49,  in  which  the  projection  of  the  plane  of  the  lateral  axes 

—  aa'a  is  the  result  of  > 

f\ 

an  elevation  of  the  eye 
9°  28'.  Referring  to 
Fig.  50,  cc'  is  the  trace 
of  the  plane  of  projec- 
tion; it  will  be  seen 
that  every  point  on 
the  horizontal  plane  in  a,  -"" 
front  of  the  plane  cc' 
will  appear  below  o,  and 
every  point  behind  cc' 
will  appear  above  o; 
the  distance  above  or 
below  o  at  which  any 
point  will  appear  de- 
pends upon  the  angle 

of  elevation  of  the  eye  and  the  distance  from  o  of  the  point  in 
question.  Take  any  point  a,  Fig.  50,  the  line  of  vision  aa'  is  9°  28' 
from  the  horizontal,  and  a  will  appear  on  the  plane  of  projection  at 
a'.  In  the  triangle  aoa',  where  oa  =  1,  oa'  =  the  tangent  of  oaa'  = 
tangent  9°  28'  =  |  oa ;  the  point  ai  will  appear  at  a/,  |  of  aio 

above  o ;  the  point  e  will 
appear  at  e',  |  oe  below  o, 
etc.  When  the  angle  of 
elevation  is  9°  28',  £  the 
distance  of  any  point  from 
the  plane  of  projection, 
measured  below  o  if  the 
point  is  in  front  and 
measured  above  o  if  the 
point  is  behind  the  plane 
of  projection,  will  deter- 
mine the  projection  of 
a" -----i/  the  point  in  question. 

FIG,  51.— The  Axial  Cross  of  the  Isometric  Construction     of     the 

System.  axial  crosses. —  Unless  to 

illustrate  some  peculiar  conditions  the  angles  18°  26'  and  9°  28' 
have  proven  the  most  satisfactory  and  are  in  general  use. 


36 


.MINERALOGY 


-C 

FIG.  52.  — Axial  Cross  of  Zircon  ;  c  =  0.64+ . 


Isometric. —  Draw  xy,  Fig.  51,  the  trace  of  the  horizontal  plane 
with  the  plane  of  projection,  and  —  a'a'  at  right  angles,  making 

o  —  a',  ao  =  the  true 
length  of  the  axis  oa. 
Draw  oa'  at  18°  26'  to 
xy,  making  oa'  =  oa; 
draw  a'a"  at  right  angles 
to  xy;  as  a'  is  in  front 
of  xy  lay  off  a  distance 
a"a'"  =  i  a'a",  draw  oa"' 
and  extend  it  to  —  a'"o, 
making  —  a'"o  =  a'"o ; 
then  -  a"V"  will  be  the 
projection  of  the  axis.  The  axis  at  right  angles  to  this  is  gotten 
by  drawing  oai  at  right  angles  and  equal  to  oa' ;  draw  a'iai  per- 
pendicular to  xy  and  make  a/'a/ 
=  -J  a/ai;  draw  a/'o,  extend  to 
-  a/',  making  --  a/'o  =  oai", 
when— a/'a/' will  be  the  projec- 
tion and  relative  length  of  the 
second  lateral  axis.  The  three 
lengths  a'"o,  a/'o,  —  a'o  all  rep- 
resent the  same  unit  of  length  as 
measured  in  turn  on  each  of  the  FlG-  52  a.— The  Unit  Pyramid  of 

i  ..        . !  .  Zircon  ;  c  =  0.640. 

axes ;  by  connecting  the  extremi- 
ties of  the  three   axes  the  clinographical  projection  of  the  unit 
form  or  octahedron  will  be  obtained,  Fig.  49. 

Tetragonal. —  The  tet- 
ragonal system  will  not 
differ  from  the  construc- 
tion in  the  isometric,  ex- 
cept that  the  c  axis  is 
not  equal  to  the  lateral 
axes.  Draw  xy,  Fig.  52, 
and  find  the  projections 
of  the  two  lateral  axes 
exactly  as  described  in  the 
isometric  cross ;  at  o  draw 
oc  =  —  co  =  oa  X  (the 
axial  ratio  of  the  mineral 
to  be  represented),  in 


FIG.  53.  —  Axial  Cross  of  Apatite  ;  c  =  0.734+. 


CRYSTALLOGRAPHY 


37 


zircon  .640.     The  unit  pyramid  of  zircon  will  be  projected  by 
connecting  the  three  axial  units,  Fig.  52  a. 

Hexagonal. —  Here  the  problem  of  the  axial  cross  differs,  as  the 
lateral  axes  are  three  and  not  at  right  angles.  Draw  xy,  Fig.  53, 
and  project  c  and  a2  as  before. 
Draw  a2'oa3'  =  60°,  making  a3'o 
=  a2'o,  then  draw  —  a'3a3  par- 
allel to  —  cc,  placing  —  a3,  |  the 
distance  from  xy  as  a'3,  when 
-  a3a3  will  be  the  projection  of 
the  axis  required.  The  third 
lateral  axis  is  found  by  laying 
off  the  angle  a/oa/'  =  60°  and 
following  the  same  construction 
as  before ;  by  connecting  the  ex- 


FIG.  54.  — The  Unit  Pyramid  of 
Apatite. 


tremities  of  all  the  axes  the  unit  pyramid  will  result,  Fig.  54. 

Orthorhombic.  —  Here  the  three  axes  are  at  right  angles,  but 
all  of  a  different  length.  Taking  barite  as  an  example,  where 
a  :  b  :  c  =  .81+  :  1 :  1.313,  to  project  the  axial  'cross.  Draw  xy, 

Fig.  55;  lay  out  ob'  at 
18°  26'  from  y  and  some 
selected  unit  in  length 
as  50  mm. ;  find  the  pro- 
jection of  b  as  before. 
The  c  axis  is  found  as  in 
the  tetragonal  system, 
oc'  =  50  mm.  X  1.313+  = 
65.  65  mm.  The  a  axis 
is  found  by  laying  off  oa' 
at  90°  to  ob',  and  = 
50  mm. X. 81  =40.5  mm., 
when  the  projection  —  aa 
is  obtained  as  before; 
connecting  the  extremi- 
ties and  Fig.  56  will  rep- 
resent the  unit  pyramid 
of  barite. 

Monoclinic. —  Here  the  problem  differs  in  that  the  clinoaxis  a 
is  not  at  90°  to  the  vertical  axis  c.     Let  it  be  required  to  draw  the 
axial  cross  of  amphibole,  a :  b  :  c  =  .55+  :  1 :  .29+,  'ft  =  73°  58'. 
Project  b  and  c  as  in  the  orthorhombic  system;  to  find  the  pro- 


FIG.  55.  — Axial  Cross  of  Barite. 


38 


MINERALOGY 


jection  of  a  it  is  necessary  to  understand  the  position  of  the  angle 
ft ;  the  position  of  ft  is  always  back,  Fig.  57,  of  the  plane  of  pro- 
jection, here  represented  by  — cc; 
its  value  is  always  taken  less  than 
90°.  Therefore  the  angle  be- 
tween the  c  and  a  axes  in  front 
will  be  greater  than  90°,  in  this 
case  106°  2',  and  the  extremity 
of  the  a  axis  in  front  will  be  be- 
low the  horizontal  plane  a  dis- 
tance oa"',  depending  upon  the 
value  of  the  angle  ft  and  the 
length  of  the  a  axis.  This  dis- 
tance is  found  by  drawing  —  aoc 
=  ft  and  extending  in  the  direc- 
tion of  a,  making  oa  =  ofe  X  9-55+ ; 
draw  aa'"  at  9°  28'  with  the  hori- 
zontal, then  the  projection  of  the 
end  of  the  clinoaxis  in  front  will 


FIG.  56.  —  The  Unit  Pyramid  of  Barite. 


appear  below  xy  a   distance   oa'",   due  to  the  angle  ft  -f  a"a'" 

(i  aa")  due  to  the  angle  9°  28'.     Therefore   lay  out  in  Fig.  58 

the  angle   aoa'  =  16°  2'   and 

oa'  =  the  unit  on  the  a  axis, 

then    the  extremity  of  the.  a 

axis  will  appear  below  xy  a  dis- 

tance aa'  due  to  the  angle  ft, 

then    by    the    revolution     of 

18°  26'  and   the  elevation   of 

the  eye  9°  28',  the  projection  of  the  extremity  will  fall  under  a'", 

a  distance  aV"  =  aa'  +  |  a"a'";  connect  a  with  o  and  extend  to  -  a 

an  equal  distance, 
when  —  a£  will  be  the 
projection  of  the  clino- 
axis; connecting  the 
extremities  of  the 
projected  axes,  when 
Fig.  59  will  represent 
the  unit  pyramid  of 
amphibole. 

Triclinic.  —  Let  it  be 


FIG.  58.  —  Axial  Cross  of  Amphibole. 


required    to    draw   the 


CRYSTALLOGRAPHY 


39 


axial  cross  of  rhodonite,  a  :  b  :  c  =  3.072  +  :  1 :  .6212  +  ;  «=  193°  18'  ; 
(3  =  108°  44' ;  y  =  81°  39' ;   and  the  angle  100A010  =  94°  26'. 

The  construction  of  the  c  and  &  axes  is  the  same  as  in  the  mono- 
clinic  system,  here  the  plane  containing  the  b  and  c  axes  is  not  at 
90°  from  that  containing  the  a 
and  c  axes,  but  as  in  this  case  is 
94°  26'.  Having  constructed  the 
projections  of  a  and  c,  lay  off  the 
angle  a"ob'  =  lOOvOlO  =  94°  26', 
make  bob'  =  a  -  90  =  13°  18'  ; 

make  ob  the  true  length  of  b,  draw  bb'  at  90°  to  ob'. 
extremity  of  the  b  axis  will  lie  below  the  horizontal  plane  a  dis- 
tance bb',  due  to  the  angle  being  13°  18'  larger  than  a  right  angle ; 
draw  b'b"  at  90°  to  xy;  the  projection  of  b  will  lie  on  the  line 


FIG.  59.  —  The  Unit  Pyramid  of  Am- 
phibole. 


The 


FIG.  60.  — Axial  Cross  of  Rhodonite. 


FIG.  61. 


b'b",  extended,  if  necessary,  a  distance  b"b  =  bb'  +  J  b'b", 
b  —  b  will  then  be  the  projection  of  the  axis  b.  Figure  61 
represents  the  combination  of  (100);  (010),  (001),  (110)  of 
rhodonite. 

Example  I  of  clinographic  projection.  After  the  axial  cross  is 
projected,  the  clinographic  projection  of  any  crystal  is  a  problem 
in  the  intersections  of  planes ;  the  inclination  of  the  faces  is  given 
by  the  parameters.  Let  it  be  required  to  project  clinographically 
the  same  forms  used  to  illustrate  the  orthographic  method  on  page 
32.  Construct  the  axial  cross  and  connect  the  extremities  of  the 
axes,  Fig.  62,  which  will  represent  the  unit  pyramid  (111) ;  the  base, 
c  =  (001),  will  truncate  the  pyramid  above  and  below  o,  and  will 
be  parallel  to  the  plane  containing  the  axes  a  and  b.  Let  it  cut 
the  c  axis  above  at  c,  below  at  ci.  This  distance  will  depend  upon 


40 


MINERALOGY 


the  development  of  the  crystal  to  be  illustrated,  as  all  faces  may 
be  moved  back  and  forth  on  any  axis,  if  their  inclination  be  not 
changed.  The  intersection  of  c  with  the  edge  of  the  pyramid  is 


found  by  drawing  c'cc"  parallel  to  the  axis  £ ;  where  this  line  inter- 
sects the  edge  of  the  pyramid  at  c'  will  be  a  point  common  to  both 
the  edge  of  the  pyramid  and  the  base ;  likewise  c",  the  intersec- 
tion of  c  and  p,  will  be  parallel  to  pp'  as  the  two  edges  belong  to  the 


CRYSTALLOGRAPHY 


41 


same  zone ;  draw  c'c'"  parallel  to  pp',  c'"c"  parallel  to  p'p",  and  so 
on  around  the  four  sides  of  the  base  above  and  below.  The  unit 
prism,  m  (110),  is  a  member  of  the 
same  zone,  and  its  intersection  with 
the  pyramid  will  be  parallel  to  the 
intersections  of  the  base  and  pyra- 
mid. If  the  prism  cuts  the  b  axis 
at  m,  then  m'm"  drawn  through  m 
parallel  to  oc  will  represent  its  edge 
in  projection,  and  m',  m"  will  be 
points  on  the  intersection  of  the 
prism  and  pyramid;  draw  m'm'" 
parallel  to  p'"p,  and  so  on  around  the  pyramid  above  and  below, 
dotting  edges  which  will  appear  behind.  To  project  the  remaining 
form  d  (102)  =  2  a:  oob  :  c,  lay  off  on  the  &  axis,  2  a,  connect  2  a 
with  c  and  —  c ;  these  lines  will  be  the  intersection  of  the  dome 
with  the  axial  plane  coa.  Move  2  ac  in  parallel  to  itself,  until  it 
cuts  the  edge  of  the  pyramid  at  d"'  and  the  base  at  e,  the  inter- 


FIG.  63. 


FIG.  64.  —  One  Octant  of  3  a  :  3  a  :  a. 


section  of  d  and  c  will  be  parallel  to  the  b  axis,  as  both  faces  are 
parallel  to  b ;    draw  ee"  parallel  to  p'p'"  and  connect  e"d'",  e"'d'", 


42 


MINERALOGY 


when  e"d'"e'"  will  represent  one  of  the  dome  faces;  the  remain- 
ing three  are  projected  in  a  similar  way.  Figure  63  represents  this 
combination  without  the  construction  lines. 

Example  II.  —  Let  it  be  required  to  draw  (113)  =  3  a :  3  a  :  a  of  the 
isometric  system.  Draw  the  axial  cross  and  extend  them  in  each 
case  to  3  a,  Fig.  64 ;  in  the  isometric  system  all  the  axes  are  inter- 
changeable, and  where  one  axis  is  cut  by  planes  at  1  and  3  the  other 
axes  must  be  cut  by  two  planes  also;  there  will  be  three  planes  in 
each  octant ;  connect  3  a,  a",  3  a',  the  three  lines  will  be  the  inter- 
section of  the  face  3  a  :  a  :  3  a  with  the  three  axial  planes  separating 
a.  the  octants;  likewise  connect 

3  a',  a,  and  3  a" ;  3  a",  3  a,  a'. 
The  intersections  of  these 
three  planes  are  3  ac,  3  a"ad, 
3a'b;  those  portions  of  the 
intersections  which  represent 
crystal  edges  are  drawn  in 
full  lines.  The  portion  of 
the  crystal  represented  is 
that  contained  in  one  octant 
or  one  eighth  of  the  whole 
form.  All  edges  in  the  three 
remaining  octants  in  front 
of  the  plane  of  projection  are 
obtained  by  the  same  method 
of  projection.  Those  in  the  four  octants  behind  the  plane  of  pro- 
jection, where  the  form  is  symmetrical  in  respect  to  a  center,  may 
be  conveniently  drawn  as  in  Fig.  65.  If  from  any  point  c  a  line 
be  drawn  to  the  center  o  and  extended  beyond  the  center  an 
equal  distance  as  —  co,  —  c  will  be  the  point  behind  the  plane  of 
projection  corresponding  to  c  in  front,  likewise  all  other  points 
may  be  located  by  drawing  lines  through  the  center  o  and  the 
faces  represented  by  connecting  these  points. 

Spherical  or  stere ©graphical  projection. —  In  the  spherical  pro- 
jection the  crystal  to  be  represented  is  placed  at  the  center  of  a 
sphere  so  that  the  intersection  of  the  crystallographical  axes  coin- 
cides with  the  center.  The  plane  of  projection  passes  through  the 
center  of  the  sphere  at  right  angles  to  the  c  axis,  and  intersects 
the  sphere  in  a  great  circle,  the  equator ;  the  c  axis  when  extended 
will  intersect  the  sphere  of  projection  at  the  north  and  south  poles. 
The  eye  is  situated  at  the  south  pole  to  view  the  faces  located  in 


-b 


FIG.  65. 


CRYSTALLOGRAPHY 


43 


the  northern  hemisphere  and  at  north  pole  to  view  those  in  the 
southern  hemisphere.  Crystal  faces  are  not  represented  by  the  pro- 
jection of  their  edges,  but  by  the  location  on  the  plane  of  projec- 
tion of  the  point  of  contact  with  the  sphere  of  a  radius  perpen- 
dicular (the  normal)  to  the  face,  as 
viewed  from  the  south  pole.  In 
Fig.  66  the  plane  of  projection  per- 
pendicular to  the  paper  is  repre- 
sented by  bb',  which  also  represents 
the  b  axis;  the  c  axis  c'e ;  the  a 
axis  perpendicular  to  the  paper  will 
be  represented  by  a  point  at  o. 
Four  faces  c,  f,  d,  b,  belonging  to 
the  zone  of  which  the  axis  a  is  the 
zonal  axis,  their  normals  when  ex- 
tended will  intersect  the  sphere  at 
their  respective  poles  c',  f,  d',  b'; 
these  poles  when  viewed  by  the  eye  at  e  will  appear  on  the  plane 
of  projection,  bb',  as  if  they  were  actually  located  at  o,  i" ,  d",  b', 
their  projections.  The  distance  from  o  at  which  the  pole  of  any 
face  will  appear  in  the  projection,  as  f,  is  proportional  to  the  tan- 
gent of  one  half  the  angle  foe ;  for  of"  =  oe  tan  oef " ;  oef "  =  \  cof ' ; 
tan  26°(foc  =  26°)  =  .23,  or  the  distance  of  f"  from  o  is  23/160  of  the 
radius.  It  may  be  seen  by  the  construction  of  Fig.  66,  that  all  the 
normals  of  any  one  zone  will  lie  in  one  plane ;  their  poles  will  all 
lie  on  the  great  circle  in  which  this  plane  intersects  the  sphere  of 
projection ;  therefore  the  arc  between  any  two  poles  is  a  measure 
of  the  angle  between  their  normals.  It  is  the  supplement  of  the 
angle  between  the  crystal  faces  which  the  normals  represent. 
The  arc  f'd'  measures  the  angle  d'of',  and  as  the  angle  of  a  =  oda 
=  90°,  then  d'of  -f-  daf  =  180°.  As  the  angle  between  the  normals 
is  the  angle  actually  measured  by  the  reflecting  goniometer,  it  is 
the  angle  reported  and  used  in  the  descriptions  of  crystals. 

All  poles  in  the  northern  hemisphere  when  viewed  from  the  south 
pole  will  fall  upon  the  plane  of  projection  within  the  equator  or 
primitive  circle.  When  the  plane  of  projection  is  a  plane  of  sym- 
metry, the  projection  of  the  northern  hemisphere  will  also  be  a  pro- 
jection of  the  southern  hemisphere  as  viewed  from  the  north  pole. 
Similar  poles  above  and  below  will  coincide  on  the  plane  of  pro- 
jection. All  zones  or  great  circles  perpendicular  to  the  plane  of 
projection  are  projected  in  diameters  of  the  primitive  circle;  all 


44 


MINERALOGY 


zones  or  great  circles  inclined  to  the  plane  of  projection  are  pro- 
jected in  arcs  of  circles  cutting  the  primitive  circle  at  the  ends 
of  a  diameter.  Before  illustrating  by  an  example  the  method  of 
drawing  the  spherical  projection  of  a  crystal,  it  is  necessary  to  have 
well  in  mind  several  problems  constantly  in  use  during  the  con- 
struction. 

If  from  the  pole  of  a  zone  circle,  lines  be  drawn  through  the  poles 
of  any  two  faces  lying  upon  the  zonal  circle,  and  extended  until  they 
intersect  the  primitive  circle,  the  arc  of  the  primitive  circle  inter- 
sected will  measure  the  angle  between  the  normals  of  the  faces. 

Problem  I.  Given  the  projection 
of  any  great  or  zone  circle,  to  find 
the  projection  of  its  pole.  Let  dsc, 
Fig.  67,  be  the  zone  circle ;  draw  the 
diameter  dc,  and  the  diameter  so 
perpendicular  to  dc,  so  will  be  the 
projection  of  a  great  circle,  with 
its  pole  at  c  and  at  90°  to  dsc; 
therefore  the  pole  of  dsc  must  lie  on 
so  90°  from  s ;  draw  cs  and  extend 
to  intersect  the  primitive  circle  at  s, 
lay  off  sa  =  90°,  connect  ac,  and 

where  it  crosses  os  at  p  will  be  a  point  on  a  great  circle  at  right 
angles  to  the  given  zone  dsc  and  90°  from  it ;  it  is  therefore  the 
pole  of  dsc. 

Problem  II.—  Given  the  projection  of  the  pole  of  any  zonal 
circle,  to  draw  the  projection  of  the  circle,  is  simply  the  reverse  of 
problem  I,  Fig.  67. 

Problem  III.  — Through 
any  two  given  poles  to  draw 
the  projection  of  the  great 
or  zonal  circle  to  which  they 
belong. 

In  Fig.  68  let  P,  S  be  the  d 
two  given  poles ;  then  draw 
the  diameter  po,  and  oa  at 
90°  to  it,  then  a  is  the  pole 
of  the  great  circle  of  which 
Po  is  the  projection  and  the  FlG'  6a 

given  pole^  P  will  lie  upon  it ;  draw  aP,  extend  it  to  P',  lay  off 
P'b  =  180°;  draw  ab  to  meet  Po  extended  at  B;  B  will  be  the 


FIG.  67. 


CRYSTALLOGRAPHY 


45 


FIG. 


projection  of  a  point  on  the  sphere  at  the  opposite  end  of  a  diame- 
ter from  P ;  therefore  draw  a  circle  through  BSP,  and  cSd  will  be 
the  projection  of  the  zonal  circle  re- 
quired. 

Problem  IV.  —  Given  the  projec- 
tion of  any  two  poles,  to  find  the 
angle  between  them.  In  Fig.  69 
let  a,  b  be  the  given  poles;  by 
Problem  III  draw  the  zonal  circle 
dabc,  and  find  its  pole  P  by  Problem 
I ;  draw  Pa  and  Pb,  extending  them 
to  meet  the  primitive  at  a'b',  then 
the  arc  a'b'  will  measure  the  angle 
between  the  normals  ab. 

Problem  V.  —  Given  the  zone  circle  and  the  projection  of  one 
pole  in  the  zone,  to  find  the  projection  of  a  face  in  the  same  zone 
at  a  given  angle  from  it. 

This  is  the  reverse  of  III.  In  Fig.  69,  let  dac  be  the  zone  circle, 
a  the  given  pole,  to  find  the  pole  b,  80°  from  it.  Find  the  pole  p 
of  dac  by  I;  draw  Paa'  and  make  a'b'  =  80°  and  draw  b'P; 

where  it  crosses  the  zonal  circle  dac 
as  at  b  will  be  the  projection  of  the 
pole  80°  from  a  as  required. 

Problem  VI.  —  To  locate  the  pro- 
jection of  any  pole,  the  axial  ratio 
and  the  indices  of  the  face  being 
given. 

The  axial  ratio  of  barite  is  &  :  b :  c 
=  .815  :  i :  1.313 ;  locate  the  pole  of 
y  =  (122)  =  2  a :  b  :  c. 

In  Fig.  70  draw  the  primitive  and 
the  two  diameters  aa',  bb'  at  90°. 
Lay  off  ob"  =  b,  the  intercept  on 
the  axis  a  is  oa"  =  2  (b  X  .815) ;  draw  b"a"  and  oc  at  90°  to  a"b", 
then  oc  will  be  the  projection  of  a  zonal  circle  at  90°  to  the  primi- 
tive with  its  pole  at  P,  on  which  the  pole  of  122  will  lie.  Lay 
off  oc'  =  b  X  1.313,  or  the  unit  on  the  vertical  axis,  and  od  =  oc, 
connect  d  and  c' ;  the  angle  c'do  =  the  angle  between  the  normal 
of  y,  (122)  and  the  normal  to  the  base  ooi ;  draw  Poc",  and  make 
c"oy'  =  c'do,  draw  y'P ;  where  it  crosses  oc  as  at  y  will  be  the 
projection  of  the  pole  of  122  as  required. 


FIG.  70. 


46 


MINERALOGY 


FIG.  71. 


Example.  —  Let  it  be  required  to  project  a  barite  crystal  with 
the  following  forms:  (100)  (010)  (001)  (110)  (102)  (Oil)  (111) 
(112)  :  1HU10  =  78°  22'  :  OOU02  =  38°  51'  :  001,011  =  52° 
43':  001*112  =  45°. 

Draw,  Fig.  71,  the  primitive  circle  within  or  upon  which  all 
poles  will  fall.  As  the  c  axis  is  projected  at  the  center  of  the  primi- 
tive, and  is  normal  to  the  base, 
ooi,  c  at  the  center  will  be  the  pole 
of  oo  i.  Draw  two  diameters  aa', 
bb'  at  90° ;  these  will  represent  the 
a  and  b  axes.  The  poles  of  all  the 
faces  belonging  to  the  zone  of  which 
c  is  the  axis  will  fall  on  the  primi- 
tive circle,  as  their  normals  will  lie 
in  the  plane  of  the  paper,  the  last 
term  of  their  indices  will  be  o. 
100  will  be  projected  at  the  ex- 
tremities of  the  axis  fi,  oio  at  the 
extremities  of  b;  the  remaining  member  of  this  zone  (no)  will  lie 
symmetrically  on  either  side  of  the  axis  b.  Draw  the  two  diame- 
ters at  78°  22',  and  the  poles  of  (no)  will  lie  at  their  extremities. 
The  form  (on)  is  a  member  of  the  zone  of  which  the  axis  &  is 
the  zonal  axis ;  its  poles  will  lie  on  the  diameter  of  the  primitive, 
perpendicular  to  &,  52°  23'  from  the  pole  c.  From  a'  lay  off  a'e  = 
52°  23',  draw  ea  and  where  it  crosses  be  is  the  pole  on  ;  on  will 
be  an  equal  distance  on  the  other  side  of  c.  The  poles  of  (102)  will 
lie  on  the  diameter  aa'  and  are  located  by  the  same  method  as  (01 1) . 
The  two  pyramids  (in),  (112)  are  members  of  the  zones  no  —  ooi 
and  Iio  —  ooi ,  their  poles  will  lie  on  these  two  diameters.  The  form 
(in)  is  also  a  member  of  the  zone  on  —  100  and  oil  —  100, 
its  poles  will  be  situated  at  the  intersection  of  the  two  zonal  circles. 
Draw  the  zonal  circle  100,  on,  loo;  where  this  intersects  the  zone 
no,  ooi,  no  as  at  in  will  be  the  pole  of  the  unit  pyramid.  Poles 
of  (112)  are  located  by  the  angle  ooiAii2  =  45°  ;  when  the  angle 
between  the  base  and  any  face  is  given,  the  zone  being  known,  its 
poles  are  quickly  found  by  the  tangent  rule,  or  by  construction, 
problem  V. 

The  advantages  of  the  stereographical  projection  are,  that  it 
shows  at  once  the  symmetry  of  the  crystal,  connects  all  faces  be- 
longing to  the  same  zone,  and  by  simple  construction  the  angle 
between  any  two  faces  may  be  measured. 


CHAPTER  III 
ISOMETRIC   SYSTEM 

THE  32  TYPES  OF  CRYSTALS 

CRYSTAL  forms,  from  a  consideration  of  their  symmetry  alone, 
are  grouped  into  32  types ;  when  referred  to  their  axes,  they  fall 
into  six  systems.  The  types  which  are  included  in  any  one  system 
are  independent,  and  one  type  cannot  combine  with  another  to 
form  crystals  even  though  they  belong  to  the  same  system ;  each 
type  possesses  a  symmetry  entirely  its  own,  a  characteristic  derived 
from  the  molecular  arrangement  and  molecule  itself.  At  the  same 
time  all  those  included  in  one  system  are  related  by  the  possession  of 
axes  or  planes  of  symmetry  to  a  large  extent  common  to  the  group. 
For  this  reason  the  old  classification  of  holohedrons,  hemihedrons, 
etc.,  is  also  given,  and  their  derivation  one  from  the  other,  as  the 
best  method  for  the  beginner  to  obtain  a  clear  understanding  of  the 
relation  of  types  and  the  influence  of  planes  and  axes  of  symmetry 
in  the  development  of  crystal  forms. 

Under  the  isometric  system  are  grouped  all  those  crystal  forms 
which  are  referred  to  three  equal  interchangeable  axes,  intersecting 
at  90°.  It  includes  five  types,  all  of  which  are  characterized  by  at 
least  four  trigonal  and  three  digonal  axes  of  symmetry.  Since  the 
crystallographical  axes  are  all  equal,  they  are  designated  by  the 
symbol  a,  and  a  becomes  the  unit  of  measurement  on  these  axes. 
The  most  general  set  of  parameters  is  therefore  na :  a :  ma,  in  which 
the  two  variables  n  and  m  may  have  any  value  between  unity, 
when  the  plane  intersects  that  axis  at  unit  length  or  a,  and  infinity, 
when  the  plane  is  parallel  to  that  axis,  or  intersects  it  at  infinity. 
From  this  standpoint  all  forms  in  the  system  may  be  considered 
as  special  cases  of  the  most  general  form. 

CLASS,  ISOMETRIC  HOLOHEDRAL,  HOLOSYMMETRIC,  OR  NORMAL 
TYPE  32,  DITESSERAL  CENTRAL 

This  type  possesses  the  highest  symmetry  possible  in  any  crys- 
tal form,  and,  as  in  all  holohedral  classes,  the  forms  are  bounded  by 

47 


48 


MINERALOGY 


pairs  of  parallel  faces.  Diagram  Fig.  72  represents  the  symmetry ; 
13  axes,  9  planes,  and  a  center.  The  value  and  position  of  the  axes 
and  planes  may  be  understood  best  by  the  consideration  of  their 

relation  to  the  edges  of  the  cube  or 
hexahedron. 

There  are  three  ditetragonal  axes 
ending  in  the  center  of  the  cube 
faces,  these  are  the  crystallographi- 
cal  axes;  four  ditrigonal  axes  end- 
ing in  the  corners;  six  didigonal 
axes  ending  in  the  middle  of  the 
edges;  three  planes  of  symmetry 
bisect  the  edges  of  the  cube,  and 
contain  the  crystallographical  axes ; 
they  intersect  in  the  center  of  sym- 
metry and  divide  space  arou  d  it  into  eight  equal  portions  (oc- 
tants). The  remaining  six  planes  contain  the  edges,  each  plane 
passing  through  the  center  and  opposite  edges.  The  nine  planes 
of  symmetry  divide  space  around  the  center  into  48  equal  tri- 
angular solid  angles. 


FIG.  72. 


Forms 

I.  Hexoctahedron ;  nararma;    (hkl),  Fig.  73. 

Here  the  values  of  the  coefficients,  m  and  n,  are  independent  of 
each  other  and  not  at  their  limiting  values,  i  or  oo.  When  n  =  2 
and  m  =  3,  yielding  the  parameters 
3  a :  a :  2  a,  they  will  locate  a  face  in 
each  one  of  the  48  triangular  segments 
into  which  the  planes  of  symmetry  di- 
vide space,  or  48  sc.alene  triangles. 
This  is  the  largest  number  of  faces 
possible  on  any  crystal  form.  Eight 
faces  are  symmetrically  grouped  around 
the  extremities  of  the  ditetragonal 
axes  (crystallographical  axes) ;  six 
around  the  ditrigonal  axes  (center  of 
the  octants) ;  four  around  the  di- 


FIG.  73.  — The  Hexahedron, 
3a:a:2a. 


digonal  axes.  All  faces  are  similar  scalene  triangles,  each  of 
which  intersects  one  axis  at  unity,  the  second  at  a  greater  dis- 
tance, the  third  at  a  still  greater  distance.  Faces,  edges,  and 


ISOMETRIC   SYSTEM 


49 


angles  of  the  form  will  vary  with  the  value  of  m  and  n,  there  is 
therefore  a  series  of  hexoctahedra,  of  which  3  a :  a :  2  a  is  one. 

The  spherical  projection  of  the 
hexoctahedron  is  represented  in  Fig. 
73  a.  The  planes  of  symmetry  are 
represented  by  the  great  circles  in 
which  they  cut  the  sphere  of  projec- 
tion. The  poles  of  the  faces  in  the 
northern  hemisphere  are  represented 
by  small  circles,  those  in  the  south- 
ern hemisphere  by  crosses.  The 
cross  within  the  circle  indicates  that 
the  two  hemispheres  are  mirror 
images  of  each  other,  and  the  type 
is  equatorial.  The  points  at  which  the  axes  of  symmetry  emerge 
are  indicated  by  the  conventional  signs. 

II.  Tetrahexahedron ;    na:a:ooa;  (hko),  Fig.  74. 

This  form  is  a  special  case  of  (hkl),  where  1  =  o ;  or  each  face 
cuts  one  axis  at  oo,  one  at  unity,  and  one  at  an  intermediate 
distance.  If  the  poles  of  (hkl)  are  moved,  so  as  to  lie  in  the 
diametral  planes,  Fig.  73  a,  two  normals  will  coincide,  as  a  with  a' 

e 


FIG.  73  a.  —  Hexoctahedron. 


FIG.    74. — The    Tetrahexahe- 
dron (320),  of  Fluorite. 


FIG.  75.  —  Tetrahexahedron 
(320). 


or  b  with  b',  producing  a  form  bounded  by  24  isosceles  triangles. 
Four  faces  are  grouped  around  the  ditetragonal  axes,  Fig.  75,  and 
six  around  the  di trigonal,  and  the  didigonal  axes  bisect  the  basal 
edges  between  adjacent  faces.  The  solid  angles  will  vary  with  the 
value  of  n,  yielding  a  series  of  tetrahexahedra,  members  of  which 
may  occur  in  combination.  The  form  may  also  be  considered  as 
derived  from  the  cube  by  replacing  each  face  with  four  triangles. 


50  MINERALOGY 

III.  Tetragonal  Trisoctahedron ;    na:a:na;    (hhl),  Fig.  76. 

If  in  place  of  moving  the  pole  of  the  most  general  form  (hkl) 
to  the  diametral  plane,  it  be  now  moved  into  the  planes  of  symmetry 
which  bisect  the  octants,  and  between  the  ditetragonal  and  ditrig- 
onal  axes ;  as  the  poles  approach  the  plane  the  angle  between  the 


FIG.  76. — The  Tetragonal  Trisocta-        FIG.  77. — The  Tetragonal  Tris- 
hedron,  (hhl).  octahedron. 

normals  constantly  diminish  until  the  plane  is  reached,  when  it 
becomes  0,  Fig.  73  a,  and  the  angle  between  the  faces  they  represent 
is  180°.  Thus  two  faces,  b  and  c,  a  and  e,  of  the  most  general  form 
will  coalesce,  producing  a  form  bounded  by  24  four-sided  faces, 
Fig.  76,  having  three  faces  entirely  within  each  octant.  Four 
faces  are  grouped  around  the  ditetragonal,  three  around  the  ditrig- 
onal,  and  four  around  the  didigonal  axes.  In  this  form,  also,  the 
solid  angles  between  faces  will  vary  with  the  value  of  n,  yielding  a 
series.  The  tetragonal  trisoctahedron  may  be  produced  by  re- 
placing each  face  of  the  octahedron  with  three  tetragonal  faces. 

IV.  Trigonal  Trisoctahedron ;   a:a:na;    (hhi),  Fig.  78. 

Let  the  poles  of  the  most  general  form  now  be  moved  till  they  lie 
in  the  plane  of  symmetry  between  the  ditrigonal  and  didigonal 
axes,  Fig.  73  a.  Again  two  faces,  a  and  b,  a'  and  b',  of  the  most 
general  form  will  fall  in  one  plane,  producing  still  a  third  form 
bounded  by  24  faces,  Fig.  78;  each  face  is  an  isosceles  triangle  with 
its  base  lying  in  the  diametral  plane.  Eight  of  its  faces,  Fig.  79,  are 
grouped  around  the  ditetragonal,  three  around  the  ditrigonal,  and 
the  didigonal  axes  bisect  the  base  of  the  triangular  face.  As  in 
the  preceding  forms  the  solid  angles  vary  with  the  value  of  n, 
producing  a  series  of  trigonal  trisoctahedra. 


ISOMETRIC  SYSTEM 


51 


V.  Hexahedron;   a :  oo  a :  <x>  a;  (ooi),  Fig.  80. 

In  the  previous  cases  the  poles  of  the  most  general  form  have  been 
moved  into  one  of  the  sides  of  the  triangle,  in  which  it  lies ;  there 
are  still  three  possibilities,  the  three  corners  of  the  triangle.  Let 


FIG.  78.  —  The  Trigonal  Trisocta- 
hedron,  (hhi). 


FIG.  79. — Trigonal  Trisocta- 
hedron,  (hhi). 


it  now  be  moved,  Fig.  73  a,  to  coincide  with  the  ditetragonal  axis, 
when  all  eight  faces  of  the  most  general  form  grouped  around  this, 
as  b,  b',  c,  etc.,  will  fall  in  one  plane,  producing  a  form  with  six 
faces,  the  hexahedron,  or  cube,  Fig.  80.  Each  face  will  cut  one 
axis  and  is  parallel  to  the  other  two.  The  ditetragonal  axes  will 


FIG.  80.— The  Hexahedron,  (100). 


FIG.  81.  —  The  Rhombic  Dodeca- 
hedron, (110). 


end  in  the  center  of  the  faces,  the  ditrigonal  in  the  corners,  and  the 
didigonal  will  bisect  the  edges.  The  angles  between  the  faces  are 
fixed  at  90°,  there  is  but  one  hexahedron  and  not  a  series.  It  is 
therefore  termed  a  fixed  form. 


52 


MINERALOGY 


VI.  Rhombic  Dodecahedron ;   a:a:ooa;    (no),  Fig.  81. 

If  the  pole  is  now  moved  to  the  didigonal  axes,  Fig.  73  a,  four  faces 

a,  a',  b,  b',  will  fall  in  one  plane,  producing  a  form  with  12  rhombic 

faces,  Fig.  81.     The  faces  are  grouped  four  around  the  ditetragonal 

axes,  three  around  the  ditrigonal,  and  the  didigonal  axes  bisect 

the  edges.  There  is  but  one  rhom- 
bic dodecahedron  with  the  angles 
fixed  at  120°.  It  is  also  a  fixed 
form. 

VII.  Octahedron;  a:a:  a;  (in), 
Fig.  82. 

The  seventh  and  last  possible 
form  in  this  type  is  where  the  pole 
is  moved  to  the  ditrigonal  axes, 
when  the  six  faces  a,  b,  c,  e,  etc., 
of  the  general  form  grouped  around 
this  axis  will  fall  in  one  plane,  pro- 
ducing a  form  bounded  by  eight 

equilateral  triangular  faces,  Fig.  82.  Four  faces  are  grouped 
around  the  ditetragonal  axes,  the  ditrigonal  axes  terminate  in  the 
center  of  the  face,  and  the  didigonal  axes  bisect  the  edges.  All 
dihedral  angles  of  the  regular  octahedron  are  fixed  at  70°  31 '  42" ; 
it  is  therefore  a  fixed  form. 


FIG.  82.  —  The  Octahedron,  (111). 


Relation  of  the  Seven  Forms 

When  any  one  of  the  48  triangular  segments  into  which  the  planes 
of  symmetry  divide  space  is  considered,  Fig.  83,  it  has  been  shown 
that  the  pole  na :  a  :  ma,  the  hexoc-  a:  a: a 

tahedron,  may  be  located  anywhere 
within  the  area,  and  when  it  ap- 
proaches the  sides  or  angles,  either 
one  or  both  of  the  variables  m  and  n 
approach  their  limiting  values  1  and 
oo.  If  the  pole  approaches  one  of 
the  sides,  only  one  of  the  variables 

approached     its     limit,     or     the     two    coa:a»a 

variables  are  of  the  same  value.     The  FlG-  83- 

hexoctahedron,  tetrahexahedron,  tetragonal  trisoctahedron,  and 
the  trigonal  trisoctahedron,  are  known  as  the  variable  forms,  since 
their  parameters  contain  a  variable.  The  position  of  the  pole  of 


ISOMETRIC  SYSTEM 


53 


any  one  form  on  the  triangle  will  depend  upon  the  value  of  the 
variable.  As  the  pole  of  any  of  the  variable  forms  approaches 
the  angle  of  the  triangle,  both  variables  approach  their  limits;  and 
upon  reaching  their  limits  the  pole  assumes  the  position  of  one  of 
the  axes  of  symmetry,  and  as  there  is  only  one  point  in  the  angle 
of  the  triangle,  therefore  there  is  only  one  possible  form  of  the 
hexahedron,  rhombic  dodecahedron,  or  octahedron,  and  they  are 
known  as  the  fixed,  or  limiting  forms.  All  substances  crystalliz- 
ing in  these  forms  must  have  the  same  angle. 

In  each  type  there  are  always  seven  possible  forms.  The  most 
general  form  is  represented  by  the  area  of  the  triangle ;  and  as  the 
number  of  points  which  the  pole  of  the  face  may  occupy  is  unlimited, 
there  are  therefore  innumerable  individuals  forming  a  series.  The 
three  sides  of  the  triangle  each  represents  a  series  of  variable  forms, 
as  here  also  there  is  a  large  number  of  points  on  each  side  between 
the  angles,  each  of  which  may  be  occupied  by  the  pole  in  turn. 
The  three  angles  of  the  triangle  represent  the  three  fixed  forms, 
as  there  is  only  one  point  in  each  of  the  three  angles.  The 
seven  forms  possible  in  each  type  are  represented  by  the  seven 
elements  of  the  triangle,  of  which  the  three  angles  represent  the 
three  fixed  forms,  and  the  three  sides  and  area  represent  the 
variable  forms. 

Combination  of  Forms 

Crystals  may  present  one  form  only,  when  the  number  of  faces 
is  very  limited ;  more  often  they  are  combinations  of  two,  three,  or 
even  all  seven  of  the  possible  forms  in  the  type,  and  in  addition 
forms  of  the  same  series ;  in  such  cases  the  number  of  faces  possible 


—*. -.kl 


FIG.  84. 


FIG.  85. 


on  a  crystal  is  very  large  and  their  relation  complex.     Such  com- 
plex crystals  are  rare  in  nature,  for  by  far  the  larger  number  are 


54 


MINERALOGY 


combinations  of  a  few  simple  forms.  The  general  appearance  or 
habit  is  fixed  by  the  simple  form  predominating  in  the  combination. 
Figure  84  is  a  combination  of  a  hexahedron  and  octahedron,  the 
former  predominating ;  Fig.  85  is  the  same  combination  with  the 
latter  predominating. 

Examples  crystallizing  in  the  Type 
Copper,  forms  (100)  (110)  (111)  (410)  (211)  (531).     Fig.  86. 


FIG.  86.  —  Combination  of  a  (001) 
and  d  (110)  on  copper. 


FIG.  87.  —  Magnetite,   a  (GUI) 
and  d  (110). 


Lead,  forms  (100)  (110)  (111)  (410)  (550). 
Silver,  forms  (110)  (111)  (310)  (751). 

Galena,  PbS,  forms   (100)   (110)   (111) 

(221).     Fig.  84. 
Magnetite,  Fe3O4,    forms    (100)    (110) 

(111)  (210)    (221)    (432).     Fig.  87. 
Fluorite,  CaF2,  forms  (100)  (111)  (421) 

(110)  (211). 
Analcite,      NaAl3(SiO3)2,  H2O,      forms 

(100)  (211)  (575). 
n 

FIG.    88.  —  Garnet ;    Combi- 
nation of  n  (221)  andd(HO). 


Fig- 


CLASS,  TETRAHEDRAL  (DIAGONAL-FACED)  HEMIHEDRONS 
TYPE  31,  DITESSERAL  POLAR 

All  forms  of  this  type  possess  four  ditrigonal  polar  axes,  terminat- 
ing in  the  center  of  the  octants.  The  conditions  surrounding  one 
extremity  of  the  axis  are  different  from  those  at  the  other  extrem- 


ISOMETRIC  SYSTEM 


55 


ity,  hence  the  term  polar.  The 
crystallographic  axes  are  didigonal 
axes.  The  six  planes  of  sym- 
metry bisect  the  octants  and  pass 
through  opposite  edges  of  the  cube, 
Fig.  89. 

Symmetry,  4  ditrigonal  polar 
axes,  3  didigonal  axes,  and  6 
planes.  There  being  no  center  of 
symmetry,  the  forms  are  not 
bounded  by  parallel  faces. 


FIG.    89.  —  Diagram    of    Axes    and 
Planes  of  Symmetry  in  Type  31. 


Forms 


I.    Hextetrahedron ; 


na  :  a  :  ma 


K  (hkl)  K  (hkl). 


In  grouping  the  faces  around  the  isometric  axes  so  as  to  conform 
to  the  symmetry  of  any  type,  it  is  necessary  to  cut  all  extremities 
of  the  crystallographical  axes  with  the  same  number  of  faces 
and  at  the  same  inclination,  since  the  axes  are  interchangeable. 
If  planes  are  grouped  on  the  axes,  fulfilling  the  symmetry  of  this 
type,  the  most  general  form  will  be  bounded  by  24  similar  scalene 
triangles,  Fig.  90;  six  faces  are  grouped  around  the  ditrigonal, 


FIG.  90.  — The  Hextetrahedron, 
K(hkl). 


FIG.  91.  —  The  Hextetrahedron, 
K(hkl). 


four  around  the  didigonal  axes.  The  spherical  projection,  Fig. 
91,  shows  that  the  poles  (circles)  in  the  northern  hemisphere  do  not 
reflect  those  in  the  southern  hemisphere  (crosses) ;  therefore  the 
plane  of  projection  is  not  a  plane  of  symmetry.  If  this  projection 
is  compared  with  Fig.  73  a,  it  will  be  seen  that  the  poles  of  the  hextet- 


56  MINERALOGY 

rahedron  correspond  to  one  half  the  poles  of  hexoctahedron.  It  is 
as  if  all  the  faces  in  alternate  octants  above  and  below  were  extended 
(Fig.  35,  the  shaded  octants)  until  they  inclosed  space  ;  the  form 
produced  would  be  the  +  hextetrahedron.  When  the  unshaded 
faces  are  extended  the  —  hextetrahedron  is  produced,  congruent 
with  the  former  by  a  revolution  of  90°.  In  all  hemihedrons  there 
are  +  and  —  ,  or  right  and  left  forms,  which  may  occur  on  crystals 
in  combination,  or  independently.  ±  forms  are  always  congruent 
by  a  revolution. 

Other  forms  of  this  type  may  be  produced,  as  in  the  holohedral 
class,  by  moving  the  pole  of  the  most  general  form  to  the  sides  and 
angles  of  the  triangle  in  which  it  lies,  yielding  in  all  seven  possible 
forms,  some  of  which  will  be  new  forms  ;  others  will  be  of  the  same 
shape  as  the  holohedral  forms. 

II.  In  Fig.  91,  if  all  the  poles  be  moved  till  they  lie  on  the  side  of 
the  triangles  between  the  two  clidigonal  axes,  they  will  occupy  the 
same  position  as,  Fig.  75,  the  poles  bf  .the  tetrahexahedron.     The 
holohedral  and  hemihedral  forms  are  of  the  same  shape,  but  the 
symmetry  of  the  two,  caused  by  the  character  or  arrangement  of 
the  molecules,  will  differ.     Where  an  apparent  holohedral  form  is 
found  in  combination  with  hemihedral  forms,  it  must  be  considered 
as  a  hemihedron  and  will  possess  the  lower  type  of  symmetry. 

The  tetrahexahedron  is  reproduced  by  extending  the  faces  which 
exist  in  alternate  octants,  as  each  face  of  the  holohedral  form 
extends  in  two  octants,  half  in  each  ;  the  half  lying  in  the  octants 
extended  will  reproduce  the  half  in  the  adjacent  octants.  There 
are  no  +  or  —  forms  in  those  cases  where  the  hemihedron  assumes 
the  holohedral  shape. 

III.  Trigonal  tristetrahedron  ;   ±  na  •  a  •  na  . 


If  the  poles  be  placed  on  the  side 
of  the  triangle  between  the  ditrigonal 
and  didigonal  axes,  two  will  coincide, 
yielding  a  new  form,  the  trigonal 
tristetrahedron,  bounded  by  12  simi- 
lar isosceles  triangles,  Fig.  92.  Three 
faces  are  grouped  around  one  ex- 
tremity of  the  ditrigonal  axis  and 
6  around  the  other.  The  didigonal 

FIG.  92.—  The  Plus  Trigraal  Tris-    axes  bisect  the  base  of  the  triangu- 
tetrahedron.  lar  faces.     This  form  may  also  be 


ISOMETRIC  SYSTEM 


57 


considered  as  produced  by  the  extension  of  alternate  octants  of  the 
tetragonal  trisoctahedron.     There  are  congruent  +  and  —  forms. 


IV.  Tetragonal  tristetrahedron ;   ± 


na  :  a  :  a 


K  (hhi)  K  (hhi). 


Let  the  poles  now  be  moved  on  the  side  of  the  triangle  between 
the  ditrigonal  axes,  when  the  tetragonal  tristetrahedron  bounded 
by  12  tetragonal  faces  will  be  pro- 
duced, Fig.  93.  Three  faces  are  grouped 
around  the  extremities  of  the  ditrig- 
onal, four  around  the  didigonal  axes. 
This  form  may  also  be  derived  by  ex- 
tending alternate  octants  of  the  trigonal 
trisoctahedron. 

The  four  tetrahedral  forms  thus  far 
considered  are  variable  forms,  as  their 
angles  will  depend  upon  the  value  of  the 
intercepts. 

V.  If  the  pole  be  placed  on  the  didigonal  axes,  the  hexahedron 
will  be  reproduced,  as  in  type  32. 

VI.  If  the   pole  be  placed  in  the  plane  of  symmetry  midway 
between  the  ditrigonal  axes,  the  rhombic  dodecahedron  will  be  re- 
produced ;  both  hexahedra  and  rhombic  dodecahedra  may  combine 
with  tetrahedral  forms. 

A       •       A       *       O 

VII.  Tetrahedron;    ±-        -  ;   K  (in)  K  (111). 


FIG.   93.  —  Tetragonal  Tris- 
tetrahedron, K  (221). 


FIG.  94.  —  The  Plus  Tetrahedron, 

K(lll). 


FIG.  94  a.  — The   Negative  Tetra- 
hedron,  K(lll). 


If  the  poles  be  placed  on  the  ditrigonal  axes,  six  faces  will  fall 
in  the  same  plane,  producing  a  form  bounded  by  four  equal  equi- 
lateral triangles,  the  regular  tetrahedron,  Fig.  94.  It  may  also  be 


58 


MINERALOGY 


considered  as  produced  by  the  extension  of  alternate  faces  of  the 
octahedron. 

Combinations 

Of  the  seven  forms  which  may  combine  in  the  tetrahedral  class, 
the  tetrahexahedron,  hexahedron,  and  rhombic  dodecahedron  are 
apparently  holohedrons  in  shape.  They  can  be  distinguished  from 
the  latter  when  not  in  combination  with  tetrahedral  forms  only  by 
special  markings,  striations,  etch  figures,  or  other  physical  proper- 
ties, which  indicate  a  symmetry  of  the  lower  type. 

Crystal  faces  often  contain  striations,  parallel  to  the  edge  of 
common  combinations  found  represented  on  the  crystals.  They 


FIG.  95. 


FIG.  96.  —  The  Plus  and  Minus 
Tetrahedrons  of  Sphalerite. 


are  attributed  to  alternations  of  growth,  in  which  the  face  is  reduced 
to  the  minimum,  and  appear  as  a  striation  parallel  to  the  common 
edge.  Fig.  95  represents  a  combination  of  the  hexahedron  and 
tetrahedron  in  sphalerite.  The  striations  on  the  hexahedral  faces 


<2± 


FIG.  97.  —  Combination  of 
K(lll),  K(211),  (110)  on 
Tetrahedrite. 


FIG.  98.  —  Combination  of 
(100),  K(lll),  K(211)  on 
Boracite. 


are  traces  of  tetrahedral  faces.     It  is  to  be  noted  that  the  striations 
on  the  cube  faces  are  symmetrical  to  planes  containing  opposite 


ISOMETRIC  SYSTEM 


59 


edges  of  the  cube,  which  mark  it  as  a  hemihedral  cube  belonging 
to  the  tetrahedral  type  of  symmetry,  even  if  the  tetrahedron  did 
not  truncate  the  corners. 

Examples  of  minerals  crystallizing  in  the  type. 

Diamond,  C,  (ill)  (100)    (321)  (210)  (320). 

Sphalerite,  ZnS,  (100)  (111)  (110)  (311)  (331)  (210).     Fig.  96. 

Tetrahedrite,  4Cu2S,  SbsSa,  (100)   (110)  (111)    (221).     Fig.  97. 

Boracite,  (100)  (110)  (111)  (410)  (531)  (221).     Fig.  98. 

CLASS,  PYRITOHEDRAL  (PARALLEL-FACED)  HEMIHEDRONS 
TYPE  30,  TESSERAL  CENTRAL 

All  forms  of  this  type  possess 
four  trigonal  axes  ending  in  the 
center  of  the  octants;  three  di- 
digonal  axes  corresponding  to  the 
crystallographical  axes;  three 
planes,  the  diametral  planes  and  a 
center  of  symmetry.  All  forms  of 
the  type  are  therefore  bounded  by 
pairs  of  parallel  faces.  • 

Symmetry.  —  4  trigonal  axes, 
3  didigonal  axes,  3  planes  and  a 
center,  Fig.  99. 

Forms 


FIG.  99. 


I.  Diploid;   ± 


na  :  a  :  ma 


IT  (hkl)  TT  (hkl.) 


If  planes  be  grouped  on  the  axes  to  conform  to  the  symmetry 
of  the  type,  it  will  be  found  that  if  half  of  the  faces  of  the  hexocta- 


FIG.  100.  — The  Minus  Dip- 
loid, IT  (hkl). 


FIG.  100  a. 


60 


MINERALOGY 


hedron  are  selected,  so  that  pairs  taken  intersecting  in  the  diame- 
tral planes  (Fig.  37,  the  shaded  faces)  are  then  extended,  they 
will  produce  a  new  form,  the  diploid,  Fig.  100,  with  24  four-sided 
faces,  three  of  which  are  grouped  around  the  trigonal  and  four 
around  the  didigonal  axes.  Figure  100  a  shows  the  symmetry  and 
poles  of  the  —  form. 

II.  Pyritohedron  ;  pentagonal  dodecahedron  ±  —  —  ; 

ir(hlo)  ir(hlo). 

When  the  pole  is  moved  to  the  side  of  the  triangle  between  the 
didigonal  axes,  and  in  the  plane  of  symmetry,  a  new  form  will  be 
produced,  the  pyritohedron,  Fig.  101,  bounded  by  12  pentagonal 
faces. 

Three  faces  are  grouped  around  the  trigonal  axes,  and  the  di- 
digonal axes  bisect  the  long  edge  between  adjacent  faces.  The 


FIG.  101. 


The  Pyritohedron, 
ir(hlo). 


FIG.  102.  —  Pyrite:  Combi- 
nation of  (100)  and  IT  (hlo). 


pyritohedron  may  be  considered  as  derived  from  the  tetrahexahe- 
dron  by  extending  alternate  faces. 

III.  Other  forms. 

Other  possible  positions  of  the  poles  are  identical  in  number 
and  positions  with  the  forms  of  type  32:  Therefore  the  tetragonal 
trisoctahedron,  trigonal  trisoctahedron,  hexahedron,  rhombic 
dodecahedron,  and  octahedron  may  be  found  in  combination  with 
the  diploid  and  pyritohedron.  They  also  reproduce  the  same 
forms  when  the  method  of  selection  to  form  hemihedrons  of  this 
class  is  applied  to  them. 

Combinations 

Geometrical  holohedral  forms  of  this  type  must  possess  the  pyri- 
tohedral  symmetry.  Pyrite,  FeSa,  crystallizes  in  all  seven  forms  of 


ISOMETRIC  SYSTEM  61 

the  type,  but  commonly  in  (111)  (010)  TT  (hlO)  with  striations  on  the 
cube  face,  parallel  to  its  edges,  due  to  alternations  of  growth  be- 
tween the  cube  and  pyritohedron,  Fig.  102.  These  striations  are 
parallel  to  the  planes  of  symmetry  which  bisect  the  cube  edges, 
and  not,  as  in  sphalerite,  type  31,  parallel  to  the  planes  which  con- 
tain the  edges  and  cross  the  face  diagonally. 

Other  representatives  of  the  type  are  smaltite,  CoAs2 ;  cobaltite, 
CoAsS. 

CLASS,  PLAGIOHEDRAL  (GYROIDAL)  HEMIHEDRONS 
TYPE  29,  TESSERAL  HOLOAXIAL 

As  the  name  implies,  this  type  possesses  all  the  axes,  3  tetragonal, 
4  trigonal,  6  digonal,  of  the  system,  but  no  planes  or  center  of 
symmetry. 

Forms 

I.    Pentagonal  icositetrahedron  (didodecahedron)  r/1—        — -  • 

T  (hkl)  T  (khl). 

If  every  other  face  of  the  hexoctahedron  around  the  ditetragonal 
axis  is  extended,  as  indicated  by  the  shaded  faces  of  Fig.  39,  the 
solid  formed  will  have  the  symmetry  of  this  type.  It  is  bounded 
by  24  pentagonal  faces,  4  of  which  are  grouped  around  the  tetrag- 
onal axes,  3  around  the  trigonal,  and  the  digonal  axes  of  symme- 
try bisect  the  edge  between  two  faces.  When  the  right  upper  face 
of  the  positive  octant  is  extended,  then  the  right  pentagonal  dido- 
decahedron  is  produced,  Fig.  103.  If  the  left  upper  face  is  ex- 


FIG.  103.  —  The  Right  Pen-  FIG.   104.— The  Left  Pen- 

tagonal Didodecahedron.  tagonal  Didodecahedron. 

tended,  the  left  pentagonal  didodecahedron,  Fig.  104,  is  produced. 
Figure  105  is  a  spherical  projection  of  the  right  form.  They  differ 
from  +  and  —  hemihedra,  as  there  is  no  way  of  revolving  one  into 


62  MINERALOGY 

a  congruent  position  with  the  other ;  they  are  not  superimpos- 
able;   one  is  a  mirror  image  of  the  other. 
Such  pairs  of  forms  are  enantiomorphous. 

....^ Other  forms  of  the   type   do  not 

.••''      /  \  \       '••  differ    geometrically    from    those  of 

\°. ••''••.         tyPe  32,   the   forms   which  may  be 

..-4    XT"0"":4--..X      \      found  in  combination  with  the  pen- 

*/o\  :  •-'  *  :°       "x ';      tagonal  didodecahedron,  the  only  new 

^:.          •  x  ,  :  -.  o  tx  form  of  the  type  will  be  the   tetra- 

''•-..  \/o  I  x  *'•!.-••        /      hexahedron  (hlo). 
\    *,;-*•.  "*"To  /  '\°    /  Tetragonal  trisoctahedron  (hhl). 

\  '/  Trigonal  trisoctahedron,  (hhi). 

--...^....-""''  Hexahedron,  (010). 

FIG.  105.— The  Right  Pentagonal        Rhombic  dodecahedron,  (110). 

Icositetrahedron.  Octahedron,   (111). 

As  six  forms  of  the  type  are  in  shape  holohedrons,  minerals  which 
crystallize  in  the  type  are  distinguished  by  a  study  of  the  symmetry 
of  the  etch  figures;  this  is  especially 
necessary  as  the  occurrence  of  the  most 
general  form  is  always  rare.  Sylvite, 
KC1,  occurs  in  combinations  of  the  cube 
and  octahedron,  Fig.  106.  Etch  figures 
appear  on  the  cube  faces  as  shallow  pits 
with  square  outline.  The  position  of  ^v 
the  square  pits  depends  upon  the  sym- 
metry of  the  point-system,  as  the  type  FIG.  106.  —  Diagram  of  Etch 
contains  no  planes  of  symmetry;  they  Figures  on  Sylvite. 

are  so  oriented  on  the  cube  that  none  of  the  possible  planes  which 
are  traced  on  the  cube  face  will  cut  them  symmetrically. 

Examples. 

Cuprite,  Cu20 ;  (111)  (100)  (110)  (211)  rarely  in  T  (hkl),  from 
Cornwall,  Eng., 

Ammonium  Chloride  NH^Cl;    (111)  (110)  (100)  r(875). 

TETARTOHEDRAL  CLASS 
TYPE  28,  TESSERAL  POLAR 

Symmetry :  crystals  of  this  type  must  conform  to  4  trigonal 
axes  ending  in  the  center  of  the  octants;  these  four  trigonal  axes 
maintain  their  position  in  all  five  types  of  the  isometric  system 
and  to  three  digonal  axes  corresponding  to  the  crystallographic 
axes.  They  have  no  planes  or  center  of  symmetry. 


ISOMETRIC   SYSTEM 


63 


na :  a :  ma 


Forms 

Tetartohedral  pentagonal  dodecahedron ;    ±  R,  ±  L 
KIT  (hkl),  KIT  (hkl),  KIT  (khl),  KIT  (khl). 

The  symmetry  of  the  type  requires  but  one  quarter  of  the  faces 
of  the  holosymmetric  form.  Figure  107  represents  the  poles  of 
the  +  right  tetartohedral  pentagonal  dodecahedron.  The  form 
is  bounded  by  12  irregular  pentagonal  faces,  three  of  which  are 
grouped  around  the  trigonal  axes;  the  digonal  axes  bisect  an  edge. 


4L        -B 


+R 
-L 


-L 

+R 


-B  :   -f-L 


FIG.  107.— The  Plus  Right  Tet- 
rahedral  Pentagonal  Dodeca- 
hedron. 


FIG.  108. 


Figure  108  represents  the  8  faces  of  the  hexoctahedron  grouped 
around  the  ditetragonal  axis,  which  axis  in  the  tetartohedral  class 
is  a  digonal  axis.  Four  tetartohedral  pentagonal  dodecahedra 
are  possible.  If  the  two  faces  +  R  are  extended,  the  +  right, 


FIG.  109.  — The  Plus  Right 
Tetartohedral  Pentagonal 
Dodecahedron,  KIT  (hkl). 


FIG.  110.  — The  Plus  Left  Te- 
tartohedral Pentagonal  Do- 
decahedron, irK(khl). 


Fig.  109,  pentagonal  dodecahedron  results ;  if  +  L  are  extended, 
the  +  left,  Fig.  110 ;  --  R  is  the  negative  right,  -  L  the  negative 
left  forms.  ±  rights  are  congruent,  for  if  the  face  +  R  is  revolved 


64  MINERALOGY 

90°  it  is  superimposed  on  —  R,  likewise  ±  lefts  are  congruent,  but 
the  rights  are  enantiomorphic  with  the  lefts,  for  in  no  way  can  the 
face  —  R  be  revolved  around  the  digonal  axis  to  bring  it  into  a 
congruent  position  with  the  face  +  L  or  —  L.  There  are  always 
four  tetartohedral  forms.  They  may  be  derived  by  superimposing 
one  hemihedral  type  of  selection  on  another  and  extending  the  faces 
remaining,  Fig.  41,  page  27. 

Other  Forms 

The  six  other  possible  forms  of  the  type  are  derived  by  a  consid- 
eration of  the  position  of  the  poles  in  the  triangle,  Fig.  107 : 

Pole  on  the  side  between  the  digonal  axes  =  +  pyritohedron. 

Pole  on  the  side  between  the  digonal  and  trigbnal  axes  =  ± 
tetragonal  tristetrahedron. 

Pole  on  the  side  between  trigonal  axes  =  ±  trigonal  tristetrahe- 
dron. 

Pole  on  the  digonal  axes  =  hexahedron. 

Pole  on  the  trigonal  axes  =  ±  tetrahedron. 

Pole  on  the  angle  between  the  digonal  axes  =  rhombic  dodeca- 
hedron. 

Combinations 

Apparent  holohedral  and  hemihedral  forms  of  more  than  one 
type  may  be  found  combined  on  the  same  crystal ;  when  this  is 
observed,  all  forms  must  be  considered  as  being  of  tetartohedral 
symmetry. 

Examples :  Minerals  crystallizing  in  the  type  are  rare. 

Ullmannite  from  one  locality  crystallizes  in  pyritohedra  and  in 
tetrahedra  from  another,  which  would  indicate  tha^  it  is  tetarto- 
hedral. There  are  a  number  of  artificial  salts,  as  barium  nitrate, 
sodium  chlorate,  strontium  nitrate,  and  sodium  bromate,  which 
crystallize  in  this  type. 


CHAPTER   IV 


TETRAGONAL   SYSTEM 

THE  tetragonal  system  embraces  all  those  crystals  referable  to 
three  axes  at  right  angles,  two  of  which,  the  lateral  axes,  are  equal 
and  interchangeable,  designated  by  the  letter  a.  The  third  or 
vertical  axis,  designated  by  c,  is  not  interchangeable  with  the  lateral 
axes.  The  most  general  parameter  of  the  system  is  na :  a :  me, 
in  which  the  value  of  n  may  vary  from  unity  to  infinity;  m  may 
vary  from  zero  to  infinity.  With  the  c  axis  held  vertical  and  one 
of  the  lateral  axes  in  the  plane  of  the  paper,  the  other  at  90°  to  it, 
the  extremities  are  designated  +  or  —  as  in  the  isometric  sys- 
tem, the  upper  right  octant  being  positive.  Seven  of  the  32 
types  are  included  in  the  tetragonal  system. 

CLASS,  TETRAGONAL  HOLOHEDRAL  (HOLOSYMMETRIC) 
TYPE  27,  DITETRAGONAL  EQUATORIAL 

Symmetry.  —  Crystals  of  this  type  possess  one  axis  of  ditetrag- 
onal  symmetry,  the  c  axis ;  four  didigonal  axes,  two  of  which  are 


FIG.    111.  — The  Planes  of 
Symmetry  in  Type  27. 


FIG.  112.  — The  Ditetragonal 
Pyramid. 


the  lateral  axes;  the  other  two,  the  intermediate  axes,  bisect  the 
angle  between  the  a  axes.     All  four  didigonal  axes  lie  in  one  plane, 
the  equatorial  plane,  at  90°  to  the  vertical  axis.     There  are  five 
P  65 


66 


MINERALOGY 


planes  of  symmetry,  Fig.  Ill,  one  of  which  is  the  equatorial  plane; 
the  other  four  intersect  in  the  c  axis  and  each  contains  one  of  the 
axes  of  symmetry  lying  in  the  equatorial  plane.  The  five  planes 
divide  space  into  16  equal  (Fig.  112)  triangular  portions,  eight 
above  and  eight  below  the  equator.  The  largest  number  of  faces 
possible  upon  any  form  of  the  tetragonal  system  will  be  16. 

Forms 

I.  Ditetragonal  pyramid  ;  na  :  a  :  me  ;  (hkl). 
When  the  values  of  n  and  m  are  between  their  limits,  the  pole 
of  the  face  will  fall  within  the  area  of  the  triangle,  Fig.  112 ;  there 
will  be  one  face  in  each  triangular  space,  yield- 
ing a  form,  the  ditetragonal  pyramid,  Fig.  113, 
bounded  by  16  scalene  triangles  (pyramid  here 
includes  the  faces  above  and  below  the  equator 
and  are  doubly  pointed).  It  has  eight  faces 
grouped  around  the  north  and  eight  around  the 
south  pole  or  c  axis ;  four  faces  grouped  around 
the  extremities  of  the  didigonal  axes.  There  is 
a  series  of  ditetragonal  pyramids,  the  shape  of 
the  face  or  the  value  of  the  interfacial  angles  of 
any  one  of  which  will  depend  upon  the  values 
of  n  and  m. 

II.    Tetragonal  pyramid   of  the   first  order; 
a :  a :  me ;    (hhl) . 
When  the  value  of  n  is  unity,  its  minimum 


FIG.  113.  — The  Ditet- 
ragonal Pyramid. 


limit,  or  if  the  pole  in  the  spherical  projection,  Fig.  112,  is  moved 
until  it  coincides  with  the  intermediate  axes,  then  two  adjacent 
poles  of  the  most  gen- 
eral form,  as  a  and  c, 
will  combine,  yielding  a 
form  bounded  by  eight 
isosceles  triangles,  Fig. 
114,  the  tetragonal  pyr- 
amid of  the  first  order. 
Its  eight  polar  edges 
are  equal.  The  crystal- 
lographical  axes  termi- 
nate in  a  solid  tetrahe- 

dral    angle;     this    char-      FIG.  lU.-Pyramid   of  the  First  Order  (111),  of 

acterizes  a  pyramid  of  Cassiterite. 

the  first  order;  in  pyramids  of  the  second  order  the  a  axes  bisect 


TETRAGONAL  SYSTEM 


67 


FIG.  115.  — Pyramid  of  the  Second  Order,  (101),  of 
Cassiterite. 


an  edge.     There  is  a  series  of  pyramids  of  the  first  order,  their 
acuteness  and  general  appearance  depending  upon  the  value  of  m. 

III.  Tetragonal  pyramid  of  the  second  order ;  a  :  oo  a  :  me ;  (hoi). 
When  the  value  of  n  is  <x> ,  its  maximum  limit,  or  if  the  pole  is 

moved  to  coincide  with  the  crystallographical  axes,  then  in  the 
resulting  form  of  eight 
faces  the  axes  will  ter- 
minate in  the  center  of 
the  equatorial  edge, 
yielding  a  pyramid  of 
the  second  order,  Fig. 
115.  In  shape  this  pyr- 
amid in  no  way  differs 
from  the  pyramid  of  the 
first  order,  with  which 
it  becomes  congruent  by 
a  revolution  of  45°  around  the  c  axis.  There  is  a  series  of  pyramids 
of  the  second  order,  depending  upon  the  value  of  m. 

IV.  Ditetragonal  prism;  na:a:<x>c;    (hko). 

When  the  value  of  n  is  between  its  limits  and  m  is  infinity,  or  if 
the  pole  in  the  spherical  projection  is  moved  to  the  primitive  circle 
between  the  extremities  of  the  didigonal  axes,  the  resulting  form 

is  the  di tetragonal  prism,  Fig.  116.    It  is  bounded 

by  eight  similar  faces. 

Each  face  will  cut  one 

of  the  lateral  axes  at 

unity,  the  other  at  a 

distance  greater  than 

unity,  and  will  be  par- 
allel to  the  c  axis;  it 

will    therefore  be   an 

open  form   extending 

to  infinity  unless  ter- 
minated by  combining 

with     another    form. 

All  prisms  are  open 
forms.  There  is  a  series  of  ditetragonal 
prisms,  the  value  of  the  interfacial  angles 
depending  upon  the  value  of  n. 

V.  Tetragonal     prism     of    the     first     FlG.  117._Prismof  the  First 
order;    a:a:ooc;    (no).  Order,  (HO). 


V-X   ;    !^^ 

L 

^\ 

~"f 

FIG.  116.  —  The  Di- 

tetragonal Prism, 

(210). 

68 


MINERALOGY 


FIG.  us.  — The  Tetragonal 

Prism  of  the  Second  Order. 


When  the  value  of  n  is  unity  and  that  of  m  is  infinity,  or  let  the 
pole  be  moved  on. the  equatorial  plane  to  coincide  with  the  inter- 
mediate axes,  then  the  resulting  form  is  the  tetragonal  prism 

^ of   the    first   order,   Fig.    117.    It  will   be 

bounded  by  four  faces,  cutting  the  c  axis  at 
infinity,  the  a  axes  at  unity.  The  lateral 
axes  terminate  in  the  middle  of  the  edges. 

VI.  Tetragonal    prism    of    the    second 
order;  ooa:a:ooc;   (oio). 

When  the  value  of  both  n  and  m  is  at 
infinity,  their  maximum  limit,  or  if  the  pole 
be  moved  in  the  equatorial  plane  to  coin- 
cide with  the  crystallographic  axes,  then  a 
rectangular   prism   results,   the  tetragonal 
prism  of  the  second  order,  which  in  shape 
differs  in  no  way  from  the   prism   of  the 
first  order  except  the  a  axes  terminate  in 
the  center  of  the  faces.     It  becomes  congruent  with  the  first  order 
prism  by  a  revolution  of  45°  around  the  c  axis,  Fig.  118. 
VII.   Basal  pinacoid;  ooa:  ooa:c;  (ooi). 

The  only  possible  position  of  the  pole  remaining  is  when  it  coin- 
cides with  the  c  axis,  when  all 
eight  faces  above  the  equato- 
rial 'plane  will  form  one  face 
and  all  the  faces  below  will  fall 
in  one  plane,  yielding  a  form 
of  two  faces,  the  tetragonal 
base  or  basal  pinacoid  which 
extends  to  infinity  on  all  sides, 
Fig.  118,  c.  All  pinacoids  cut 
but  one  axis  and  are  parallel  to 
the  other  two ;  in  combination 
with  prisms  they  inclose  space. 
The  fixed  forms  of  the  tetrago- 
nal system  are  the  base  and  the 
prisms  of  the  first  and  second 
orders ;  these  correspond  to  the 
fixed  forms  of  the  isometric  system,  as  do  also  the  position  of  their 
poles  in  the  triangle,  viz.,  the  corners  or  angles;  as  here  there  is 
evidently  only  one  position  for  the  pole,  there  is  only  one  form 
possible. 


FIG.  119.  —  Combination  of   (111) 
(110)  (100)  of  Cassiterite. 


(101) 


TETRAGONAL  SYSTEM 


69 


Combinations.  —  The  pyramids  and  prisms 
of  the  first  'and  second  orders  truncate  each 
other's  edges  symmetrically;  Fig.  119  repre- 
sents these  four  forms  of  cassiterite.  Several 
members  of  a  series  of  a  variable  form  may 
occur  in  combination  with  the  fixed  forms, 
Fig.  120,  Zircon. 

Examples  of  minerals  crystallizing  in  the  di- 
tetragonal  equatorial: 

Cassiterite,  Sn02 ;  (110)  (100)  (310)  (111) 
(101). 

Zircon,  ZrSi04;  (110)  (100)  (111)  (331) 
(311). 

Rutile,  TiO2;   (110)  (100)  (310)  (111)  (101). 

Vesuvianite,  Ca«(Al,  OH)  Al2(Si04)5 ;  (100) 
(110)  (310)  (210)  (001). 


FIG.  120.  —  Combina- 
tion of  (111)  (331) 
(311),  (110)  and  (010) 
in  Zircon. 


CLASS,  SPHENOIDAL  (DIAGONAL-FACED)  HEMIHEDRONS 
TYPE  26,  DITETRAGONAL  ALTERNATING 

Symmetry.  —  Crystals  of  this  type  possess  a  ditetragonal  alter- 
nating axis,  the  c  axis;    two  digonal  axes,  the  a  axes;  and  two 

planes  of  symmetry  intersecting  in 
the  c  axis  and  each  containing  one 
of  the  intermediate  lateral  axes;  Fig. 
121  represents 
the  symmetry  of 
the  type. 

this  class  of 
hemihedrons  in 
the  tetragonal 
system  corre- 
sponds to  the 
tetrahedral  class 

FIG.  121.  —  Type  26:  Ditetragonal  .,  ,    . 

alternating.  m  the    ISOmetriC 

system  and  may 
be  considered  as  derived  from  the  holohedral    FIG.  122.— The  Minus 

f  ,  ,.  „    .,        »  |,  Scalenohedronic(hkl). 

forms  by  extending  all  the  faces  in  alternate 

octants,  Fig.  122;  the  shaded  faces  when  extended  will  produce 

a  —  form  as  drawn. 


70 


MINERALOGY 


I.  Tetragonal   scalenohedron ;     ± 


na  :  a  :  me 


K(hkl)K(hkl). 


FIG.    123.  — The  Plus 
Sphenoid  ic(121). 


This  form  is  bounded  by  eight  similar  scalene  triangles,  Fig. 
123,  four  of  which  are  grouped  around  each  extremity  of  the  c 
axis,  with  the  edges  lying  in  the  planes  of  sym- 
metry. The  lateral  axes  terminate  in  the  middle 
of  an  edge.  There  are  +  and  —  forms  which 
become  congruent  by  a 
revolution  of  90°  around 
the  c  axis. 

II.  Sphenoid;  ±  a :  a:  me; 
K(hhl)K(hhl). 

When  the  pole  is  in  the 
plane  of  symmetry  repre- 
sented by  the  full  lines, 
Fig.  121,  two  faces  of  the 
scalenohedron  will  fall  in 
one  plane,  as  e  and  e'  and 

b,  b',  and  the  form  will  be  bounded  by  four  FlG  124  _  The  Plus 
isosceles  triangles,  Fig.  124,  producing  the  tet-  Tetragonal  Sphenoid  of 
ragonal  sphenoid  of  the  first  order  in  which  the  the  First  Order,  K(IH). 
lateral  axes  bisect  the  four  equal  edges.  The 
c  axis  bisects  the  remaining  two  edges.  This 
form  may  also  be  considered  as  derived  from 
the  tetragonal  pyramid  of  the  first  order  by 
extending  alternate 
faces  above  and  below 
until  they  inclose 
space. 

III.  Other  forms  of 
the  type.  —  All  other 
positions  of  the  pole 
will   in  turn   produce 
FIG.  125.— Combination    the  holohedral  shapes 
of  (122)  and  (772)  of    of  type  27,  here  as  in 

Chalcopyrite.  , ,       . 

the -isometric  system; 
here,  also,  they  must  be  considered  as  of 
lower  symmetry,  which  will  be  recognized 
by  their  combination  with  the  two  new 
forms  of  the  type,  or  by  their  striations  and  Fl^  126'  ~~  Chalc°pyrite : 

etch  figures.  Combination      of     (111) 


TETRAGONAL  SYSTEM 


71 


The  possible  forms  to  be  found  in  combination  in  this  type  are  : 
The  positive  and  negative  tetragonal  scalenohedrons,  K  (hkl), 
K(hkl). 

The  positive  and  negative  sphenoids  of  the  first  order,  K(hhl), 

K  (hffl). 

The  pyramid  of  the  second  order,  (ohl).    ^' 
The  ditetragonal  prism,  (hko). 
The  prism  of  the  first  order,  (hho). 
The  prism  of  the  second  order,  (oho). 
The  basal  pinacoid,  (ooi). 
Examples  of  minerals  crystallizing  in  the  type. 
Chalcopyrite,  FeCuS2 ;  (111)  (111)  (122)  (772).     Figs.  125  and 
126. 

CLASS,  HOLOHEDRAL  HEMIMORPHIC 
TYPE  25,  DITETRAGONAL  POLAR 

Hemimorphic  forms  may  be  considered  as  derived  from  an  equa- 
torial form  by  the  suppression  of  all  the 
faces  around  one  extremity  of  the  c 
axis  and  the  development  of  those 
around  the  opposite  extremity,  as  an 
independent  form,  or  if  the  equatorial 
form  is  cut  along  the  equatorial  plane, 
it  will  yield  an  upper  hemiform  and 
lower  hemiform. 

Symmetry.  —  All  the  elements  of 
symmetry  in  the  equatorial  plane  are 
lost,  namely  the  four  axes,  the  equa- 
torial plane,  and  the  center.  The  type 
is  symmetrical  in  regard  to  a  ditetrag- 
onal axis,  the  c  axis  and  four  planes  of  symmetry  intersecting  in 
the  c  axis,  Fig.  127. 

Forms 

Each  pyramid  of  the  ditetragonal  equatorial  type  yields  an  upper 
and  lower  hemipyramid,  each  of  which  is  independent.  The  basal 
pinacoid  is  also  divided  into  upper  and  lower  forms. 

The  possible  forms  to  combination  in  this  type  are  therefore  : 

The  upper  and  lower  ditetragonal  pyramid;  u/1  -        — • ; 

(hkl)  (hid). 


FIG.  127.  — Type  25:    Ditetrag- 
onal Polar. 


72 


MINERALOGY 


The  upper  and  lower  pyramid  of  the  first  order ;  u/1 
(hhl)   (hhf). 
The  upper  and  lower  pyramid  of  the  second  order ;  u/1 

(ohl)  (ohl). 


a  i  a !  me 


GO  a :  a :  me  . 


FIG.  128. — The  Upper  Di- 
tetragonal  Hemipyramid. 


FIG.  129.— The  Upper  Hemi- 
pyramid of  the  First  Order. 


The  ditetragonal  prism ;  na  :  a  :  oo  c ;   (hko). 
The  prism  of  the  first  order ;  a  :  a :  me ;   (110). 

The  prism  of  the  second  order ;  oo  a :  a :  oo  c ; 
(010). 
The    upper    and    lower    basal    pinacoid; 

u/l*":ao>:c;    (001) 


Examples.  —  There  are  as  yet  no  minerals 
known  to  crystallize  in  this  type,  but  several 
organic  and  artificial  salts  belong  here.     Fig. 
FIG.  130.  —  Silver  Fluor-    ^39  represents   a  crystal  of  silver  fluoride, 

in  com- 


(113). 


bination. 


CLASS,  PYRAMIDAL  (PARALLEL-FACED)  HEMIHEDRONS 
TYPE  24,  TETRAGONAL  EQUATORIAL 

Symmetry.  —  Crystals  of  this  type  have  an  axis  of  tetragonal 
symmetry,  the  c  axis ;  one  plane,  the  equator,  and  a  center,  Fig. 
131. 

The  forms  may  be  considered  as  derived  from  the  ditetragonal 
equatorial  type  by  extending  alternate  pairs  of  faces  which  inter- 
sect in  the  equator,  as  is  shown  in  Fig.  132,  where  the  shaded  faces 
when  extended  produce  the  —  third  order  pyramid,  Fig.  133. 


TETRAGONAL  SYSTEM 


73 


Forms 
I.  Tetragonal  pyramid  of  the  third  order ;   ± 


na : a :  me 


(hkl) 


ir(hkl). 

The  faces  represented  by  the  poles  in  Fig.  131  form  a  pyramid 
similar  in  shape  to  the  tetragonal  pyramids  of  the  first  and  second 

orders,    differing 

only  in  that  the 

lateral    axes 

terminate  in  the 

equatorial    edge 

between  the  tet- 

rahedral  angle 

and  the  middle. 

A  diagram  of  the 

equatorial  plane 

is  represented  in 

Fig.   134;    aa  is 

the  pyramid  of  the  first,  ee  is  the  pyramid 
of  the  second  order,  both  being  fixed  forms ; 
ab  is  the  ditetragonal  pyramid,  a  variable  form  lying  between  the 
fixed  forms  as  its  limiting  forms ;  the  dotted  line  represents  the  ex- 
tension of  one  half  of  the  faces,  as  ab  to  d,  where  it  intersects  at 


FIG.  131.  —  Type  24 :  Tetragonal 
Equatorial. 


FIG.  132. 


FIG.  133.  —  Pyramid  of  the  Third 
Order  of  Scheelite,  «•  (313). 


FIG.  134. 


90°  with  the  extension  of  the  alternate  face  ad ;  note  that  the  axes 
in  this  pyramid  terminate  not  in  the  angle,  nor  in  the  middle  of  the 
edge,  but  at  a  point  on  the  edge  somewhere  between  them.  This 
diagram  will  represent  the  relation  of  the  prisms  equally  well. 


MINERALOGY 


II.  Tetragonal  prism  of  the  third  order;   ±-   '—^ ;    IT  (hko) 

IT  (hko). 

If  the  pole  be  placed  on  the  primitive  circle  in  an  asymmetric 
position  in  the  octant,  or  if  every  other  face  of  the  ditetragonal 


FIG.  135.  —  Prism  of  the  Third  Order 
of  Stolzite,  «•  (340). 


FIG.  135  a.  —  Combination  of  the 
First  Order  Prism  with  the  Third 
Order  Pyramid. 


prism  is  extended,  a  new  form  will  result,  the  tetragonal  prism  of 
the  third  order,  Fig.  135,  similar  in  shape  to  the  1st  and  2d  order 
prisms,  except  the  lateral  axes  end  in  the  face  between  the  center 
of  the  face  and  the  middle  of  the  edges. 

All  other  positions  of  the  pole  will  yield 
holohedral  shapes. 

The  possible  forms  in  combination  will 
therefore  be  the  — 

Positive  and  negative  pyramids  of  the  3d 
order,  IT  (hkl)  ir(hkl). 

Tetragonal  pyramid  of  the  1st  order,  (hhl). 
Tetragonal  pyramid  of  the  2d  order,  (hoi) . 
The  positive  and  negative  prism  of  the  3d 
order  IT  (hko),  (hko). 

Tetragonal  prism  of  the  1st  order,  (hho). 
Tetragonal  prism  of  the  2d  order,  (oho). 
Basal  pinacoid,  (001). 


FIG.  136.  —  Combination 
of  (101)  (111)  and  (313) 
of  Scheelite. 


TETRAGONAL  SYSTEM 


75 


Examples.  —  In  combinations  the  two  new  forms,  the  3rd  order 
pyramid  and  prism  will  give  the  crystals  an  asymmetric  appearance 
as  in  Fig.  135  a. 

Examples.  — Scheelite;    CaWO4:  (101)  (111)  (313).     Fig.  136. 

CLASS,  TETRAGONAL  TRAPEZOHEDRAL  (PLAGIOHEDRAL)  HEMI- 

HEDRONS 

TYPE  23,  TETRAGONAL  HOLOAXIAL 

Symmetry.  —  As  the  name  implies,  this  type  has  all  the  axes  of 

symmetry  of  the  tetra- 

...-••••T- gonal  system,  but  no 

'•y        planes  or  center.    The 
c  axis  is  a  tetragonal 
\    axis    and    the    lateral 
I    and  intermediate  axes 
o     :    are    digonal    axes    of 
/    symmetry.     As  in  all 
.,    /       holoaxial  types,  there 
S         are    right     and     left 
enantiomorphic  forms. 


:    O 


FIG.  137.  —  Type  23  :  Tetragonal 
Holoaxial. 


Forms 


r/l 


na : a :  me 


I.  Tetragonal    trapezohedron; 

T(hkl)  T(khl). 

If  alternate  faces  of  the  di- 
tetragonal  pyramid  around  the 
north  pole,  represented  by  circles 
in  Fig.  137  and  shaded  in  Fig. 
138,  and  faces  alternating  with 
these  around  the  south  pole,  rep- 
resented by  +  in  Fig.  137,  are 
extended,  the  right  trapezohedron, 
Fig.  139,  will  result;  if  the  un- 
shaded faces  of  Fig.  138  are  ex- 
tended, the  left  trapezohedron 
will  result,  Fig.  139  a. 

The  form  is  bounded  by  eight 

13Tetrag£Kiai   traPezoidal  faces>  four  of  which 
Trapezohedron.      are  grouped  around  each  extrem- 


FIG.  138. 


FIG. 


FIG.  139  a.— The 
Left  Tetragonal 
Trapezohedron, 

r(212). 


76  MINERALOGY 

ity  of  the  c  axis.  The  equator  is  represented  by  eight  zigzag  edges. 
The  lateral  axes  bisect  opposite  edges. 

II.  Other  forms.  —  All  other  positions  of  the  poles  will  yield 
holohedral  shapes.  The  trapezohedron  is  the  only  new  form  of  the 
type,  and  it  has  never  been  found  on  a  crystal.  All  substances 
crystallizing  in  this  type  have  been  placed*  here  as  a  result  of  a  study 
of  their  etch  figures. 

Forms  possible  to  combine  in  the  type  are : 

The  right  and  left  tetragonal  trapezohedron,  r(hkl)  r(khl). 

The  tetragonal  pyramid  of  the  first  order,  (hhl). 

The  tetragonal  pyramid  of  the  second  order,  (ohl). 

The  ditetragonal  prism,  (hko). 

The  tetragonal  prism  of  the  first  order,  (hho). 

The  tetragonal  prism  of  the  second  order,  (oho). 

The  tetragonal  base,  (001). 

Examples.  —  There  are  no  minerals  of  this  type.  The  artificial 
nickel  sulphate,  NiSCX,  6  H20,  is  placed  here,  also  the  sulphate  of 
strychnine. 

CLASS,  TETRAGONAL  SPHENOIDAL  (TETARTOHEDRAL) 
TYPE  22,  TETRAGONAL  ALTERNATING 

The  c  axis  in  this  type  is  a  tetragonal  alternating  axis ;  there  is 
no  plane  or  center  of  symmetry.  The  forms  of  the  type  may  be 

considered  as  derived  from  the  holohe- 
dral forms  by  an  extension  of  one  quarter 
of  the  faces  selected  as  illustrated  in  Fig. 
140,  which  also  shows  the  alternating 
character  of  the  c  axis. 

I.  Sphenoid  of  the  third  order ;  r/1  ± 

^  '  a  :  mC  ;    TTK(hkl),    TTK(khl),   TTK(hkl), 

irK(khl). 

The  faces  represented  by  the  poles  of 
Fig.  141  produce  the  +  R  sphenoid  of 
FIG.  140.  — The  Plus  Left  Te-   the  third  order,  Fig.  142.     The  -f  L  form 
tragonai    Sphenoid  of   the   is  shown  in  Fig.  140,  in  its  relation  to  the 
ditetragonal   pyramid,    where  the  four 

faces  selected  include  the  left-hand  face  of  the  +  octant.  There 
are  four  sphenoids  of  the  third  order  possible,  bearing  the  same 
relation  to  each  other  as  in  the  tesseral  polar  type.  The  lateral 


TETRAGONAL  SYSTEM 


77 


axes  terminate  asymmetrically  in  the  face,  on  a  line  connecting 
the  middle  points  of  the  four  equal  edges. 

I.   Sphenoid  of  the  second  order  ;  ±  —  -  ;  iTK(hol),  TTK(ohl). 

When  the  poles  of  Fig.  141  are  moved  into  the  diametral  planes, 
the  +  right  and  —  left  sphenoids  will  fall  in  one  form,  the  -f- 
sphenoid  of  the  second  order,  Fig.  142,  in  which  the  lateral  axes 


o     : 


FIG.  141.  — Type  22:  Tetragonal 
alternating. 


FIG.  142.  — The  Te- 
tragonal Sphenoid  of 
the  Second  Order. 


FIG.  142  a.  — The  Plus 
Right  Tetragonal 
Sphenoid  of  the  Third 
Order. 


terminate  in  the  central  point  of  the  line  joining  the  middle  of  the 
four  equal  edges.  Sphenoids  of  the  second  order  may  be  considered 
as  derived  from  the  tetragonal  pyramid  of  the  second  order  by 
extending  alternating  faces  above  and  below. 

II.  Other  forms.  —  No  new  forms  are  produced  by  the  other 
possible  positions  of  the  poles.  Forms  possible  to^  combine  in  the 
type  are : 

±  right  and  ±  left  sphenoids  of  the  third  order,  irK(hkl), 
TTK(khl),  TTK(hkl),  "irK(khl).  . 

±  sphenoid  of  the  second  order,  K  (ohl),  K  (hoi). 

±  sphenoid  of  the  first  order,  K  (hhl),  K  (hiil). 

±  prism  of  the  third  order,  IT  (hko),  IT  (kho). 

Prism  of  the  second  order,  (oho) . 

Prism  of  the  first  order,  (hho). 

Basal  pinacoid,  (001). 

Example.  —  There  is  yet  no  representative  of  this  type,  either 
among  minerals  or  artificial  compounds. 


78 


MINERALOGY 


CLASS,  HEMIHEDRAL  HEMIMORPHIC 
TYPE  21,  TETRAGONAL  POLAR 

Symmetry.  —  There  is  one  axis  of  tetragonal  symmetry ;  all  the 
symmetry  of  the  equatorial  plane  is  lost.     This  type  is  related  to 

the  tetragonal  equatorial  in  the  same 
way  as  the  ditetragonal  polar  is  to  the 
di tetragonal  equatorial,  Fig.  143. 

Forms 

o      \ 

I V •;        i  Tetragonal  hemipyramid  of  the  third 


order,  u/1 


na :  a :  me 


ir(hkl). 


FIG.  143.  — Type  21:  Tetragonal 
Polar. 


The  tetragonal  pyramid  of  the  third 
order  yields  the  only  new  forms  of  the 
type,  the  positive,  Fig.  144,  and  nega- 
tive upper  and  lower  hemipyramids  of 
the  third  order. 

Forms  possible  to  combine  in  the  type  are  : 
±  upper  and  lower  hemipyramids,  of  the  third  order,  ir  (hkl) 
ir(khl),  IT  (hkl),  ir(khl). 

Upper  and  lower  hemipyramids  of 
the  second  order,  (ohl),  (ohl). 

Upper  and  lower  hemipyramids  of 
the  first  order, 
(hhl),  (hhl). 

±  prisms  of 
the  third  order, 
ir(hko),  ir(kho). 

Prism  of  the  second  order,  (oho). 
Prism  of  the  first  order,  (hho). 
Basal  pinacoid,  (001),  (001). 
Examples. -rWulfenite,  PbMo04,  (111)  (111) 
(001)  (430),  Fig.  145. 

Etch  figures.  —  In  the  seven  types  of  the 
tetragonal  system  it  will  be  noted  that  two 
forms,  the  prisms  of  the  first  and  second  orders,  are  common  to  all. 
They  are  of  the  same  shape  and  possess  the  same  number  of  faces 
in  each  type ;  and  in  external  appearance  the  prism  of  one  type  is 
not  to  be  distinguished  from  that  of  another.  The  same  condition 
exists  in  the  case  of  the  cube  and  rhombic  dodecahedron  in  the  five 


FIG.  144.  —  The  Upper  Hemipyra- 
mid of  the  Third  Order,  w(hkl). 


FIG.  145.  — Wulfenite 
(111)  (111)  (001)  (430) 


TETRAGONAL  SYSTEM 


79 


types  of  the  isometric  system.  While  outwardly  these  seven  prisms 
of  the  first  order  are  exactly  alike,  yet  inwardly  they  all  possess 
their  distinctive  symmetry.  The  physical  properties  are  distributed 
on  the  face  of  each  prism  in  accord  with,  and  they  conform  to,  the 
symmetry  of  the  type.  The  seven  prisms  of  the  first  order  will 
differ  then  in  their  symmetry. 

When  a  solvent  is  applied  to  a  crystal  face  or  a  natural  crystal- 
line surface,  as  a  cleavage  surface,  the  crystal  will  not  pass  into  solu- 
tion equally  or  with  the  same  speed  in  all  directions.  T-he  rapidity 
with  which  molecules  pass  into  solution  or  are  torn  off  from  the  crys- 
talline network  will  depend  upon  the  symmetry  of  the  network. 
The  solvent's  action  will  not  act  evenly  all  over  the  surface,  but  will 
start  at  points  scattered  over  the  crystal  face ;  and  solution  will  begin 
at  each  one  of  these  points  as  a  nucleus  or  center,  the  molecules 


FIG.  146.  —  Photograph  of  Etch  Figures  on  Halite,  enlarged  Five  Diameters. 

going  into  solution  one  after  the  other,  with  a  speed  that  varies 
with  the  direction.  If  the  solvent's  action  is  stopped  after  a  very 
short  time  and  the  crystalline  surface  is  examined,  in  many  cases,  if 
the  concentration  and  character  of  the  solvent  has  been  favorable 
and  it  has  not  been  allowed  to  act  too  long,  the  surface  will  be  pe- 
culiarly pitted.  All  of  these  pits  or  etch  figures  will  be  of  the  same 
shape  on  all  faces  of  the  same  crystal  form.  They  are  bounded  by 
straight  or  slightly  curved  lines  and  are  of  the  nature  of  negative  or 
reentrant  crystals,  with  their  equivalent  faces  and  axes  arranged 
parallel,  as  is  shown  in  Fig.  146.  These  etch  figures  may  be  produced 
by  plunging  a  crystal  into  a  solvent  for  a  very  short  time,  as  they 
are  the  result  of  the  first  action  of  the  solvent  on  the  crystal  face. 

r 


MINERALOGY 


If  solution  is  allowed  to  continue  for  a  longer  time,  the  outlines 
of  the  individual  pits  will  meet  and  the  face  is  then' often  covered 
with  characteristic  hillocks,  representing  points  where  the  walls  of 


*.. 


[N 


the  etch  figures  have  not  as  yet  joined.  The  sides  of  the  etch  fig- 
ures represent  possible  crystal  faces,  or  more  correctly  vicinal  faces, 
and  they  therefore  reflect  and  conform  to  the  symmetry  of  the 
face.  Their  shape  will  depend  upon  the  chemical  compound,  the 


TETRAGONAL  SYSTEM  81 

strength  and  character  of  the  solvent,  as  well  as  upon  the  crystal 
form  etched ;  but,  however  produced,  if  produced  under  like  con- 
ditions, the  etch  figures  on  all  faces  of  any  one  form  will  be  alike 
and  conform  to  the  symmetry  of  the  face.  Etch  figures  are  there- 
fore one  of  the  most  reliable  means  of  determining  the  symmetry  of 
any  crystal  face,  and  often  decide  the  type  to  which  a  crystalline 
compound  will  belong  when  the  crystal  forms  or  combination  of 
forms  fail,  and  when  the  most  general  or  distinctive  form  of  the 
type  is  absent.  Nickel  sulphate,  NiSO4,  6  H2O,  has  been  placed  in 
the  tetragonal  holoaxial  type  from  the  symmetry  of  its  etch  figures 
alone,  while  the  most  general  or  distinctive  form  of  this  type,  the 
tetragonal  trapezohedron,  has  never  been  observed  on  any  crystal, 
and  from  the  combination  of  forms  alone  it  might  belong  to  type 
27,  ditetragonal  equatorial,  or  to  type  24,  the  tetragonal  equatorial. 
As  an  illustration  of  the  method  of  determining  the  symmetry  of 
apparently  holohedral  hemihedrons,  Fig.  146  a  represents  the  seven 
tetragonal  prisms  of  the  first  order,  with  diagrammatic  etch  figures  on 
each,  conforming  to  the  symmetry  of  the  face  in  each  of  these  seven 
possible  prisms  in  the  tetragonal  system.  The  planes  of  sym- 
metry where  they  cross  the  prism  face  are  represented  by  dotted 
lines  and  where  they  cross  the  etch  figure  by  a  white  line.  In  27, 
the  ditetragonal  type,  the  prism  face  is  symmetrical  to  two  planes 
of  symmetry,  the  vertical  and  equatorial  planes  and  a  center  of 
symmetry.  The  shape  of  the  etch  figures  on  this  face  a,  as  indi- 
cated, must  be  symmetrical  to  planes  parallel  to  these  two  planes 
and  an  axis  of  symmetry ;  furthermore,  when  the  etch  figure  a  is 
revolved  90°  about  the  vertical  axis  c,  it  must  become  congruent 
with  the  etch  figures,  as  a',  on  the  adjacent  prism  face  of  the  same 
form.  If  all  these  conditions  are  fulfilled,  then  the  prism  is  of 
type  27. 

In  type  26,  the  ditetragonal  alternating,  where  the  prism  face  is 
crossed  by  the  vertical  plane  of  symmetry  only  and  the  equatorial 
plane  of  symmetry  is  absent,  the  etch  figures  will  be  of  a  different 
shape,  as  represented,  from  those  on  the  prism  of  type  27.  They 
will  be  without  an  axis  of  symmetry,  but  will  be  symmetrical  to 
the  vertical  plane ;  and  when  revolved  90°  around  the  vertical 
axis  c  and  reflected  over  the  equatorial  plane,  they  will  become 
congruent  with  the  etch  figures,  as  a,  on  the  adjacent  prism  face. 

In  this  type  an  axis  of  digonal  symmetry  ends  in  the  edge ;  the 
etch  figure  a,  if  revolved  around  this  axis  180°,  will  be  congruent 
with  a'.  They  are  oppositely  oriented  on  adjacent  faces. 


82  MINERALOGY 

In  type  25,  the  ditetragonal  polar,  where  there  is  a  vertical  plane 
of  symmetry  crossing  the  prism  face  and  the  vertical  axis  is  a  dite- 
tragonal axis  and  no  equatorial  plane  or  digonal  axes,  the  etch 
figure  on  each  face  is  symmetrical  to  a  vertical  plane  only ;  and  re- 
volved around  the  vertical  axis  c  90°  will  become  congruent  with 
those  on  the  adjacent  face,  as  a',  a  will  also  be  a  reflection  of  a', 
across  the  plane  of  symmetry  containing  the  edge  between  them. 

In  type  24,  the  tetragonal  equatorial,  where  the  vertical  axis  is  a 
tetragonal  axis  of  symmetry  and  has  an  equatorial  plane,  the  etch 
figure  will  be  symmetrical  to  the  equatorial  plane  By  a  revolution 
of  90°  around  the  vertical  axis  it  will  become  congruent  with  a7, 
on  the  adjacent  prism  face,  but  a  is  not  a  reflection  of  a',  as  here 
there  is  no  plane  of  symmetry  between  them. 

In  type  23,  the  tetragonal  holoaxial,  where  there  are  no  planes  of 
symmetry  and  the  vertical  axis  is  a  tetragonal  axis  and  there  are 
four  digonal  axes  in  the  equatorial  plane,  the  etch  figure  must 
have  a  center  of  symmetry;  and  if  revolved  90°  around  the  vertical 
axis  or  180°  around  the  crystallographical  axis,  it  must  become  con- 
gruent with  a',  the  etch  figures  on  the  adjacent  crystal  faces.  The 
figure  a'  is  not  a  reflection  of  a,  as  there  is  no  plane  of  symmetry 
between  them. 

In  type  22,  the  tetragonal  alternating,  where  there  are  no  planes 
of  symmetry  and  the  vertical  axis  is  a  tetragonal  alternating  axis, 
the  etch  figure  a  will  be  asymmetric  ;  and  if  revolved  around  the 
vertical  axis  c  90°,  then  reflected  over  the  equatorial  plane,  will 
become  congruent  with  a7,  on  the  adjacent  prism  face. 

In  type  21,  the  tetragonal  polar,  where  the  vertical  axis  is  a  te- 
tragonal axis,  the  etch  figure  a  will  be  asymmetric  and  will  become 
congruent  with  a'  by  a  rotation  of  90°  around  the  vertical  axis  c. 

From  the  several  diagrams  it  will  be  seen  that  each  of  the  seven 
possible  prisms  of  the  first  order,  though  alike  in  outward  form, 
possesses  the  symmetry  of  the  type,  which  is  revealed  by  the  shape 
and  relation  of  the  etch  figures  on  the  form. 

Crystalline  elements.  —  In  the  isometric  system,  as  the  axes  are 
all  interchangeable,  the  crystalline  characters  are  fixed  and  are  the 
same  for  all  substances  crystallizing  in  the  system.  In  the  tetrag- 
onal system  the  axial  ratio  -  varies  with  the  substance ;  its  value 

is  constant,  however,  for  each  chemically  pure  substance.  The 
axial  ratio  is  calculated  from  the  angles  of  the  fundamental  forms, 
or  those  forms  which  intercept  the  axes  at  unity. 


TETRAGONAL  SYSTEM 


83 


Example.  —  The  axial  ratio  of  zircon  is  calculated  directly  from 
the  pyramid  of  the  second  order  (101),  Fig.  147;  the  angle  cao  is 
found  by  measurement  to  be 
32°  38'  4",  in  the  triangle  coa 
right-angled  at  o.  Tan  cao  = 

— ,  but  oc  =  c  and  oa  =  a ;  tan 
ca 

cao  =  -  =  .6404.  In  the  tetrag- 
a 

onal  system  the  lateral  axis  a 
is  assumed  as  the  unit  of  meas- 
urement, therefore  c  =  .6494 
is  the  axial  ratio  of  zircon. 

When  the  pyramid  of  the  first  order  is  the  fundamental  form 
in  which  the  angle  is  measured  (111),  Fig.   148  at  a',  at  right 


FIG.  147.  — Zircon,  (101). 


angles  to  aai ;  tan  ca'o  = 


oc 
oa7'' 
oc  =  oa'  (tan  ca'o),  also  oa'  = 


(tan  ca'o). 

In  rutile  ca'o  is  42°  10'. 
Log  tan  42°  UK  =  9.959616 +10 
Log  jV2  =  1.849485 
Log  c  =  9.809101 -10 
c  =  .644+,  the  axial 
ratio  of  rutile. 

FIG.  148.  —  Cassiterite,  (111).  . 

When    the -axial    ratio    is 

known,  it  is  an  easy  problem  to  calculate  the  value  of  the  variables 
m  and  n  in  any  set  of  parameters ;  thus  in  rutile  there  is  a  pyra- 
mid of  the  second  order  in  which  the  angle  corresponding  to  cao, 

Fig.  147,  is  78°  15' ;  tan  78°  15'  =  -  =  -  =  4.8  or  nearly  5 ;  its  pa- 

a      i 
rameters  would  be  (a :  <x>  a :  5  c),  and  indices  (501). 


CHAPTER   V 


HEXAGONAL    SYSTEM 

THE  hexagonal  system  includes  all  those  crystals  which  may  be 
referred  to  four  axes,  three  of  which,  the  lateral  axes,  lie  in  one 

plane,  the  equatorial 
plane.  They  are  equal 
and  interchangeable 
and  inclined  60°  to  each 
other.  They  are  all 
designated  by  the  let- 
a  ter  a.  The  order  of  + 

and  —  extremities  are 
as  shown  in  Fig.  149. 
The  fourth  or  c  axis  is 
the  vertical  axis  and  is 
at  right  angles  to  the  a 
axes  and  not  inter- 
changeable with  them. 
It  may  be  either  longer 
or  shorter. 

In  the  hexagonal  system  the  parameters  have  four  terms, 
and  are  written  in  the  following  order,  nai :  pa2 :  a3 :  me.  As 
all  three  lateral  axes  are  in 
the  equatorial  plane  and  all 
faces,  except  the  base,  in- 
tersect this  plane  in  straight 
lines,  two  lateral  axes  and 
the  c  axis  will  fix,  the  in- 
clination of  any  face,  for 
the  straight  line  dd',  Fig. 
150,  is  fixed  by  the  inter- 
cepts on  the  axes  ai  and  a2. 
The  value  of  the  intercept 
a2,  or  the  coefficient  p  in 
the  general  set  of  parame- 
ters, when  one  intercept  is 


FIG.  149. 


—a 


84 


FIG.  150. 


HEXAGONAL  SYSTEM 


85 


unity  p,  is  a  function  of  the  other  intercept  n.     The  value  of  p  ex- 
pressed in  terms  of  n  is  —  -  ;  -       -  will  increase  as  n  decreases, 


until  n  is  unity,  when 


n  —  i 


becomes  infinity,  or  if  the  value  of  n 


increases, 


decreases,  and  when  n  =  2, 


=  2  ;  the  value 


n  —  i  n  —  i 

of  n  may  vary  between  i  as  its  minimum  limit  and  2  as  its  maxi- 
mum limit ;  m,  the  coefficient  of  c,  is  independent  of  n  and  may 
vary  between  0  and  oo .  There  will  also  be  four  terms  in  the  in- 
dices of  any  plane,  thus  hkil,  where  i  represents  the  smallest  inter- 
cept. Of  the  three  indices  hki,  two  will  always  be  of  the  same 
sign  and  the  third  of  the  opposite  sign,  and  the  algebraic  sum  of 
these  three  indices  is  always  zero.  Their  relative  values  are 
i>h>k  and  h  -j-  k  =  i.  In  writing  the  indices  1  always  stands 
last  and  represents  the  c  axis  not  interchangeable  with  the  lateral 
axes.  Twelve  of  the  thirty-two  types  are  included  in  the  hexag- 
onal system,  all  of  which  possess  at  least  one  axis  of  trigonal 
symmetry. 

CLASS,  HEXAGONAL  HOLOHEDRAL  (HOLOSYMMETRIC) 
TYPE  20,  DIHEXAGONAL  EQUATORIAL 

Symmetry.  —  Crystals  of  this  type  possess  one  dihexagonal  axis, 
the  c  axis,  6  didigonal  axes,  all  lying  in  the  equatorial  plane  and 
inclined  at  an  angle  of  30°  to  each  other,  three  of  which  are  the 


FIG.  151. 


FIG.  152.  —  Type  20,  Dihexagonal 
Equatorial. 


86 


MINERALOGY 


lateral  crystallographical  axes.  The  remaining  three  bisect  the 
angles  between  the  lateral  axes.  There  are  seven  planes  of  sym- 
metry, one  of  which,  the  equatorial  plane,  contains  the  didigonal 
axes  and  is  at  right  angles  to  the  c  axis.  The  other  six  planes  all 
intersect  in  the  c  axis  and  each  contains  one  of  the  didigonal  axes. 
They  are  therefore  inclined  to  each  other  at  an  angle  of  30°,  Fig.  151. 
These  seven  planes  of  symmetry  divide  space  into  24  equal  por- 
tions or  solid  angles.  The  largest  number  of  faces  on  any  hexag- 
onal form  will  be  24,  or  one  face  in  each  solid  angle.  There  is 
also  a  center  of  symmetry  and  the  forms  of  this  type  will  all  be 
bounded  by  pairs  of  parallel  faces,  Fig.  152. 


Forms 


I.  Dihexagonal  pyramid ;    na  : 


n  —  i 


a  :  a  :  me ;  (hkil) . 


This  form  is  represented  by  one  face  in  each  of  the  24  solid  angles 
and  is  bounded  by  24  scalene  triangular  faces,  Fig.  153 ;  each  face 


FIG.  153.  —  Dihexagonal  Pyramid, 


na: 


a:  a:  me;   (hkil). 


PIG.  154. —  Hexagonal  Pyra- 
mid of  the  First  Order, 
a  :  oo a  :  a  :  c,  (hohl). 


cuts  the  lateral  axes  at  a  different  distance.     The  equatorial  edges 
are  all  equal,  and  the  alternate  polar  edges  are  equal. 

II.  Hexagonal  pyramid  of  the  first  order ;  a  :  oo  a  :  a  :  me ;  (hohl). 

If  the  potes  in  Fig.  152  be  moved  into  the  intermediate  planes 
of  symmetry  so  as  to  lie  on  that  side  of  the  triangle  between  the 
intermediate  and  the  hexagonal  axes,  then  the  number  of  faces  will 
be  reduced  to  12,  and  a  new  form  will  result,  the  hexagonal  pyra- 
mid of  the  first  order,  Fig.  154,  bounded  by  12  isosceles  triangles. 


HEXAGONAL  SYSTEM 


87 


Each  face  cuts  two  of  the  lateral  axes  at  an  equal  distance  and  is 
parallel  to  the  third.  The  axes  therefore  end  in  the  tetrahedral 
angles,  making  it  a  pyramid  of  the  first 
order. 

III.  Hexagonal  pyramid  of  the  second 
order;  2  a  :  2  a  :  a  :  me ;  (hh2hl) . 

If  the  poles  in  Fig.  152  are  moved  into 
the  diametral  planes,  then  the  faces  of 
the  most  general  form  will  be  reduced  to 
12  isosceles  triangles,  Fig.  155,  each  face 
cutting  two  of  the  lateral  axes  at  an 
equal  distance  and  the  third  at  one  half 
that  distance.  The  a  axes  will  bisect  the  T 

FIG.    155.  — Pyramid  of   the 

equatorial  edges,  making  the  form  a  pyra-      second  Order,  2  a :  2  a :  a :  c, 
mid  of  the  second  order.  (hh2hi). 


IV.  Dihexagonal  prism;  na  : 


n  —  i 


a  :  a:  ooc;  (hkio). 


The  poles  are  now  moved  to  the  equatorial  plane  between  the  axes 
of  symmetry,  when  the  faces  will  be  reduced  to  12,  all  of  which  are 


FIG.  156.  —  Dihexagonal  Prism, 
na :  —^—  a :  a :  me,  (hklo). 


FIG.  157.  —  Hexagonal  Prism  of  the 
First  Order,  a  :  oo  a  :  a  :  oo  c,  (hoho). 


parallel  to  the  c  axis,  yielding  an  open  form,  Fig.  156,  the  dihex- 
agonal  prism,  alternate  edges  of  which  are  similar. 


88 


MINERALOGY 


V.  Hexagonal  prism  of  the  first  order  ;<  a  :  oo  a :  a  :  oo  c ;  (ho hi). 
Three  possible  positions  of  the  poles  now  remain,    the  three 

angles  of  the  triangle,  each  position  yielding  one  of  the  three  fixed 
forms.  The  four  forms  already  developed  represent  the  variable 
forms,  there  being  a  series  of  each.  If  the  poles  coincide  with  the 
intermediate  axes,  Fig.  152,  four  faces  of  the  most  general  form  will 
fall  in  one  plane,  producing  an  open  form,  the  hexagonal  prism  of 
the  first  order,  bounded  by  6  faces,  all  of  which  are  parallel  to  the 
c  axis,  Fig.  157.  Each  face  cuts  two  lateral  axes  at  the  same 
distance  and  is  parallel  to  the  third. 

VI.  Hexagonal    prism    of    the    second    order ;    2  a  :  2  a  :  a  :  oo  c  ; 
(hh^ho). 

In  this  form  the  poles  will  coincide  with  the  lateral  crystallo- 
graphical  axes.  It  is  bounded  by  6  faces,  each  of  which  cuts  two 
lateral  axes  at  the  same  distance,  and  the  third  at  one  half  that 
distance.  The  axes  will  therefore  terminate  in  the  center  of  the 
faces,  Fig.  158. 

VII.  Basal  pinacoid ;  oo  a  :  oo  a  :  oo  a  :  me,  (oooi). 

If  the  poles  are  moved  to  the  c  axis  the  number  of  faces  will  be 
reduced  to  a  single  pair  of  faces  parallel  to  the  equatorial  plane. 
They  terminate  the  prisms  as  shown  in  Fig.  158. 


FIG.  158.  — Hexagonal  Prism 
of  the  Second  Order  (hh2ho). 


•*  f — 

FIG.  159. — Beryl,  a  Combi- 
nation of  m  (10TO),  u  (2021), 
s  (1121),  p(10ll),c  (0001). 


The  forms  possible  to  combine  on  crystals  of  the  dihexagonal 
equatorial  type  are : 


HEXAGONAL  SYSTEM 


89 


Dihexagonal  pyramid,  (hkil). 

Hexagonal  pyramid  of  the  first  order,  (hohl). 

Hexagonal  pyramid  of  the  second  order,  (hh2hl). 

Dihexagonal  prism,  (hkio). 

Hexagonal  prism  of  the  first  order,  (hoho). 

Hexagonal  prism  of  the  second  order,  (hh2hb). 

Hexagonal  base,  (0001). 

Examples.  —  Few  minerals  crystallize  in  this  type;  as  a  rule  the 
holosymmetric  class  in  other  systems  is 
the  most  important  class  in  the  system. 

Beryl,  Be3Al2(Si03)6,  Fig.  159,  repre- 
sents a  combination  of  five  forms  on  a 
crystal  of  beryl. 

Hanksite,    9  Na2SO4 . 2  Na2C03 .  KC1, 


FIG.  160.— •Hanksite,  c(0001), 
m(1010),p(10Tl). 


Fig.  160,  represents  a  combination  of  three  forms  on  a  crystal  of 
hanksite. 

CLASS,  RHOMBOHEDRAL  HEMIHEDRQNS 
TYPE  19,  DIHEXAGONAL  ALTERNATING 

Symmetry.  —  Crystals  of  this  type  possess  one  dihexagonal  al- 
ternating axis,  the  c  axis;  three  didigonal  axes,  the  lateral  crys- 
tallographical  axes,  three  planes  of  symmetry,  intersecting  in  the 
c  axis  and  each  containing  one  of  the  intermediate  lateral  axes,  and 


FIG.  161.  — Type  19,  Dihex- 
agonal Alternating. 


FIG.  162. — The  Positive  Scale- 
nohedron,  k  (hhfl). 


also  a  center  of  symmetry,  Fig.  161.  It  is  to  be  noted  here  that  the 
equatorial  plane  of  type  20  is  lost  and  the  equatorial  edge  in  the 
two  new  forms  of  the  type  is  represented  by  a  zigzag  edge.  Forms 
of  this  type  may  be  derived  from  the  holohedral  forms  of  type  20 


90 


MINERALOGY 


by  the  extension  of  all  planes  in  alternate  dodecants  above  and 
below  the  equatorial  plane,  as  the  shaded  faces  of  Fig.  162. 

Forms 

n 
na  :  —         a  :  a  :  me 

I.  Scalenohedron  ±         n~  J .  ;  K(hkil),  K(khil). 

2 

The  faces  represented  by  the  poles 
in  Fig.  161  when  extended  will  yield 
the  minus  scalenohedron,  Fig.  163,  a 
form  bounded  by  12  similar  scalene 
triangles.  Six  faces  are  grouped 
around  the  extremities  of  the  c  axis. 
Alternate  edges  are  equal  both  as  to 
length  and  angle.  The  equatorial 
edge  of  the  dihexagonal  pyramid  is 
replaced  by  6  equal  zigzag  edges, 
each  of  which  is  bisected  by  the  ex- 
tremity of  a  lateral  axis. 

II.  Rhombohedron  of  the  first  order,  ±  — — ;  K(hohl), 


FIG.  163.  —  The  Minus  Scaleno- 
hedron, K  (hkfl). 


K(ohhl). 

When  the  poles  of  Fig.  161  are  moved  into  the  planes  of  sym- 
metry, a  form,  the  rhombohedron  of  the 
first  order,  Fig.  164,  bounded  by  6  simi- 
lar rhombic  faces,  is  the  result.  The  form 
is  plus  if  the  pole  is  moved  into  the  plane 
of  symmetry  lying  between  the  ai  and  a3 
axes  in  front,  and  the  minus  form  is  pro- 
duced when  they  are  moved  into  the 
plane  between  a2  and  a3.  There  are  three 
faces  grouped 
around  each  ex- 
tremity of  the  c 
axis,  and  three 
equal  edges.  The 
six  zigzag  edges 
which  are  bi- 
sected by  the  ex- 

FIG.  164.— The  Minus  Rhom-     ,          ...    '        -    ,,        FIG.  164  a.  —  The    Rhombo- 
bohedron  of  the  First  Or-    1  hedron  of  the  Middle  Edge 

der,  K  (ohfil).   .  lateral     axes    are       and  Scalenohedron. 


HEXAGONAL  SYSTEM 


91 


FIG.  165.  —  Combination  of 
the  Scalenohedron  and  the 
Rhombohedron  of  the  Mid- 
dle Edge. 


equal  in  length  to  the  polar  edges  but  differ  in  angle.     These 
edges  correspond  in  position  to  the  zigzag  edges  of  the  scaleno- 
hedron.      For   each  scalenohedron  there 
is    a   corresponding  rhombohedron;  Fig. 
164  a,  termed  the  rhombohedron  of  the 
middle  edges.     When  in  combination,  it 
bevels   symmetrically  the    edges  of   the 
scalenohedron,  Fig.  165. 

Other  forms.  —  All  other  possible  po- 
sitions of  the  poles  yield  forms  similar  in 
shape  to  the  holohedral  forms  of  type  20, 
but  when  found  in  combination  with  a 
rhombohedron  or  a  scalenohedron  they 
must  be  considered  as  of  hemihedral  sym- 
metry. 

The  possible  forms  to  combine  in  this  type  are  : 
Plus  and  minus  scalenohedron,  K  (hkil),  K  (khil). 

Plus  and  minus>  rhombohedron  of  the 
first  order,  K  (hohl)  ,  K  (phhl)  . 

Hexagonal  pyramid  of  the  second  order, 
(hh2hl). 

Dihexagonal  prism,  (hkio). 
Hexagonal  prism  of  the  first  order,  (hoho). 
Hexagonal    prism  of   the  second  order, 
(hhiho). 

Hexagonal  base,  (0001). 
Examples.  —  The     rhombohedral    class 
is  the  most  important  class  of   the   hex- 
agonal system, 
as  a  large  num- 
ber of  common 
and     commer- 

™P°f 

minerals 
belong  here. 
Calcite,  CaCO3,  Fig.  166,  is  a  com-  FIG.   167.—  Hematite,  c(oooi), 


FIG.     166.  —  Calcite,    Com- 
bination       of      R  (1011), 

m(ioro),  v(2i3i).  tant 


bination  of  R(lOll),  m(10lO),  v(2131). 


1}'  e 


Hematite,  Fe2O3,  Fig.  167,  is  a  combination  of  a  base  and  the 
plus  and  minus  rhombohedron. 

Corundum,  A1203;  Siderite,  FeC03;  Arsenic;  Antimony; 
Brucite,  MgO  .  H2O,  —  also  crystallize  in  this  type. 


92 


MINERALOGY 


CLASS,  DIHEXAGONAL  HEMIMORPHIC 

TYPE  18,  DIHEXAGONAL  POLAR 

Symmetry.  —  By  a  polar  development  of  the  holohedral  forms 
of  type  20,  all  the  symmetry  lying  in  the  equatorial  plane  is  lost, 

leaving  the  dihexagonal  axis  and  the 
six  vertical  planes  intersecting  in  the  c 
axis  as  the  symmetry  of  this  type, 
Fig.  168. 

Forms.  —  The  new  forms  would  be, 
upper  and  lower  dihexagonal  hemi- 
pyramids,  Fig.  169.  Also  upper  and 
lower  hexagonal  hemipyramids  of  the 
first  and  second  orders,  Fig.  170. 

Possible  forms  to  be  found  in  com- 
FIG.  168.— Type  is,  Dihexag-    bination  on  crystals  of  this  type  would 

onal  Polar.  TL 

Upper  and  lower  dihexagonal  hemipyramid, 


na : :  a :  me 

n  — i 


;  (hkfl),  (hkil). 


Upper  and    lower  hemipyramid   of  the  second  order, 
,./^2a:2a:  a:mc 

u/v  ~^~ 


FIG.  169.  —  The  Upper  Dihex- 
agonal Plemipyramid,  (hkH.) 


FIG.  170.— The  Upper  Hex- 
agonal Hemipyramid  of 
the  First  Order,  (hofil). 


Upper     and     lower      hemipyramid     of     the     first     order, 
u/,(ii^iM).  (hoSl),  (hoS). 


Dihexagonal  prism,  na :  :  a :  oo  c ;  (hkio) . 


Hexagonal  prism  of  the  first  order,  a :  oo  a :  a :  oo  c ;  (1010) 
Hexagonal  prism  of  the  second  order,  2a:2a:a:ooc;  (i  120) . 


HEXAGONAL  SYSTEM 


93 


Upper  and  lower  base,  ooa:  ooa:  ooa:  c;  (oooi),  (oooi). 

Examples.  —  Greenockite,  CdS,  Fig.  171,  is  a  combination  of  five 
forms. 

Iodide  of  Silver,  Agl,  Fig.  172,  is  a  top-shaped  combination  of 
an  upper  and  a  lower  hemipyramid  with  the  first  order  prism. 


FIG.  171.  —  Greenockite,  a 
Combination  of  p  (lOTl), 
p  (1011),  m  (101 0),  z  (2021), 
c(OOOT). 


FIG.  172.— m  (1010), p(iori), 
z  (2021). 


Wurtzite,    ZnS,    and  Zincite,  ZnO,  are  other  minerals   which 
crystallize  in  this  type. 

CLASS,  HEMIHEDRAL  PYRAMIDAL  (PARALLEL-FACED  HEMIHEDRONS) 
TYPE  17,  HEXAGONAL  EQUATORIAL 

Symmetry.  —  Crystals  of  this  type  are  symmetrical  in  regard 
to  one  axis  of  hexagonal  symmetry,  the  c  axis,  one  plane  of  sym- 


FIG.  173.— Type  17,  Hexagonal 
Equatorial. 


FIG.  174. 


94 


MINERALOGY 


metry,  the  equatorial  plane,  and  a  center,  Fig.  173.  The  forms  are 
parallel-faced  hemihedrons,  and  may  be  considered  as  derived  from 
the  holohedrons  of  type  20  by  extending  alternate  pairs  of  faces 
which  intersect  in  the  equatorial  edge,  as  the  shaded  faces  in  Fig. 
174:,  which  will  produce  a  minus  hemihedron. 

Forms 
I.  Hexagonal  pyramid  of  the  third  order, 


na: 


a:  a:  me 


n  — i 


;  -Tr(hkil),  ir(khil). 


The  faces  represented  by  the  poles  of  Fig.  173  bound  a  pyramid, 
which  in  shape  does  not  differ  from  the  hexagonal  pyramid  of  the 
first  or  second  order  except  here  the  lateral  axes  do  not  terminate 
either  in  the  center  of  the  faces  or  bisect  the  edges,  but  on  the  line 

\ 


FIG.  175.  —  The  Hexagonal  Pyra- 
mid of  the  Third  Order. 


FIG.  176. 


connecting  these  two  points,  Fig.  175,  also  Fig.  176,  which  is  a  plan 
of  the  first,  second,  and  third  order  pyramids  and  prisms  drawn  on 
the  equatorial  plane. 

II.  Hexagonal  prism  of  the  third  order, 


na: 


a:a:  ooc 


;  ir(hkio),  ir(khio). 


If  the  poles  in  Fig.  173  are  moved  so  as  to  lie  upon  the  equator, 
between  the  a  axes  and  the  intermediate  axes,  then  two  faces  of  the 


HEXAGONAL  SYSTEM 


95 


pyramid  of  the  third  order  will  fall  in  one  plane,  producing  a  new 
open  form,  the  prism  of  the  third  order,  Fig.  177,  in  which  the  a 

axes  neither  terminate  in  the  edges  or 
in  the  center  of  the  faces,  but  on  the 
line  drawn  between  these  two  points. 

III.  Other  forms  of  this  type  are 
like  the  hexagonal  holohedral  in  shape. 
The  possible  forms  to  be  found  in 
combination  will  be : 

Plus  and  minus  hexagonal  pyramid 
of  the  third  order,  Tr(hkil),  ir(khil). 

Hexagonal 
pyramid  of  the 
first  order, 
Tr(hohl). 

Hexagonal 
pyramid  of  the 
second  order, 
ir(hh2hl). 

Plus  and  minus  hexagonal    prism   of  the 
third  order,  ir(hkio),  ir(khio). 

Hexagonal  prism  of  the  first  order,  Tr(hoho). 
Hexagonal    prism    of    the    second    order, 
ir(hh2ho). 

Hexagonal  base,  ir(OOOl). 

Examples.  —  Apatite,  Ca5(FCl)(PO4)3,  Fig.  178,  shows  a  combina- 
tion of  the  three  pyramids,  a  prism,  and  the  base. 

Pyromorphite,  Pb5Cl(PO4)3;  Mimetite,  Pb5Cl(PO4)3,  and  Vana- 
dinite,  Pb5Cl(PO4)3,  —  also  crystallize  in  this  group. 


FIG.  177.  —  Hexagonal  Prism  of 
the  Third  Order. 


FIG.  178.  —  Apatite, 
a  Combination  of 
p  (1011),  u  (1231), 
s  (1121),  m  (1010). 


CLASS,  TRAPEZOIDAL  (PLAGIOHEDRAL)  HEMIHEDRONS 
TYPE  16,  HEXAGONAL  HOLOAXIAL 

Symmetry.  —  Crystals  of  this  type  possess  all  the  axes  of  the  di- 
hexagonal  equatorial  type,  but  no  planes,  or  center  of  symmetry. 
They  have  therefore  one  axis  of  hexagonal  symmetry,  the  c  axis, 
and  six  digonal  axes  corresponding  to  the  lateral  and  intermediate 
axes,  lying  in  the  position  of  the  equatorial  plane,  Fig.  179.  The 
forms  are  plagiohedral  and  may  be  derived  from  the  holohedral 
forms  by  extending  alternate  faces  around  the  poles,  Fig.  180. 


96 


MINERALOGY 


Forms.  —  Hexagonal  trapezohedrons, 


na: 


a :  a :  me 


r/1 


n—  i 


r(hkil),  T(khil). 


'•,  O 


0 


: 


The  faces  represented   by  the  poles  in  Fig.  179  yield  a  form 
bounded  by  12  similar  trapezoidal  faces,  Fig.  181,  the  right  trape- 
..—--...  zohedron.     If  the  shaded  faces  are  ex- 

tended, the  left  trapezohedron,  Fig 
180,  is  formed.  Six  faces  are  grouped 
around  each  extremity  of  the  c  axis, 
making  equal  angles  and  equal  polar 
edges.  The  median  edges  are  alter- 
nately long  and  short;  the  crystallo- 
graphical  axes  terminate  in  the 
middle  of  the  long  edges.  In  look- 
ing at  a  south  polar  edge,  with  the 
crystal  form  vertical,  if  the  long 
median  edge  is  to  the  right,  it  is  a 
right-handed  form ;  if  to  the  left,  it  is  a  left-handed  form. 

Other  forms.  —  All  other  possible  positions  of  the  poles,  as  the 
sides  and  angles  of  the  triangles  in  Fig.  179,  will  yield  holohedral 


FIG.   179.  —  Type  16,  Hexagonal 
Holoaxial. 


FIG.  180.  —  The  Left  Hexagonal 
Trapezohedron. 


FIG.  181.  — The  Right  Hexago- 
nal Trapezohedron,  r  (hkil). 


shapes.     The  possible  forms  therefore  to  be  found  in  combination 
on  crystals  of  this  type  will  be  : 

Right  and  left  hexagonal  trapezohedrons,  r(hkil),  r(khil). 

Hexagonal  pyramid  first  order,  T(hohl). 

Hexagonal  pyramid  second  order,  T(hh2hl). 


HEXAGONAL  SYSTEM 


97 


Dihexagonal  prism,  r(hkio). 

Hexagonal  prism  first  order,  r(hohl). 

Hexagonal  prism  second  order,  r(hh2ho). 

Hexagonal  base,  r(OOOl). 

Examples.  —  There  has  been  as  yet  no  mineral  assigned  to  this 
type ;  in  fact,  the  trapezohedron  has  never  been  observed  on  any 
crystal.  There  are,  however,  several  salts  included  here,  as  barium 
stibiotartrate,  Ba(C4H406)2SbO2,  KN03,  which  from  the  symmetry 
of  its  etching  figures  must  crystallize  with  an  hexagonal  holoaxial 
symmetry. 

CLASS,  RHOMBOHEDRAL  TETARTOHEDRONS 
TYPE  15,  HEXAGONAL  ALTERNATING 

Symmetry.  — Crystals  of  this  type  '•••*„• 

possess  an  alternating  hexagonal  axis,          ..-•''   \  xao  /  "\ 

the  c  axis,  and  a  center  of  symmetry,       /          \  /  <3, 

but  no  planes  of  symmetry,  Fig.  182.     / 
The  forms  of  this  type  are  tetarto-    ,/_     Ol  V' 

hedrons,    derived    by   superimposing    1  /  \  *»   / 

type  17  upon  type  19,  and  extending     \ 
the  faces  not  selected  by  these  two        \      x24/ 
types  to  produce  a  new  form.     If  the  ?   \  ..••'' 

dihexagonal  pyramid   is    rolled   out, 

the  faces  being  numbered ;  the  equa-    FlG-  182.— Type  15,  Hexagonal 
torial  edge  represented  by  a  horizon- 
tal line,  and  the  terminations  of  the  a  axes  marked  by  a  vertical 
line,  as  here  represented : 

a3          a2          ai 
t 


16 


#U9 


8 
20 

9     10 

n  n 

n_  n 

23    24 

FIG.    183.  —  The    Plus 
Rhombohedron  of  the 
Third  Order,  (hkil). 
H 


If  a  line  be  drawn  through  those  faces 
which  are  extended  by  the  method  of  selec- 
tion used  to  produce  forms  of  type  19,  and 
a  line  drawn  under  those  selected  in  type 
17,  then  it  will  be  seen  that  there  are  three 
faces  above  and  three  faces  below  the  equa- 
torial plane  not  marked. 

The  faces  2,  6,  and  10  do  not  lie  sym- 
metrically above  the  three  faces,  16,  20, 
and  24,  neither  do  these  faces  lie  symmet- 


98 


MINERALOGY 


rically  between  the  lateral  axes.  When  these  six  faces  are  extended, 
the  plus  right  tetartohedron  will  be  formed,  but  it  is  easily  seen  that 
the  superposition  of  the  two  hemihedrons  may  be  so  arranged  that 
in  place  of  2  being  selected,  1  and  the  corresponding  faces  could 
have  been  taken,  forming  the  plus  left  rhombohedron  or  13  could 
have  been  selected,  the  minus  right ;  or  14,  the  minus  left,  —  thus 
yielding  four  possible  forms,  the  plus  and  minus  right,  congruent 
forms,  and  the  plus  and  minus  left,  also  congruent  forms.  The 
rights  are  not  congruent  with  the  lefts. 

Forms 
I.  Rhombohedrons  of  the  third  order, 


±r/l 


na: 


a :  a :  me 


n— i 


;  +r(Uril),  H-l(ikhl),  -  r(khil),  -  l(ihkl). 


When  the  faces  represented  by  the  poles  of  Fig.  182  are  extended, 
they  will  inclose  space  and  yield  a  rhombohedron  of  the  third  order, 

Fig.  183,  which  does  not  differ  in 
shape  from  the  rhombohedron  of 
the  first  order;  the  lateral  axes, 
however,  do  not  end  in  the  central 
point  of  the  edges,  but  terminate 
asymmetrically  in  the  faces  on  the 
line  drawn  between  the  central 
points  of  the  zigzag  edges.  As  is 
shown  above,  there  are  four  rhom- 
bohedrons  of  the  third  order.  Fig. 


FIG.  184.  — The  Plus  Left  .Rhom- 
bohedron of  the  Third  Order, 
(ikfil). 


183  is  a  plus  right  and  Fig.  184  is  a  plus  left  rhombohedron  of  the 
third  order. 


II^Rhombohedron    of      the    second    order    ±f2&:  2a-:  a:mc\ 
(hhffl),  (2hhhl). 

If  the  poles  in  Fig.  182  are  moved 
into  the  diametral  planes,  it  will  be 
the  same  as  revolving  the  rhombohe- 
dron of  the  third  order  until  the  lateral 
axes  terminate  in  the  median  point 
of  the  line  drawn  between  the  centers 
of  the  zigzag  edges.  In  this  position 
the  right  form  will  coincide  with  the  , 

loft-     OYI^    ™Ur      Y  j        •  FlG-    185-  —  Rhombohedron    of 

left,  and  only  phis  and  minus  forms        the  Second  Order,  (hh2hi). 


HEXAGONAL  SYSTEM 


99 


will  remain.     These  are  the  rhombohedrons  of  the  second  order, 
Fig.  185,  derived  from  the  hexagonal  pyramid  of  the  second  order. 

III.   Rhombohedrons  of  the  first  order. 

If  the  poles  are  moved  into  the  plane  containing  the  intermediate 
axes,  the  resulting  form  is  the  rhombohedron  of  the  first  order. 

The  three  rhombohedrons  differ  only  in  the  position  of  the 
lateral  axes :  in  the  first  order  they  end  in  the  central  point  of  the 
zigzag  edges ;  in  the  second  order  they  end  in  the  median  point 
of  the  line  connecting  the  central  points  of  the  zigzag  edges ;  in 
the  third  order  they  end  asymmetrically  on  this  same  line  between 
the  above  two  points. 

Other  forms.  —  When  the  poles  are  moved 
to  the  equatorial  plane  the  first,  second,  and 
third  order  prisms  are  formed. 

The  possible  forms  to  combine  on  crystals 
of  this  type  will  be  : 

Plus  and  minus  right  and  plus  and  minus 
left_rhombohedrons  of  the  third  order,  (khil), 
(hkil),  (ihkl),  (ikW). 

Plus  and  minus  rhombohedrons  of  the  sec- 
ond order,  (hhThl),  (2hhhl). 

Plus  and  minus  rhombohedrons  of  the  first 
order,  (hkil),  (khil). 

Plus  and  minus  prisms  of  the  third  order, 
(hoho),  (ohho). 

Hexagonal  prism  of  the  second  order, 
(hh2ho). 

Hexagonal  prism  of  the  first  order,  (hoho). 

Hexagonal  base,  (0001). 

Examples.  —  Dolomite,  MgCa(CO3)2 ;  Phenacite,  Be2Si04 ; 
Willemite,  Zn2SiO4;  and  Dioptase,  H2CuSi04,  —  crystallize  in  this 
type,  Fig.  186. 

CLASS,  PYRAMIDAL  HEMIMORPHIC 
TYPE  14,  HEXAGONAL  POLAR 

Symmetry.  —  Crystals  of  this  type  possess  an  axis  of  hexagonal 
symmetry,  the  c  axis,  and  no  plane  or  center  of  symmetry.  It 
is  a  polar  development  of  the  hexagonal  equatorial  type.  Fig.  187 
shows  the  symmetry. 


FIG.  186.  —  Phenacite,  a 
Combination  of  the 
Negative  Right  Rhom- 
bohedron of  the  Third 
Order  and  the  Prism  of 
the  First  Order. 


100 


MINERALOGY 


Forms 
I.  Hexagonal  hemipyramid  of  the  third  order, 


±u/l 


na: 


a:a:mc 


n  — i 


;  (hkil),  (khil),  (hkil),  (khil). 


The  upper  or  lower  half  of  the  pyramid  of  the  third  order  may 
occur  independently,  Fig.  188. 

Other  forms  are  the  same  as  in  type  18.  The  possible  forms  to 
combine  in  this  type  will  be  — 

Plus  and  minus  upper  and  plus  and  minus  lower  hemipyramids 
of  the  third  order,  (hkil),  (khil),  (hkil),  (khil). 


/  o 


o     .' 


o  ,- 


FIG.   187.  —  Type  14,  Hexagonal 
Polar. 


FIG.  188.  — The  Upper  Hexagonal 
Hemipyramid  of  the  Third  Order. 


Upper  and  lower  hemipyramid  of  the  first  order,  (hkil),  (hkil). 
Upper  and  lower  hemipyramid  of  the  second  order,  (hhzhl), 
(hhahi). 

Plus  and  minus  hexagonal  prism  of  the  third  order,    (hkio), 
(khio). 

Hexagonal  prism  of  the  first  order,  (1010). 
Hexagonal  prism  of  the  second  order,  (1120). 

Hexagonal  base,  upper  and  lower,  (0001), 
(OOOl). 

Examples.  —  Crystals  in  this  type  are 
rare  and  the  hemipyramid  of  the  third 
order,  which  is  the  only  form  characteristic 
of  the  type,  does  not  occur  on  any  min- 
eral, but  from  the  symmetry  of  the  etch- 
ing figures,  nephelite,  KoNaeAlgSiaO^,  is 

FIG.  189.  — Combination  of  ,,  , 

p(ion)  p(ioil  m(ioTo)      Placed  here;   as  are  also  the  double  sul- 


c(oooi),  c'(oooi). 


phate  of  the  alkali  metals,  potassium  and 


HEXAGONAL  SYSTKM 


'1.01 


lithium,  KLiSO4 ;  lithium,  ammon  um,  LiNH4SO4 ;  and  lithium 
and  rubidium,  LiRbSO4.  Fig.  189  shows  the  appearance  of  these 
combinations. 

CLASS,  TRIGONAL  HEMIHEDRONS 
TYPE  13,  DITRIGONAL  EQUATORIAL 

Symmetry.  —  Crystals  of  this  type  possess  one  ditrigonal  axis, 
the  c  axis,  three  didigonal  axes,  the  intermediate  lateral  axes,  and 
four  planes,  three -of  which  intersect  in  the  c  axis  and  each  con- 
tains one  of  the  didigonal  axes.  The  fourth  is  at  right  angles 
to  these  and  contains  the  a  axes.  Fig.  190  illustrates  this  sym- 


FIG.    190.  — Type  13,   Ditrigonal 
Equatorial. 


FIG.  191.— The  Plus  Di- 
trigonal Pyramid  (hkll). 


metry.  The  forms  may  be  considered  as  hemihedrons,  derived 
from  type  20  by  extending  all  the  faces  in  alternate  dodecants 
around  the  north  pole  and  dodecants  below,  which  intersect  with 
these  in  the  equator,  as  the  shaded  faces  in  Fig.  191. 


Forms 


I.  Ditrigonal  pyramid,  ± 


na: 


a :  a :  me 


n— i 


(hkil),  (ihkl). 


The  faces  represented  by  the  poles  in  Fig.  190  or  shown  in  Fig. 
191,  in  their  relation  to  the  hexagonal  pyramid,  when  extended, 
form  the  plus  ditrigonal  pyramid.  A  form  bounded  by  12  scalene 
triangles,  meeting  in  six  equal  equatorial  edges.  There  are  12 
polar  edges,  six  around  each  extremity  of  the  c  axis,  alternate 
edges  are  similar  both  as  to  length  and  angle.  Fig.  192  represents 
the  —  ditrigonal  pyramid. 


10'.' 


MINERALOGY 


a :  oo  a :  a :  me 


II.  Trigonal  pyramid  of  the  first  order,  ±  f  -  —  J  ;  (hohl), 

(ohhl). 

When  the  poles  of  Fig.  190  are  moved  into  the  vertical  planes  of 
symmetry,  two  adjacent  faces  of  the  ditrigonal  pyramid  will  fall 
in  one  plane,  producing  a  form,  the  trigonal  pyramid  of  the  first 
order,  bounded  by  six  isosceles  triangles,  Fig.  193;  here  the  rela- 


FIG.  192.  — The  Negative  Di- 
trigonal Pyramid. 


FIG.  193.  —  Trigonal  Pyra- 
mid of  the  First  Order, 
(Olll). 


tion  of  the  faces  to  the  hexagonal  pyramid  of  the  first  order  is  also 
shown.  The  lateral  crystallographical  axes  terminate,  two  in  each 
equatorial  edge,  dividing  it  into  three  equal  parts. 

n 


III.  Ditrigonal  prism,  ± 


na: 


n— i 


a:a:  oo  c 


;  (Mdo),  (khio). 


If  the  poles  are  moved  to  the  equator  between  the  a  axes  and  the 
intermediate  axes,  the  ditrigonal  prism  results,  Fig.  194,  a  form 


—  >  "~~ 

^ 

j 

••—  ' 

-.  

'" 

,...-J 

- 

FIG.    194.  —  Ditrigonal 
Prism,  (hkll). 


FIG.  195.  — The  Trigonal 
Prism  of  the  First  Order, 
(ohho). 


HEXAGONAL  SYSTEM 


103 


bounded  by  six  faces.  Alternate  solid  angles  are  equal,  three  being 
less  than  120°  and  three  greater.  The  crystallographical  axes 
bisect  the  edges. 

IV.  Trigonal    prism    of     the     first     order, 
(hoho),   (ohho). 

When  the  poles  are  moved  on  the  primitive  circle  to  coincide  with 
the  didigonal  axes,  the  resulting  form  is  the  trigonal  prism  of  the 
first  order,  Fig.  195,  bounded  by  three  equal  faces,  the  lateral 
axes  terminating,  two  in  each  face,  as  indicated  in  Fig.  195. 

Other  forms.  —  Other  possible  positions  of  the  poles  will  produce 
apparent  holohedral  forms,  i.e.  the  hexagonal  pyramid  and  prism 
of  the  second  order  and  the  base. 

Forms    possible    to    combine    on 
crystals  of  this  type  will  be  — 

Plus  and  minus  ditrigonal  pyra- 
mids, (hkil),  (ihkl). 

Plus  and  minus  trigonal  pyramids 
of  the  first  order,  (hohl),  (ohhl). 

Hexagonal    pyramid  of  the  sec- 
ond order,  (hh2ho). 

Plus  and  minus  ditrigonal  prisms,  (hkio),  (ihko). 

Plus  and  minus  trigonal  prisms  of  the  first  order,  (hoho),  (ohho). 

Hexagonal  prism  of  the  second  order,  (hh2ho). 

Hexagonal  base,  (0001). 

Example.  —  There  is  only  one  example  of  a  substance  crystalliz- 
ing in  this  type,  the  mineral  benitoite,  BaTiSiaOg,  Fig.  196. 

CLASS,  DITRIGONAL  HEMIMORPHIC 
TYPE  12,  DITRIGONAL  POLAR 

Symmetry.  —  The  c  axis  is  an  axis  of  ditrigonal  symmetry,  which 
is  also  polar,  with  three  planes  of  symmetry  intersecting  in  it.  The 
forms  are  derived  from  the  ditrigonal  equational  type  by  a  polar 
development  of  the  c  axis,  Fig.  197. 

Forms 

I.    Ditrigonal  hemipyramids, 
n 


FIG.  196.  —  Benitoite,  Combina- 
tion of  p(10Tl),  p'(Olll),  m(ioro), 
r(10T2),  c(0001). 


±  u/1 


na: 


a :  a :  me 


n  — i 


;  (hkil),  (ikhl),  (hkil),  (ihkl). 


104 


MINERALOGY 


Fig.  198  represents  that  portion  of  the  ditrigonal  pyramid  above 
the    equatorial    plane.      In   this  hemipyramid   the   lateral   axes 


a ' 


FIG.  197.  —  Type  12,    Ditrigonal 
Polar. 


FIG.  198.  — The  Upper  Plus 
Ditrigonal  Hemipyramid 
(hkil). 


hold  the  same  relation  to  the  edges  as  in  the  ditrigonal  equatorial 
types. 

II.  Trigonal  hemipyramids  of  the  first  order, 


;  (hohl),  (ohhl),  (hohl),  (ohhl). 


The  trigonal  hemipyramid  appears  here  as  a  new  form ;  Fig.  199 
represents  the  upper  minus  trigonal    hemipyramid  of   the    first 
order. 

Other  forms  in 
'  the  type  are  simi- 
lar to  the  forms 
of  the.  ditrigonal 
equatorial,  except 
the  hexagonal 
FIG.  199.  — Upper  Negative  pyramid    of    the 

Trigonal    Hemipyramid  of    seCond  Order  and 
the  First  Order,  (ohfil).  ,     , 

the  base  are  upper 

and  lower  forms  which  have  appeared  in 
the  dihexagonal  polar  type. 

Combinations.  —  The  possible  forms  to 
combine  in  this  type  are : 

Upper  and  lower,  plus_and  minus  ditrig- 
ona_l  hemipyramid,  (hkil),  (khil),  (hkfi), 
(khil). 

Upper    and    lower,    plus    and    minus 


FIG.  200.  — Tourmaline,  a 
Combination  of  the  Upper 
and  Lower  Trigonal  Pyra- 
mids r ;  Hexagonal  Prism, 
Second  Order  a;  the  Lower 
Base  c,  and  the  Tngonal 
Prism,  First  Order  m. 


HEXAGONAL  SYSTEM 


105 


trigonal  hemipyramid  of  the  first  order,  (hohl),  (ohhl),  (hohl), 
(ohhl). 

Upper  and  lower  hexagonal  hemipyramid  of  the  second  order, 
(hh2hl),  (hhihf). 

Plus  and  minus  ditrigonal  prism,  (hkio),  (khio). 

Plus  and  minus  trigonal  prism  of  the  first  order,  (hoho),  (ohho). 

Hexagonal  prism  of  the  second  order,  (hh2ho) . 

Upper  and  lower  base,  (0001),  (0001). 

Example.  —  The  common  and  important  mineral  tourmaline 
belongs  to  this  type ;  Fig.  200  represents  a  combination  of  forms  as 
found  on  this  mineral. 


CLASS,  TRIGONAL  TETARTOHEDRAL 


TYPE  11,  TRIGONAL  EQUATORIAL 

Symmetry.  —  Crystals  of  this  type 
have  an  axis  of  trigonal  symmetry, 
the  c  axis,  and  one  plane  of  symmetry 
perpendicular  to  it,  Fig.  201.  The 
class  may  also  be  considered  as  te- 
tartohedral  derived  by  superposing 
type  19,  scalenohedral  hemihedral, 
upon  type  13,  trigonal  hemihedral. 

Forms 
I.  Trigonal  pyramids  of  .the  third       FIG.  201.  — Type  11,  Trigonal 

Order,  Equatorial. 


±r/l 


na : a :  a :  me 

n— i 


;  (hkil),  (khil),  (ikhl),  (ihkl). 


The  faces  represented  by  the  poles  in  Fig.  201  bound  the  right 
plus  trigonal  pyramid  of  the  third  order,  a  form  having  six  isosceles 
triangular  faces.  The  a  axes  terminate  asymmetrically  in  the  equa- 
torial edges,  as  represented  in  Fig.  202.  There  are  four  pyramids 
of  the  third  order :  the  plus  and  minus  right,  two  congruent  forms ; 
the  plus  and  minus  left,  also  two  congruent  forms.  The  rights  and 
lefts  are  enantiomorphic.  Fig.  203  is  a  minus  left  form. 


106 


MINERALOGY 


FIG.  202.  — Diagram  of  the  Equatorial  Plane,  showing  the  Relation  of  the  Trigonal 
Prisms  and  Pyramids  to  the  Lateral  Axes. 


2  a:  2  a:  a:  mc\ 


II.  Trigonal  pyramids  of  the  second  order,  ± 
(hh2hl),  (2hhhl). 
When  the  poles  in  Fig.  201  are  moved  so  as  to  lie  on 

dotted   line   representing 


the 
the 


FIG.  203.  — The  Minus  Left  Trigonal  Pyra- 
mid of  the  Third  Order. 


lateral  axes,  a  new  form,  the 
trigonal  pyramid  of  the  second 
order,  is  the  result,  Fig.  204 ;  a 
form  which  will  not  differ  from 
the  trigonal  pyramids  of  the 
third  or  first  orders  in  appear- 
ance, but  differs  in  its  relation 
to  the  lateral  axes,  which  ter- 
minate in  the  central  point  of 
each  equatorial  edge  and  in 
the  opposite  angle,  as  is  shown 
in  Fig.  202. 

III.    Trigonal  prisms  of  the 
third  order, 


±r/l 


na :  — -  a :  a :  oo :  c 
n  — i 


;  (hMo),(khlo),(ikho),(ihko). 


When  the  poles  in  Fig.  201  lie  on  the  primitive  circle,  between  the 
points  a;  the  terminations  of  the  lateral  axes  and  the  points  p,  the 


HEXAGONAL  SYSTEM 


107 


terminations  of  the  intermediate  axes,  a  new  form,  Fig.  205,  the 
trigonal  prism  of  the  third  order,  will  result.     When  the  pole  is  to 


FIG.  204.  — The  Positive  Trigonal  Pyramid  of 
the  Second  Order,  (hhahl). 


FIG.  205.  — The  Negative  Left 
Trigonal  Prism  of  the  Third 
Order,  (ikEl). 


the  right  of  the  point  p,  it  is  a  right  form;  when  to  the  left  of  p, 
it  is  a  left  form;  when  between  a  and  a3,  a  plus,  and  between  a3  and 
a2,  a  minus  form. 

IV.  Trigonal  prisms   of  the  second  order,  ± 
(hhlho),  (2hhho). 

When  the  poles  are  at  the  points  a,  Fig.  201,  a  new  form,  Fig. 
206,  the  trigonal  prism  of  the  second  order,  will  result,  in  which  the 
lateral  axes  terminate  in  the  central  point  of  the  face  and  bisect 
the  opposite  edges,  as  illustrated  in  Fig.  206. 
The  relation  of  the  trigonal  pyramids  and 
prisms  to  the  lateral  axes  is  shown  in  Fig. 
202,  which  is  a  plan  of  the  equatorial  plane. 

Other  forms.  —  All  other  positions  of 
the  poles  will  yield  forms  of  the  ditrigonal 
equatorial  type. 

The  possible  forms  to  combine  in  this 
type  will  be : 

Right  and  left  plus  and  minus  trigonal 
pyramids_of  the  third  order,  (hkil),  (khil), 

(ihkl),  (ikhl). 

,       .          x  .  .j       r,i          FIG.  206.  — The  Plus  Trig- 

Plus  and  minus_tngonal_pyramids  of  the       onal  Prism  of  the  Second 

second  order,  (hhihl),  (2hhhl).  Order,  (hhifio). 


108  MINERALOGY 

Plus  and  minus  trigonal  pyramid  of  the  first  order,  (hotil),  (ohhl). 

Right  and  left  plus  and  minus  trigonal  prisms  of  the  third  order, 
(hkio),  (khio),  (ihko),  (ikho). 

Plus  and  minus  trigonal  prisms  of  the  second  order,  (hh2ho), 
(2hhho). 

Plus  and  minus  trigonal  prisms  of  the  first  order,  (hoho),  (ohho). 

Base,  (0001). 

Examples.  —  As  yet  there  are  no  representatives  of  this  type. 

CLASS,  TRAPEZOHEDRAL  TETARTOHEDRAL 
TYPE  10,  TRIGONAL  HOLOAXIAL 

Symmetry.  —  Crystals  of  this  type  are  symmetrical  in  regard  to 
one  trigonal  axis  of  symmetry,  the  c  axis,  and  three  digonal  axes, 
the  a  axes.  Fig.  207  illustrates  the  symmetry  of  the  type. 

It  may  also  be  considered  as  a  tetartohedral  class,  and  the  forms 
are  derived  by  superimposing  the  rhombohedral  hemihedrons, 
type  19,  upon  the  trapezohedral  hemihedrons,  type  16,  and  extend- 
ing the  faces  not  thus  marked,  as  below  : 

2  6  10 

$^Z      J  _  6  .    J>^        0    10 
~  ~ 


23    24     ' 
15  19  23 

In  the  rhombohedral  method  of  selection,  alternating  dodecants 

above  and  below  the  equator  are  crossed  out  and  suppressed  ;  in  the 

.......  trapezohedral    method    every  other 

.*'  ^^  face  above  and  below  the  equator  is 

*  /       \        crossed   out,  as   suppressed.      There 

\      will  remain  of  the  24  faces  of  the 

o          \    /  \     dihexagonal     pyramid,     represented 

\'"  —  *—  "  !    above,  6  faces,  2,  6,  and  10  above, 

/    \  /     and  15,  19,  and  23  below  the  equator. 

\   x  When   this    method    of  selection  is 

o   \     /'         compared     with    the    rhombohedral 

*^...._         ,,-^'  tetartohedral    method,    page    97,   it 

FIG.  207.  —  Type  10,    Trigonal      ^U  be  S6en  that  there  the  faces   be" 

Holoaxiai.  low  lie  symmetrically  between  those 

above,  but  here  they  are  not  sym- 

metrically located,  as  15  is  nearer  2  than  to  6,  which  in  the  new 
form  will  produce  a  long  inclined  edge  between  6  and  15  and  a 
short  edge  between  2  and  15.  These  corresponding  edges  are  equal 


HEXAGONAL   SYSTEM 


109 


in  type  15,  forming  the  rhombohedron  of  the  third  order.  As  in  all 
other  tetartohedral  classes,  there  are  here  also  four  new  forms. 
In  the  selection  above,  face  2  and  those  corresponding  are  taken, 
forming  the  plus  right  form.  The  selection  may  be  so  arranged  that 
1,  or  13,  or  14,  and  corresponding  faces  should  be  selected ;  2  and  13 
are  the  plus  and  minus  right  congruent  forms  and  1  and  14  are  the 
plus  and  minus  left  congruent  forms.  By  applying  the  method  as 
above  in  turn  to  each  of  the  holohedral  forms  of  type  20,  the  new 
forms  of  this  type  will  be  produced. 


Forms 


I.  Trigonal  trapezohedrons, 


±  r/1 


na: 


a :  a :  me 


n— i 


KT(hkil),  KT(khil),KT(ihkl),  KT(ikhl). 


From  the  dihexagonal  pyramid  the  four  trigonal  trapezohedrons 
are  derived,  of  which  the  plus  right  is  represented  in  Fig.  208,  and 
the  plus  left  in  Fig.  209.  There  are  six  equal 
polar  edges,  three  at  each  pole,  three  long 
and  three  short  median  zigzag  edges,  which 
are  bisected  by  the  terminations  of  the  a 
axes. 

II.   All  other  positions  of  the  poles  in  Fig. 
207   will   yield  forms   already  described,  as 
the  trigonal   trapezohedron  is  the  only  new 
form  of  the  type.     The 
possible    forms  to  com- 
bine in  the  type  are  : 

Plus  and  minus  right 
and  left  trigonal  trape- 

PIG.  208.  -The  Positive     zohedrons,    KT(hkll), 

KT(klnl),       KT  (ihkl), 
KT  (ikhl). 

Plus  and  minus  rhombohedron  of  the  first 
order,  KT  (hohl),  (ohhl). 

Plus  and  minus jtrigonal  pyramid  of  the  sec- 
ond order,  KT  (hh2hl),  KT  (ihhhl). 

Plus  and  minus  right  and  jeft  ditrigonal 
prisms,  KT  (hkio) ,  KT  (khio) ,  KT  (ihko) ,  KT  (ikho). 

Plus   and  jninus    trigonal    prisms    second    FIQ  2Q9  _The  plus 

Order,  KT  (hh2ho) ,  KT  (2hhho) .  Left  Trapezohedron. 


Right  Trigonal  Trape- 
zohedron. 


110 


MINERALOGY 


Hexagonal  prism  first  order,  KT  (hoho). 
Base,  (0001). 

Examples.  —  Quartz,  SiO2,  crystallizes  in  this  type,  of  which 
Fig.  210  is  a  combination  of  the  hexagonal  prism,  first  order,  plus 


FIG.  210.  —  Right-handed 
Quartz :  Combination  of 
m(10lO),  r(1011),  z(OlTl), 
,  x(5161). 


FIG.  211.  — Lef  t-hand_ed 
Quartz,  m(ioro),  r(10n), 
z  (Olll),  8(2111),  x(65Tl). 


and  minus  rhombohedrons,  first  order,  plus  trigonal  pyramid,  and 
the  right  plus  trigonal  trapezohedron,  —  a  right-hand  crystal.  Fig. 
211  is  a  left-hand  crystal.  Cinnabar,  HgS,  also  belongs  here. 


CLASS,  TRIGONAL  HEMIMORPHIC 
TYPE  9,  TRIGONAL  POLAR 

*">..  Symmetry.  —  Crystals  of  this  type 

possess  one  axis  of  trigonal  sym- 
metry, the  c  axis,  Fig.  212.  The  type 
is  a  polar  development  of  the  trigonal 
equatorial,  in  which  the  hemitrigonal 
pyramids  may  occur  independently, 
yielding  two  new  forms. 


FIG.  212.  —  Type  9,  Trigonal 
Polar. 


na: 


:a:mc 


±  r/1,  u/1 


n  — i 


Forms 

I.  Trigonal    hemipyramids    of   the 
third  order, 

;  (Uril),   (khtt),  (ikhl),   (ihkl),  (hkU), 


(khil),  (ikhl),  (ihkl). 


HEXAGONAL  SYSTEM 


111 


Derived  from  the  trigonal 
pyramid  of  the  third  order 
there  are  eight  trigonal  hemi- 
pyramids,  of  which  Fig.  213 
represents  the  upper  left 
minus  hemipyramid,  and  the 
poles  in  Fig.  212  are  of  the 
upper  right  plus  form. 

II.  Trigonal  hemipyramids 
of  the  second  order, 


FIG.  213.  — The  Upper  Left  Negative  Trigo- 
nal Hemipyramid  of  the  Third  Order. 


±11/1 


2a: 2a: a:  me 


(hh2hl),  (hh2hl),  (2hhhl),  (2hhhl). 


There  are  four  trigonal  hemipyramids  of  the  second  order,  de- 
rived from  the  trigonal  pyramid  of  the  second  order,  all  of  which 
may  occur  independently.  Fig.  214  is  the  upper  minus  trigonal 
hemipyramid  of  the  second  order. 

The  forms  possible  to  combine  in  this  type  are  as  follows  : 
Plus  and  minus  right  and  left  upper  and  lower  hemipyramids  of 
the  third  order,  (hkil) ,  (khil) ,  (ihkl) ,  (ikhl) ,  (hkii) ,  (khil) ,  (ihkl) ,  (ikhl) . 


FIG.  214.  —  The  Upper  Negative  Trigonal  Hemi- 
pyramid of  the  Second  Order. 


FIG.  215.  —  Combination  of 
Forms  showing  the  Polar 
Development  of  Crystals  of 
Sodium  Periodate. 


Plus  and  minus  upper  and  lower  trigonaHiemipyramids  of  the 
second  order,  (hhihl),  (2hhhl),  (nh2HT),  (2hhhT). 

Plus  and  minus  upper  and  lower  trigonal  hemipyramids  of  the 
first  order,  (hohl),  (ohhl),  (hohl),  (ohhl). 

Plus  and  minus  right  and  left  trigonal  prisms  of  the  third  order, 
(hklo),  (ihko"),  (khio),  (ikho). 

Plus  and  minus  trigonal  prisms  of  the  second  order,  (hoho) ,  (ohho) . 

Plus  and  minus  trigonal  prisms  of  the  first  order,  (hh2ho),  (2hhho). 

Upper  and  lower  base,  (0001),  (OOOl). 


112  MINERALOGY 

Example.  —  Sodium  periodate,  NaKX,  3  H2O,  crystallizes  in  trig- 
onal hemipyramids.  Fig.  215  is  a  combination  of  two  plus  upper 
hemipyramids  of  the  second  order  and  one  of  the  first  order  with 
the  lower  base. 

Crystalline  Characters 

Like  the  tetragonal  system,  the  hexagonal  system  has  but  one 
variable  crystalline  character,  the  axial  ratio,  - .  This  is  calcu- 
lated in  a  similar  way,  by  means  of  the  angle  between  the  basal 
pinacoid  and  the  pyramid  face  of  the  first  or  second  order. 

I.  When  the  pyramid  of  the  second  order  is  used, 

-  =  tan  (oooi)A(ii22). 

Example.  —  In  the  mineral  beryl,  the  angle  between  the  normal 
to  the  base  and  that  of  the  unit  pyramid  of  the  second  order  is  26° 

31',  -  =  0.4988+,   and   as    a  =  1,  c  =  0.4988+,  which  is   the  unit 
a 

of  measurement  on  the  c  axis. 

II.  When  the  pyramid  of  the  first  order  is  used, 

-  =  tan  (0001)A(1011)  X  1/2V3. 
a 

Figure    216,     CO  =  c    and 
OA  =  a  =  i. 

^rk 

tan  CDO  = 


CO  =  tan  CDO  X  DO. 

In  the  triangle  aDO,  right- 
angled  at  D,  and  DOa  =  30°, 
DO  =  l/2V3,then  c  =  tan  CDO 
X  1/2V3,  but  the  angle  CDO 

is  equal  to  the  angle  between  the  poles  of  the  base  and  the  unit 
pyramid  of  the  first  order,  (0001J011). 

In  the  mineral  beryl  the  angle  0001J011  is  29°  56', 

c  =  tan  29°  56'  X  1/2V3. 
Log  tan  29°  56'  =  9.760272  +  10 
'  Log  1/2V3"=  1.937530 
Log  c  +  10  =  9.697802 
c=    .4988+ 


CHAPTER  VI 

THE   ORTHORHOMBIC,  MONOCLINIC,   AND   TRICLINIC   SYSTEMS 
THE  ORTHORHOMBIC  SYSTEM 

CRYSTALS  of  this  system  possess  three  crystallographical  axes; 
all  at  right  angles,  none  of  which  are  interchangeable.  The 
vertical  axis  is  represented  by  c.  The  longer  lateral  axis 
or  macro-axis  is  represented  by  b  and  is  placed  horizontally 
from  right  to  left,  while  the  short  or  brachy-axis,  a,  is  at 
right  angles  to  b.  Included  in  the  system  are  three  types  all  of 
which  have  at  least  one  axis  of  digonal  symmetry.  The  forms 
fall  into  three  groups,  according  to  the  relation  of  their  faces 
to  the  axes.  If  the  faces  cut  all  three  axes,  it  is  a  pyramid  and 
there  will  be  no  0  in  its  indices,  as  here  there  are  no  pyramids  or 
prisms  of  the  second  order ;  if  the  face  cuts  two  axes  and  is  paral- 
lel to  the  third,  it  is  a  prism  and  there  will  be  one  0  in  its  indices ; 
when  parallel  to  a  lateral  axis  it  is  a  dome  and  receives  the  name 
of  the  lateral  axis  to  which  it  is  parallel,  as  macrodome.  A  dome  is 
a  prism  parallel  to  a  lateral  axis.  When  the  face  is  parallel  to  two 
axes,  it  is  a  pinacoid  and  there  will  be  two  zeros  in  its  indices ;  when 
parallel  to  the  lateral  axes  it  is  a  basal  pinacoid ;  when  parallel  to  c 
and  one  of  the  lateral  axes,  it  takes  the  name  of  the  lateral  axis  to 
which  it  is  parallel,  as  brachypinacoid. 

CLASS,  ORTHORHOMBIC,  HOLOSYMMETRIC,  OR  HOLOHEDRAL 
TYPE  8,  DIDIGONAL  EQUATORIAL 

Crystals  of  this  type  possess  three  didigonal  axes,  correspond- 
ing to  the  crystallographical  axes;  three  planes,  the  diametral 
planes,  and  a  center  of  symmetry,  Fig.  217.  The  largest  number  of 
faces  possible  on  any  crystal  form  of  the  type  will  be  8,  one  in  each 
octant  into  which  the  three  planes  of  symmetry,  Fig.  218,  divide 
space. 

i  113 


114 


MINERALOGY 
Forms 


I.   Orthorhombic  pyramids,  na  :  b  :  me;  (hkl). 
The  poles  in  Fig.  217  represent  the  orthorhombic  pyramid;  it  is 
bounded  by  8  similar  scalene  triangular  faces,  which  inclose  space, 


FIG.  217.  — Type  8,  Didigonal 
Equatorial. 


FIG.  218.  —  The  Orthorhombic  Axes  and  Planes 
of  Symmetry. 


Fig.  219.     The  crystallographical  axes  terminate  in  the  tetrahedral 
angles.     There  are  three  series  of  pyramids  : 

a.  The  unit  series,  &  :  b  :  me,  (hhl),  where  the  variable  lies  on  the 
c  axis ;  Fig.  220  represents  the  unit  series  of  pyramids. 


FIG.  219.  — The  Unit  Pyramid  of 
Barite. 


FIG.  220.  — The  Unit  Series  of  Pyra- 
mids. 


b^  Macro  series  of  pyramids,  &  :  nb  :  c,  (hlh),  when  the  intercept 
on  b  is  greater  than  unity. 

c.  Brachy  series  of  pyramids,  n&  :  b  :  c,  (Ihh),  when  the  intercept 
on  the  a  axis  is  greater  than  unity. 


THE  ORTHORHOMBIC  SYSTEM 


115 


II.  Prisms,  r£  :  b  :  oo  c ;  (hko). 

Prisms  are  parallel  to  the  c  axis;  the  poles  of  Fig.  217  will  lie 
on  the  primitive  circle  between  the  digonal  axes.  When  the  pole 
is  nearer  a  the  form  will  be  of  the 
macro  series,  a :  nb :  oo  c,  as  its  in- 
tercept on  b  will  be  larger  than 
unity,  and  when  near  b,  the  form 
will  be  of  the  brachy  series ;  Fig.  221 
is  the  unit  prism,  which  is  the  limit- 
ing form  connecting  the  two  series. 

III.  Domes.      Macro    series    of 
domes,  na  :  oo  b  :  me ;  (hok). 

When  the  poles  lie  in  the  diam- 
etral plane  containing  the  c  and 
&  axes,  the  faces  will  be  parallel  to 
the  macro  axis,  and  the  form  will 
be  a  macrodome  Fig.  222,  an  open 
form  bounded  by  four  similar  faces. 
There  will  be  a  series  of  macrodomes 
the  angles  of  which  and  the  inter- 
cepts on  the  axes  will  depend  upon 

...  .    ,  ,  FIG.  221.  —  The  Unit  Prism  of  Barite. 

the  position  of  the  poles. 

Brachy  series  of  domes,  oo&:  nb :  me;  (ohk). 

When  the  poles  lie  in  the  plane  of  symmetry  containing  the  c 


-,-f 

'j 

i 


FIG.  222.  —  The  Unit  Macro- 
dome  of  Barite. 


FIG.  223.  —  The  Brachy  Series  of  Domes. 


and  b  axes,  the  faces  will  be  parallel  to  the  brachyaxis,  and  the 
form  will  be  a  brachydome,  of  which  there  is  also  a  series,  Fig. 
223. 


116 


MINERALOGY 


IV.  Pinacoids.  —  Three  other  positions  of  the  poles  are  possible, 
which  represent  the  fixed  forms,  that  is  when  the  poles  take  the 

position  of  the  angles  of 
the  triangle,  or  coincide 
with  the  crystallograph- 
ical  axes. 

Basal    pinacoid, 
ooa:  oobrc,  (001). 


FIG.  224.  —  The  Unit  Brachydome  of  Barite. 


FIG.  225.  —  Combination  of  the 
Three  Pinacoids. 


When  the  pole  coincides  with  the  c 
axis,  the  face  will  be  parallel  to  a  and  b, 
the  form  consisting  of  two  faces,  one 

above  and  one  below  the  equatorial  plane,  which  will  produce  the 
basal  pinacoid. 

Macropinacoid,  &  :  oob  :  ooc  ,  (100). 

Here  the  pole  coincides  with  the  &  axis,  when  the  face  is  parallel 

to  b  and  c. 

Brachypinacoid,  oo  a  :  b  :  oo  c,  (010). 
Here  the  pole  coincides  with  the  b 
axis  and  the  faces  are  parallel  to  a  and 
c;  Fig.  225  represents  the  three  pina- 
coids  in  combination. 
Forms  in  combination.  —  The  possible  forms  to  combine  in  this 
type  therefore  are  : 


FIG.  226.  — Barite. 


Pyramids,  series  (hkl). 
Prisms,  series  (hko). 
Macrodome,  series  (hok). 
Brachydome,  series  (ohk). 


Basal  pinacoid,  (001). 
Macropinacoid,  (100). 
Brachypinacoid,  (010), 


Examples.  —  A  large  number  of,  arid  especially  important,  rock- 
forming  minerals  crystallize  in  this  type: 


THE  ORTHORHOMBIC  SYSTEM 


117 


Olivine,  (Mg,Fe)2SiO4. 
Enstatite,  MgSi03. 
Aragonite,  CaCO3. 


Topaz,  Al[Al(O.F2)]SiO4. 
Barite,  BaSO4. 


Fig.  226  is  a  combination  of  the  base,  brachydome,  macrodome, 
and  macropinacoid  in  barite. 


CLASS,  ORTHORHOMBIC  HEMIMORPHIC 
TYPE  7,  DIDIGONAL  POLAR 

Symmetry.  —  Crystals  of  this  type  possess  one  didigonal  axis, 
the  c  axis,  and  two  planes  of  sym- 
metry intersecting  in  the  c  axis,  Fig. 
227.  It  is  a  polar  development  of 
the  didigonal  equatorial,  with  a  loss 
of  all  the  symmetry  lying  in  the  equa- 
torial plane  of  that  type. 


Forms 


I.  Hemipyramids,    u/1 


na:b:  me 


(hkl),  (hkl). 

There  would  be  upper  and  lower 
pyramids  of  each  series,  Fig.  228. 


FIG.  227.  — Type  7,  Didigonal 
Polar. 


II.  Domes.       Macrodomes,    u/1     -  -;    (hok),    (hok), 


Fig.  229. 

Brachydomes,  u/l(°°2:  °b:  mc)  ;  (ohk),  (ohk),  Fig.  230. 


FIG.  228.  —  The  Upper  Hemipyramid. 


FIG.  229.  —  The  Upper  Macrodome. 


Both  domes  would  be  modified  by  the  symmetry,  yielding  hemi- 
domes,  while  the  prisms  would  suffer  no  apparent  change. 


118 


MINERALOGY 


III.  Of  the  three  pinacoids,  the  base  would  yield  hemi  forms,  the 


upper  and  lower  base,  u/l 


°°*: 


'  C 


,  (001),  (001). 


Possible  forms  to  combine  in  the  type  would  be  : 

Pyramids  upper  and  lower,  (hkl)  ,  (hkl)  . 

Prisms,  two  series,  (hko). 

Domes  ;  upper  and  lower  macrodomes,  (hok)  ,  hok)  . 

Domes  ;  upper  and  lower  brachydomes,  (ohk)  ,  (ohk)  . 

Macropinacoid,  (100). 

Brachypinacoid,  (010). 

Upper  and  lower  base  (001,)  (001). 

Examples.  —  Calamine,  ZnSiO3,  2H2O.      Fig.  231  represents  a 
combination  of   two  upper  hemimacro-  and    brachydomes,   the 


FIG.  230.  —  The  Upper  Brachydome. 


FIG.  231.  —  Calamine. 


unit  prism,  the  lower  unit  hemipyramid,  the  macro-  and  brachy- 
pinacoids,  and  the  upper  base. 

Struvite,  NH4MgP04,  6  H20,  also  crystallizes  in  this  type. 

CLASS,  SPHENOIDAL  HEMIHEDRONS 
TYPE  6,  DIGONAL  HOLOAXIAL 

Symmetry.  —  Crystals  of  this  type  possess  three  axes  of  digonal 
symmetry  corresponding  to  the  crystallographical  axes,  but  no 
planes  of  symmetry.  Fig.  232  illustrates  the  symmetry  of  the  type. 


Forms 


I.  Sphenoids, 


The  poles  of  Fig.  232  represent  the  general  form  of  the  type. 


THE  ORTHORHOMBIC  SYSTEM 


119 


The  right  sphenoid,  with  four  similar  scalene  triangular  faces, 
Fig.  233  and  Fig.  234,  is  the  complementary  left  form. 


O 


FIG.  232.  —  Type  6,  Digonal 
Holoaxial. 


FIG.  233.  — The  Right 
Sphenoid. 


Other  forms  are  similar  in  appearance  to  those  of  type  8. 

Forms  possible  to  combine  in  this  type  will  therefore  be : 

Right  and  left  sphenoids,  K(hkl) ,  K(hkl) . 

Orthorhombic  prisms,  (hko).         Macropinacoids,  (100). 

Macrodomes,  (hok).  Brachypinacoids,  (010). 

Brachydomes,  (ohk).  Basal  pinacoid,  (001). 

Examples.  —  Sulphur,  S,  Fig.  235,  represents  a  combination  of 
the  right  and  left  sphenoids  and  the  base  on  sulphur. 

Epsomite,  MgSO4 . 7  H2O,  Fig.  236,  is  a 
combination,  of  the  prism  and  right  sphenoid 
as  found  on  crystals  of  this  mineral. 


FIG.  234.  —  The 
Left  Sphenoid. 


FIG.  235.  —  Sulphur. 


FIG.  236.  —  Epsomite: 
Combination  of  (110) 
and  (111). 


120 


MINERALOGY 


Crystalline  Characters 

In  the  orthorhombic  system,  where  the  unit  on  each  axis  is  a 
different  one,  there  are  two  axial  ratios,  fe  and  - ,  b  being  the  unit 

of  comparison,  or  as  they  are  generally  written,  a  :  b  :  c  =  .8152  + 

1 :  1.31359+,  the  axial  values  of  barite. 

On  calculating  the  axial  ratios  it 
will  be  necessary  to  measure  the  angle 
of  the  unit  prism,  or  the  angle  be- 
tween the  unit  prism  and  either  the 
macro-  or  brachypinacoid,  when  a  in 
terms  of  b  may  be  calculated.  To 
determine  c,  the  angle  of  the  unit 
dome  or  the  angle  between  the  dome 
and  a  pinacoid  must  be  measured. 

Example.  —  In  the  mineral  stauro- 
lite,  the  angle  100,110  =  25°  20';  as 
this  is  the  angle  between  the  poles, 
the  actual  angle  of  the  right-angled 
triangle  is  25°  20',  with  the  side  a 

opposite,  therefore,  tan  25°  20'  =  £  =  £  =  .4734+,  as  b  =  1. 

b      1 

In  calculating  the  value  of    c,  the  angle  101J01  =  110°  32', 
which  being  the  angle  between  the  poles,  the  actual  angle  between 

the  faces,  Fig.  237,  cao  =  1/2  dod'  =  55°  16' ;  -  =  tan  55°  16',  c  = 


FIG.  237. 


tan  55°  16';  a  =  .4734. 

Log  tan  55°  16' 
Log  .4734 


10.159083  -  10 
1.675228 


Log  c  =   9.834311  -  10 

c  =      .6829 
Axial  ratio  of  staurolite,  a  :  b  :  c  =     .§£§£:  1  :  .6829. 


THE  MONOCLINIC  SYSTEM 

The  monoclinic  system  includes  all  those  crystals  referable  to 
three  axes,  two  of  which,  c  the  vertical,  and  b  the  orthoaxis,  are  at 
right  angles  ;  the  third,  or  clinoaxis,  is  at  right  angles  to  b  and  in- 
clined to  c,  and  is  designated  by  a.  Here  the  three  diametral  planes 


THE   MONOCLINIC   SYSTEM 


121 


no  longer  divide  space  into  eight  equal  octants,  but  into  octants  of 
two  different  sizes,  four  of  which  are  large,  or  obtuse,  and  four 
smaller,  or  acute.  The  two  upper  front  and  the  two  lower  back 
octants  are  large  and  designated  —  octants ;  the  smaller  are  the  + 
octants.  As  the  inclination  of  a  to  c  varies  with  the  substance, 
the  angle  between  these  two  axes,  measured  in  the  +  octants  is 
designated  by  (3,  and  is  therefore  less  than  90°,  which  added  to  the 

two  axial  ratios  -  and  -  make  three  crystalline  characters  for  the 

D  D 

system. 


CLASS,  HOLOHEDRAL  (HOLOSYMMETRIC) 
TYPE  5,  DIGONAL  EQUATORIAL 

Symmetry.  —  Crystals  of  this  type  possess  one  digonal  axis, 
the  b  axis ;   one  plane  of  symmetry,  the  equatorial  plane,  at  right 

angles  to  the  c  axis  and 
containing  a  and  c,  and 
a  center.     Figure   238 
represents  the  relation 
of  the  axes  and  plane 
of  symmetry,  and  also 
the  general  position  of 
_    the  monoclinic  crystals 
relation  to  the  ob- 


+b 


in 


+0 

FIG.  238.  —  The  Monoclinic  Axes  and  Plane  of 
Symmetry. 

server.      Fig.  239    representing    the 

symmetry  of  the  type,  it  differs  from 

others  in  that  the  b  axis  and  not  c  is 

perpendicular   to    the    plane  of    the 

paper.     In  viewing  a  crystal  of  this 

type  held  in  the  general  position,  the  equatorial  plane  will  be 

vertical. 


FIG.  239.  —  Type  5,  Digonal  Equa- 
torial. 


122 


MINERALOGY 
Forms 


I.  Monoclinic  pyramids  differ  from  those  heretofore  considered, 
as  the  octants  subtended  by  the  faces  are  large  and  small,  yield- 
ing faces,  represented  by  the  same  parameters  or  indices,  of  two 
sizes,  of  which  the  larger  faces  form  the  minus  and  the  smaller  form 
the  plus  pyramids.  Monoclinic  pyramids  do  not  inclose  space; 


FIG.  240.  —  The  Minus  Pyramid.    The  Plus  Pyramid.    Combination  of  the  Plus  and 

Minus  Pyramid. 

the  combination  of  the  plus  and  minus  pyramids  is  equivalent  to  a 
single  orthorhombic  pyramid,  and  incloses  space,  Fig.  240. 

As  in  the  orthorhombic  system,  there  are  three  series  of  pyramids 
holding  the  same  relation  to  the  axes  as  in 
that  system. 

_Unit  series  of  pyramids,  ±  a  :  b  :  me,  (hhl), 
(Mil). 

Ortho  series  of  pyramids,    ±  &  :  nb  :  mC, 
(hkl),  (hkl). 
-  -'•"  ------       _Clino  series  of  pyramids,  ±  n£  :  b  :  me,  (khl), 

(km). 

The  unit  or  fundamental  pyramid  is  the 
connecting  link  between  the  three  series. 
When  the  form  represented  by  the  spherical 
projection  is  a  pyramid,  the  poles  will  fall 
within  the  triangle,  Fig.  239. 


. 

circle  at  right  angles  to  the  c  axis,  the  form 

is  a  prism,  of  which  there  are  two  series  ;  when  the  pole  is  nearer 
the  b  axis,  it  is  a  prism  of  the  clino  series,  ni  :  b  :  o>  c,  (hko).     When 


THE  MONOCLINIC  SYSTEM 


123 


its  position  is  nearer  the  extremity  of  the  a  axis,  it  is  of  the  ortho 
series,  &  :  nb  :  oo  c,  (kho).  Prisms  are  not  plus  and  minus  forms,  as 
each  face  subtends  two  octants,  one  above  and  one  below,  Fig.  241. 

III.   Clinodome,  oo  £  :  nb  :  me,  (ohl). 

When  the  pole  lies  in  the  plane  at  right  angles  to  the  a  axis,  the 
faces  are  parallel  to  the  clinoaxis  and  the  form  is  the  clinodome. 


FIG.  242.  — The  Plus  and  Minus  Ortho- 
domes. 


FIG.  243.  —  Combination  of 
the  Three  Pinacoids. 


IV.  Orthodome.  —  If  the  poles  lie  in  the  equatorial  plane  the 
faces  will  be  parallel  to  the  orthoaxis  and  the  form  is  the  ortho- 
dome,  of  which  there  are  two  forms: 

the  plus  orthodome,  &  :  oob  :  me,  (hoi), 
formed  by  the  two  faces  subtending 
the  four  small  octants ;  and  the  minus 
orthodome,  a  :  b  :  me,  (hoi),  formed  by 
the  two  faces  subtending  the  four  large 
octants,  Fig.  242. 

V.  Orthopinacoid,  a :  oo  b :  oo  c,  (100), 
when  the  poles  lie  in  the  plane  of  sym- 
metry at  90°  from  c. 

VI.  Clinopinacoid,  oo  a :  b :  oo  c,  (010), 
when  the  poles  lie  on  the  b  axis. 

VII.  Basal     pinacoid,     oo  a  :  oo  b  :  c, 
(001),  when  the  poles  lie  on  the  c  axis. 
Figure  243  is  a  combination  of   the 
three  pinacoids. 

Combinations.  —  The  possible  forms 
to  combine  in  this  type  are  : 


FIG.  244.  —  Combination  of 
mdlOXaClOO),  b(010),  u(lll), 
and  y(101),  in  Augite. 


Pyramids,  three  series,  plus  and  minus,  (hkl),  (hkl). 
Prisms,  two  series,  (hko). 


124 


MINERALOGY 


Ortho  series  of  domes,  plus  and  minus,  (hok),  (ho.k). 

Clino  series  of  domes,  (ohk). 

Orthopinacoid,  (100). 

Clinopinacoid,  (010). 

Basal  pinacoid,  (001). 

Examples.  —  Some  of  the  most  important  rock-forming  minerals 
are  members  of  this  type,  as  : 

Orthoclase,  KAlSi3O8.  Epidote,  HCaaAlaSisOis. 

Augite.  Gypsum,   CaSO4,  2H20. 

Amphibole. 

Fig.  244  represents  a  combination  of  the  unit  prism,  (110), 
orthopinacoid,  (100),  clinopinacoid,  (010),  minus  unit  pyramid, 
(111),  and  the  plus  orthodome,  (101),  of  augite. 


CLASS,  MONOCLINIC  HEMIMORPHIC 
TYPE  4,  DIGONAL  POLAR 

Symmetry.  —  Crystals  of  this  type  possess  one  digonal  axis  of 
symmetry,  the  b  axis,  Fig.  245.    The  forms  may  be  considered  as  pro- 


o 


o 


FIG.  245.  — Type  4,  Digonal  Polar. 


FIG.  246. 


duced  by  a  polar  development  of  type  5,  along  the  orthoaxis ;  thus 
the  plane  and  center  of  symmetry  is  lost,  and  yielding  hemi  forms 
either  side  of  the  plane  of  symmetry  each  of  which  may  occur 
independently,  Fig.  246. 


THE  MONOCLINIC  SYSTEM 
Forms 


125 


The  new  forms  would  be : 

I.    Right  and  left  plus  and  minus  hemipyramids,  of  two  faces 
each,  (hid),  (hid),  (hid),  (hid),  Fig.  247. 


FIG.  247.— The  Right 
Plus  and  Minus 
Hemipyramids. 


FIG.  248.  — Right  Hemi- 
prism. 


FIG.  249.  —  The  Right 
Hemiclinodome. 


II.    Right  and  left  hemiprisms  of  two  faces  each,  (hko),  (hko), 
Fig.  248. 

JII.    Right  and  left   hemiclinodome  of  two  faces  each,  (ohk), 
(ohk),  Fig.  249. 

IV.  Right  and  left  clinopinacoid, 
(010),  (OlO). 

The  orthodome  and  the  ortho  and 
basal  pinacoids  are  not  modified,  and 
combine  with  the  above  new  forms  on 
crystals  of  this  type. 

Examples.  —  A    number    of    organic 
compounds  crystallize  in  this  type,  as,   FIG.  250.  — Milk  Sugar:  Com- 
tartaric  acid,  C2H606.  bination  of  b«no),  b'(olo), 

Fig.  250  is  a  combination  of  the  right 
and  left  clinopinacoids,  right  and  left 
prism,  orthopinacoid  and  left  clinodome  on  milk  sugar. 


and  a  (100) 


126 


MINERALOGY 
CLASS,  DOMATIC  HEMIHEDRONS 


TYPE  3,  EQUATORIAL 
Symmetry.  —  Crystals  of  this  type  possess  a  plane  of  symmetry 


only,  Fig.  251.     They  may  be  considered  as  hemihedrons  formed 


FIG.  251.  — Type  3,  Equatorial. 


FIG.  252. 


by  selecting  pairs  of  faces  of  type  5,  which  meet  in  the  equatorial 
plane,  as  the  shaded  faces  of  the  minus  pyramid  of  Fig.  252. 

Forms 
I.  Upper  and  lower  plus   and  minus   hemipyramids   of  each 

series  of  two  faces  each,  ±  (  — — 1— ^Ju/1,   K(hkl),    K(hkl), 

I  •     V        2        J 

K(hkf),  K(hkl),  Fig.  253. 

II.  Front  and 
|                         rear  hemiprisms  of 

two  faces  each, 

f/r(n*:  V  °°  C)» 
K(hko),  K(hko). 
Fig.  254. 

III.  Upper    and 
lower     clinodomes, 

„   /oo  at :  nb  :  D 
u/1  (-   — 

K(ohl),  (ohl),  Fig. 
255,  of  two  faces 

FIG.  253.  —  The  Lower  Minus     eacn- 

Hemipyramid.  IV.    Upper  and  lower,  back  and  front 


FIG.  254.  — Front 
Hemiprism. 


THE  MONOCLINIC  SYSTEM  127 

hemiorthodome  of  one  face  each,  ±  u/1  (no" :  oo  b  :  me),  (hoi),  (hoi), 
(hoi),  (hoi),  Fig.  256. 

V.   The  ortho  and  basal  pinacoids,  from  their  relation  to  the  axis 
of  symmetry,  will  each  be  represented  by  one  face. 


I 

FIG.  255.  —  The  Upper  Semiclinodome. 


FIG.  256.  —  The  Upper  Front  Hemi- 
orthodome, (hoi). 


The  clinopinacoid  will  appear  as  in  type  5  with  two  faces. 

Examples.  —  A  single  mineral,  clinohedrite,  H2ZnCaSi05,  crys- 
tallizes in  this  type.  Fig.  257  is  a  combination  of  the  plus  and 
minus  hemipyramid,  (111), 
plus  upper  pyramid,  (771), 
plus  upper  pyramid,  (161), 
and  the  back  hemiprism,  (110) 
as  found  on  this  mineral. 


Crystalline  Elements 

In  addition  to  the  two  axial 
ratios,  in  the  monoclinic  sys- 
tem the  angle  p  is  required  to 
fix  the  characteristics  of  any  FlG-  257.  -  Combination  of  p(iii),  q(iii), 

,    ,    f  rp,,  .       :          m(110),  t(771),  z(161)  on  Clmohednte. 

crystal   form.     The    angle   is 

measured  in  the  diametral  plane,  and  in  the  plus   octants;   its 

value  is  therefore  always  given   as  less  than  90°.     It  may  bo 


128 


MINERALOGY 


measured  directly,  being  the  angle  between  the   base  and  the 

orthopinacoid. 

Four  angles  are  sufficient  to  determine  the  elements:  (100*001), 

(101  A  100),  (001.011)  and  (110*100). 

Calculations.  —  When   p   is  not  measured    directly,   generally 

(001  *  110)  and  (110*110)  can 
be  obtained,  Fig.  258.  In  the 
right-angled  spherical  triangle, 
right-angled  at  A,  the  two  angles 
B(001A110)andC  =  l/2(110A110) 
are  known,  the  two  sides  c  and 
b  can  be  calculated.  C  is  the 
angle  of  the  right-angled  triangle 
of  which  a  is  one  leg  and  b  the 
other,  also  p  =  180°  -  b. 

Example.  —  In  orthoclase  the 
angle  (001*110)  =  67°  47'  and 
the  angle  (110*110)  =  61°  13'; 
as  these  are  the  angles  between 
the  poles,  in  each  case  subtract- 


FIG. 258. 

triangle  =  112°  13'  and  C 


ing  from  180°,  B  in  the  spherical 

1/2  (118°  47'). 


cosB      cos  (112°  13') 


C  sinC  "sin  (59°  24') 

Log  cos  112°  13' =  9.577618 
Log  sin     56°  24' =  9.934783 
Log  cos  b  =  9.642745 
b  =  116°  3'. 
P  =180°  -  (116°  3')  =  63°  57'.  ' 

The  axial  ratio.  —  In  the  spherical  triangle  ABC,  with  the  side 
b  and  the  angle  C  known,  being  right  angled  at  A,  the  side  c  is 
calculated  by  Napier's  rule. 


tan  c 


sinb 


sin  (116°  3') 


cot  C     cot  (59°  23' 
Log  sin  116°  3' =  9.953475 
Log  cot  (59°  23'  30")  =   9.772312 
Log  tan  c  =  10.181 163 
c  =  56°37'. 


30") 


THE   TRICLINIC  SYSTEM  129 

In  the  triangle  doe,  right-angled  at  o  and  with  the  side  oe  =  b  = 
i  and  the  angle  ode  known : 

^P  =  *-  =  cot  ode  =  cot  56°  37'  =  .658  +  or  a  =  .658  +. 
oe      b 

For  the  value  of  c  the  angle  (001 A 101)  =  50°  16'. 
In  the  triangle  coa,  P  =  63°  37'  and  oca  =  180°  -  (50°  16')  - 
(63°  37')  =  65°  47'. 

In  the  triangle  aoc,  in  which  the  angles  and  one  side  oa  =  a  = 
.658  +  are  known,  oc  =  c  may  be  calculated. 
oc :  oa :  sin  oac :  sin  oca,  or  c :  a  : :  sin  59°  16' :  sin  65°  45'. 

a  X  (sin  59°  16') 

sin  (65°  450 

Log  a  =1.818226 

Log  sin  50°  16' =  9.885942 

9.704168 

Log  sin  65°  45' =  9.959852 
Log  c  =  1.744316 
c=    .555 +. 

The  crystalline  constants  of  orthoclase  would  be  expressed  as 
calculated,  a  :  b:  c  =  ,658+  :  1 :  .555+  :  p  =  63°  57'. 


THE  TRICLINIC  SYSTEM 

In  the  triclinic  system  all  axes  are  inclined,  and  none  of  the  five 
crystalline  elements  are  fixed ;  the  axes  are  unequal  and  designated, 
&:  b:  c,  as  in  the  orthorhombic  system.  Generally  the  unit  plane 
has  been  chosen  so  that  the  unit  on  c  is  smaller  than  that  on  b, 
but  this  may  not  be  so  in  all  species.  Here  the  diametral  planes 
divide  space  into  octants  of  four  different  sizes ;  of  which  opposite 
octants  through  the  center  are  similar;  thus  the  pyramids  of  the 
triclinic  system  will  consist,  at  the  most,  of  a  single  pair  of  parallel 
faces  each  subtending  octants  of  the  same  size.  The  four  possible 
pyramids  are  equivalent  to  the  orthorhombic  pyramid  and  in 
combination  inclose  space.  The  axial  angles  are  either  greater 
or  less  than  90°  and  are  measured  in  the  plus  octant,  the  upper  right- 
hand  octant.  The  angle  between  b  and  c  is  designated  a,  that 
between  a  and  b,  -y,  and  that  between  &  and  c,  P,  Fig.  265. 


130  MINERALOGY 

CLASS,  HOLOHEDRAL  (HOLOSYMMETRIC) 
TYPE  2,  CENTROSYMMETRIC 

All  forms  of  this  type  possess  a  center  of  symmetry  only  and  each 
form  is  composed  of  a  single  pair  of  parallel  faces,  Fig.  259. 
Forms.  —  Pyramids. 


FIG.  259.  —  Type  2,  Centrosym- 
metric. 


FIG.  260.  —  Two  Faces  forming 
a  Triclinic  Pyramid. 


Unit  series  of  pyramids, 


|a:b:mc,  (hkl). 
a:b:mc,  (hkl). 
a:b:mc,  (hkl). 
a:b:mc,  (hkl). 


FIG.  261.  —  Two  Faces  forming  a  Triclinic  Prism. 


THE  TRICLINIC  SYSTEM 


131 


Macro  series  of  pyramids, 


a:nb:mc,  (hkl). 
a:nb:mc,  (hkl). 
arnbrmc,  (hkl). 
a:nb:mc,  (hkl). 


FIG.  262.  —  The  Two  Faces  which  form  a  Triclinic     FIG.  263.  —  Combination  of  the 
Dome.  Three  Pinacoids. 


Brachy  series  of  pyramids, 


na  :  b  :  me,  (hkl) 
narbrmc,  (hkl) 
na:b:mc,  (hkl) 
na:b:mc,  (hkl). 


Fig.  260. 


FIG.   264.  —  Axinite:    Combination    of 
m(110) 
•(201), 


FIG.  265.  —  Combination  of  the  Three 
Pinacoids  in  Rhodonite. 


Macro  series  of  prisms, 


a  :  nb  :  oo  c,  (hko) . 
a  :  nb  :  ooc,  (hko). 


132 


MINERALOGY 


fn£  :  b  :  ooc,  (hko).     ^.      OC1 
Brachy  series  of  prams,      |na  .  b  .  «, C)  (hio).     F'g-  261 


Macro  series  of  domes, 
Brachy  series  of  domes, 


na :  oo  b :  me,  (hoi) . 
na  :  oo  b  :  me,  (hoi) . 

f  oo  a  :  nb  :  me,  (ohl). 
I  oo  a  :  nb  :  me,  (ohl). 


Fig.  262. 


,     Macropinacoid,  a  :  oo  bj^  oo  c,  (100) . 

Brachypinacoid,  oo  a  :  b_:  oo  c,  (010). 

Basal  pinacoid,  ooa:  oob  :  c,  (001),  Fig.  263. 

Examples.  —  The  plagioclase  feldspars  crystallize  in  this  type. 
Fig.  264  represents  a  combination  ofJUO)  (110)  (100)  (111) 
(201)  on  a  crystal  of  axinite,  in  which  a  :  b  :  c  =  .492+:  1 :  .479+  and 
a  =  82°  54',  p  =  91°,  y  =  131°  32',  100  A  010  =  48°  21'. 

Fig.  265  is  the  combination  of  the  three  pinacoids  in  rhodonite, 
MnSi03,  in  which  a  =  103°  18',  p  =  108°  44',  ?  =  81°  39'. 

CLASS,  TRICLINIC  HEMIMORPHIC 
TYPE  1,  ASYMMETRIC 

Symmetry.  —  Crystals  of  this  type  possess  no  symmetry  what- 
ever, and  each  form  of  the  type  is  composed  of  a  single  face,  Fig. 

266.  Any  collection  of  faces,  how- 
ever irregularly 
grouped,  may 
belong  to  this 
type,  provided 
they  conform  to 
the  law  of  ra- 
tional indices. 

All    forms    of 
the   type    may 
be  considered  as 
being    produced 
by  the  selection  of  one  face  of  the  pair  form-  _ 

FIG.  26/. — Calcium  Thio- 

ing  the  corresponding  holohedron  of  type  2. 
Forms,  —  Possible  forms  will  be  the  same 
as  in  type  2,  except  they  will  all  be  hemi 
forms  consisting  of  one  face  each,  and  designated  as  the  upper 
right  front  hemipyramid,  or  as  the  lower  back  hemimacrodome,  etc. 


FIG.  266.  —  Type  1,  Asymmetric 


sulphate  :  Combinations 
ofc(001),b(010),m(110), 
q(011),  andh(HO). 


THE  TRICLINIC  SYSTEM  133 

Examples.  —  There  are  no  minerals  crystallizing  in  the  type, 
but  several  salts,  as  strontium  bitartrate,  Sr(C4H406)2  .  5  H2O. 

Calcium  thiosulphate,  CaS2O3 .  6  H2O,  Fig.  267,  represents  the 
combination  of  (001)  (010)  (HO)  (Oil)  (110)  common  on  this  salt. 

The  Crystalline  Elements  of  the  Triclinic  System 

In  the  triclinic  system,  where  no  elements  are  fixed,  all  five  are 
calculated  from  measured  angles,  as  none  of  the  axial  angles  can 
be  measured  directly;  at  least  five  angles  must  be  measured. 
The  angles  generally  chosen  are  the  pinacoidal  angles,  001 A 100  ; 
001  A  010;  100  A  010,  and  the  angles  between  the  unit  form  and  a 
pinacoid,  as  100  A 1 10 ;  001 A 101 ;  001 A  01 1 .  When  any  five  of  these 
angles  are  measured,  the  axial  ratios  and  axial  angles  may  be 
calculated. 


CHAPTER  VII 


RELATION   OF  INDIVIDUAL  CRYSTALS 


CRYSTALS  as  found  in  nature  are  rarely  simple,  or  composed  of  one 
individual.  During  the  process  of  formation  they  must  necessarily 
come  hi  contact  with  each  other ;  this  contact  modifies  them,  not 
only  producing  distortions  and  irregularities  in  external  form,  but 
reentrant  angles  are  formed.  The  angles  of  all  simple  crystals 
must  be  less  than  180°,  and  whenever  an  angle  greater  than 
180°  or  a  reentrant  angle  appears,  it  is  proof  that  the  crystal  is  of  a 
compound  nature  or  consists  of  more  than  one  individual.  At  the 
time  of  separation,  one  crystal  may  have  an  influence  upon  the 
position  or  direction  of  the  axes  of  its  neighbor,  and  this  influence 
may  show  itself  in  various  ways.  Minerals  totally  different  in 
composition  and  crystalline  structure  are  sometimes  so  placed  that 
certain  axes  and  edges  are  parallel  in  the  two  species,  as  in  case 
of  staurolite  and 


cyanite  from  St. 
Gothard,  Fig.  268 ; 
while  belonging  to 
different  crystal 
systems,  these  two 

v^S  £$  ^WHH  species    are    often 

*jj#j-  v  \   ^r  so      placed      that 

»|t,  ;fek|.  40Q  "'•*  their    c    axes    are 

^It  -i^-if  W^  $  parallel.     Such 

v*§£  "•"^~W  ''*  parallel     growths, 

however,  occur  the 
more  often  be- 
tween individuals 

of  the  same  species,  or  between  species  belonging  to  the  same 
isomorphic  group;  in  such  cases  large  aggregates  will  have  all 
their  crystalline  directions  parallel  as  in  Fig.  269,  sulphur,  or  as  in 
Fig.  270,  microcline  from  Pike's  Peak.  In  such  parallel  growths 
equivalent  faces  will  reflect  light  or  appear  bright  at  the  same 
time. 

134 


FIG.  268.  —  Cyanite  and  Staurolite  in  Parallel  Position 
from  St.  Gothard,  Switzerland. 


RELATION   OF   INDIVIDUAL   CRYSTALS 


135 


It  often  happens  that  the  faces  of  a  large  crystal  may  have  a  mat- 
like  surface,  caused  by  a  layer  of  small  individuals  which  have  been 
deposited,  generally 
in  parallel  position, 
upon  the  surface  of 
the  larger  crystal. 
They  either  repre- 
sent a  secondary 
generation,  or  are 
the  result  of 
changed  condition 
during  crystalliza- 
tion, causing  a  more 
rapid  deposition, 
Fig.  271. 

Drusiness  of  faces 
is  also  produced 
either  by  a  second 
generation  of  the 
same  species,  or  by 
secondary  minerals 
formed  by  the  decomposition  of  the  surface  of  the  mineral  upon 

which  the  small 
crystals  are 
placed. 

Complete  par- 
allelism exists  the 
more  often  be- 
tween individ- 
uals of  the  same 
species,  or  the 
species  of  an  iso- 
morphous  group. 
The  dividing  line 
between  individ- 
uals is  not  al- 
ways distinct,  for 
as  each  individ- 
ual is  reduced  in 


FIG.  269.  —  Parallel    Aggregate    of    Sulphur    Crystals. 
Girgenti,  Sicily. 


FIG.    270.  —  Parallel  Aggregate   of   Microcline  from  Pike's 
Peak,  Colorado. 


size,    each    may 
vanish  as  a  line, 


136 


MINERALOGY 


and  will  be  repre- 
sented by  a  striation 
running  across  the 
crystal  face  in  a 
fixed  direction. 
Wherever  these 
striations  appear  on 
the  face  of  a  crys- 
tal, they  must  be 
considered  as  the 
boundary  between 
two  individuals.  In 
Fig.  272  a,  a  com- 
plex quartz  crystal, 
the  individuals  are 
well  marked  and 
apparent ;  but  in 
Fig.  272  b,  a  quartz  crystal  with  striations  running  across  the  prism 
face  parallel  to  the  intersection  of  the  prism  and  rhombohedron, 
each  striation  represents  a  reentrant  angle  between  individuals, 
or  the  crystal  in  its  growth  may  be  said  to  oscillate  between  the 


FIG.  271.  —  Quartz  Crystals  in  Parallel  Position  on  Or- 
t  thoclase. 


b  a 

FIG.  272.  —  Smoky  Quartz  from  Disentis,  Switzerland. 


RELATION   OF   INDIVIDUAL  CRYSTALS 


137 


prism  and  the  rhombohedron.     Striations  are  very  characteristic 
of  certain  crystal  faces  in  various  mineral  species.     The  cube  face 


FIG.  273.  —  Crystals  of  Pyrite  showing  Striations  on  the  Cube  and 
Pyritohedron. 

in  pyrite  is  striated  at  right  angles  to  a  pair  of  edges,  Fig.  273, 
representing  an  oscillatory  growth  between  the  cube  and  the  pen- 
tagonal dodecahedron  or  py- 
ritohedron.  In  tourmaline, 
Fig.  274,  the  prism  faces  are 
striated  lengthwise  the  crys- 
tal, which  represents  an  os- 
cillation between  the  trigonal 
and  hexagonal  prisms. 

Twins.  —  In  a  large  num- 
ber of  unions  of  crystals,  all 
crystallographic  equivalent 
directions  are  not  parallel,  as 
in  parallel  growths;  some 
may  be  parallel  and  others 
at  an  angle,  as  if  rotated 
around  an  axis  180°,  or  as 
if  reflected  across  a  plane. 
Fig.  275  is  a  diagrammatical  representation  of  6  molecules.  In 
a,  b,  and  c  the  equivalent  directions  are  all  parallel,  as  in  a  simple 
crystal,  but  x,  y,  and  z  are  reversed,  as  if  reflected  over  the  plane 


FIG.  274.  —  Tourmaline  from  Pala,  Cali- 
fornia, showing  Longitudinal  Striations  on 
the  Prism. 


138 


MINERALOGY 


mn.  Again,  a,  b,  and  c,  when  revolved  around  mn  as  an  axis 
180°,  will  become  congruent  with  x,  y,  and  z.  The  molecules  a, 
b,  c  are  said  to  occupy  a  twinning  position  in  regard  to  x,  y,  z/and 
the  two  individuals  are  said  to  be  twins.  The  axis  of  revolution 

is  the  twinning  axis,  and  a 
plane  at  right  angles  to  the 
twinning  axis  is  the  twin- 
ning plane.  The  plane 

m !— _ ! ' n  separating  the  two  indi- 
viduals is  the  composition 
or  contact  plane ;  this  with 
rare  exceptions  is  parallel  to 
a  possible  crystal  face. 

The  twinning  axis  is  either 
parallel  to  a  possible  crystal 

edge  or  perpendicular  to  a  possible  face.  It  can  never  be  an  axis  of 
even  symmetry,  as  by  a  revolution  of  180°  around  such  an  axis  the 
two  individuals  would  be  congruent  and  form  a  simple  crystal. 
Fig.  276  represents  a  simple  crystal  of  gypsum ;  Fig.  277  is  a  twin 


FIG.  276.  —  Gypsum  Crystal  showing  the  Position  of  the  Twinning  Plane. 

crystal  of  gypsum  in  which  the  twinning  axis  is  parallel  to  the 
vertical  axis  c.  Fig.  278  is  a  tw'nned  crystal  of  gypsum  in  which 
it  may  be  seen  that  one  individual  has  been  revolved  around  the 


RELATION  OF  INDIVIDUAL   CRYSTALS 


139 


twinning   axis  c  180°,  and  also 

that  the  twinning  axis  is  parallel 

to  the  edge  in  the  prism  zone. 

The  shaded  plane  is  the  contact 

plane  and  is  parallel  to  the  or- 

thopinacoid.     The  trace  of  the 

twinning   plane   in   the    crystal 

is  usually  marked  by  a  reentrant 

angle,  as  xyz,  or,  where  this  is 

reduced  to  a  minimum,  by  stri- 

ations   on  the   crystal  face,   as 

yy',  Fig.  278. 

A  reentrant  angle  is  not  al- 
ways present  to  distinguish  the 

crystal  as  a  twin,  and  often  when 

absent,  as  is  shown  in  the  epi- 

dote    crystals    from    Prince    of 

Wales  Island,  Alaska,  Fig.  280, 

where  the  twinning  axis  is  per-     FlG.  277.  -  Gypsum  Twins  from  near 

pendicular  to  the  orthopinacoid  Paris,  France. 

and  the  composition  and  twin- 
ning plane  is  the  orthopinacoid,  and  which  after  a  revolution  of 

180°,  leaves  on  these  crystals  no  indication  of  the  twinning. 

Striations  on  the  clino- 
pinacoid  due  to  parallel 
growth  are  indicated  by 
the  parallel  lines,  and  the 
effect  of  twinning  on  these 
striations  is  shown.  The 
striations  meet  the  twin- 
ning plane,  yy',  from  each 
•  individual  at  the  same  in- 
clination, and  the  trace  of 
the  twinning  plane  on  the 
crystal  face  bisects  the 
angle  between  them. 

The  complexity  of  some 
apparently  simple  crystals 

FIG.  278.  —  Gypsum  Twins.  J  , 

is  often  only  revealed  by 

the  microscope  and  polarized  light,  as  in  the  twinning  bands  of 
the   plagioclase   feldspars,   Fig.  281.     In   enantiomorphic   types, 


140 


MINERALOGY 


where  there  are  right-  -and  left-handed  forms,  it  is  not  possible  to 
revolve  one  individual  around  an  axis  into  a  congruent  position. 


FIG.  279.  — Simple   Crystal   of 
Epidote. 


FIG.  280.  —  Twinned  Epidote  from  Prince 
of  Wales  Island,  Alaska. 


and  a  twinning  axis  is  therefore  not  always  possible;  but  such 
crystals  are  twinned  by  reflection,  as  some  twins  of  quartz.  Twins 
formed  by  the  union  of  plus  and  minus,  upper  and  lower,  or  right 


FIG.  281. -Twinning  Lamellae  of  Plagioclase,  between  Crossed  Nicols. 
Much  enlarged. 

and  left  forms  are  supplementary  twins;  and  when  as  contact 
twins,  and  equally  developed,  the  individuals  will  possess  a  pseudo- 


RELATION   OF   INDIVIDUAL   CRYSTALS 


141 


FIG.  282.  —  Supplementary  Twins  of  Pyrite. 


symmetry  or  a  symmetry 

of  a  higher  type,   as   the 

upper  and  lower  forms  of 

a  polar  type  when  joined 

along  the  plane  of  the  base 

will  possess  the  symmetry 

of  an  equatorial  type. 
The    plus     and    minus 

forms  may  penetrate  each 

other  and  be  distinguished 

as  complex  individuals  by 

the  reentrant  angles.    Fig. 

282  is  a  drawing  of  the 

supplementary    twins     of 

pyrite,  formed  by  the  plus 

and  minus  pyritohedrons ;  while  Fig.  283  is  a  photograph  of  these 
twins  from  Prussia.  In  interpenetrating 
twins  there  is  no  marked  plane  of  contact 
between  individuals,  but  a  very  irregular 
and  ill-defined  area  separates  the  two  indi- 
viduals internally. 

In  the  growth  of  crystals  the  twinning 
position  may  have  been  assumed  at  the 
very  outset,  in  the  nucleus  of  crystalliza- 
tion, and  the  complex  nature  existed  at  the 
beginning  of  crystallization;  or  again  the 

individuals  may  have  developed  as  simple  crystals*  when  through 

a   changed    condition    mole- 
cules   have   separated    in    a 

twinned    position    and    the 

axis  of  the  simple  crystal  is 

abruptly    changed.      Where 

there  is  but  one  angle  in  the 

axis  of  an  elongated  crystal 

they  are  often  termed  genic- 

ulate  twins,  as  the  geniculate 

twins  of  rutile,  Fig.  284. 
This  bending  or  angle  in 

the  axis  of  the  crystal  may 

be     repeated     either     in     the      FlG.  284._  Geniculate  Twins  of  Rutile  from 
Same  direction  Or  in  the  Op-  Lancaster  County,  Pennsylvania. 


FIG.    283.  —  Supplemen- 
tary Twins  of  Pyrite. 


142 


MINERALOGY 


FIG.  285.  —  Cyclic  Twins  of  Marcasite  from 
Folkstone,  England. 


posite  direction.     When  repeated  in  the  same  direction  a  number 
of  times,  the  complex  individual  is  circular  and  is  termed  cyclic 

twins,  as  in  marcasite,  Fig. 
285 ;  or  the  twinning  may 
be  repeated  first  in  one  di- 
rection and  then  in  the 
other,  with  a  zigzag  effect, 
as  in  rutile,  Fig.  286. 

When  the  twinning  is 
repeated  many  times  at 
very  short  intervals,  each 
individual  of  the  complex 
structure  will  be  confined 
to  a  very  thin  sheet  pass- 
ing through  the  aggregate, 
parallel  to  the  composition 
plane,  and  indicated  on  the 
crystal  externally  by  a  re- 
entrant angle  as  illustrated  in  Fig.  287,  a  twin  crystal  of  albite  in 
which  the  clinopinacoid  is  the  composition  plane  and  the  twinning 
axis  is  perpendicular  to  this  face.  Fig.  288  is  a  crystal  of  albite 
composed  of  several  individuals 
twinned  after  this  same  law ;  each 
individual  is  indicated  by  the 
reentrant  angle  passing  around 
the  crystal  parallel  to  the  twin- 
ning plane.  Each  individual  may 
be  reduced  to  extreme  thinness, 
when  only  a  striation  on  the 
crystal  face  will  remain  to  mark 
the  plane  of  contact  separating 
individuals,  the  whole  complex 
structure  building  up  an  appar- 
ently simple  crystal.  When  often 
repeated  in  this  manner,  the 
twinning  is  termed  polysynthetic.  *IG'  286>  ~  zigzag-twins  of  Rutile- 
The  striations  produced  by  polysynthetic  twinning  are  quite  dif- 
ferent .from  those  caused  by  parallel  growths;  the  former  pass 
through  the  body  of  the  crystal  and  are  caused  by  the  arrange- 
ment of  the  molecules  and  will  therefore  appear,  not  only  on  crystal 
faces,  but  also  on  all  cleavage  faces  intersecting  the  twinning  planes, 


RELATION   OF  INDIVIDUAL  CRYSTALS 


143 


as  in  the  plagioclases.  The  latter  are  confined  to  the  crystal  face 
and  are  not  caused  by  a  change  in  the  molecular  arrangement,  and 
will  therefore  not  appear  on  cleavage  faces  or  be  indicated  below 
the  surface. 

Twinning  in  the  isometric  system.  —  In  isometric  minerals  the 
spinel  law  is  the  most  common.     In  this  method  of  twinning  the 


FIG.  287.  —  Twins  of 
Albite. 


FIG.  288. 


FIG.  289.  —  Spinel  Twins. 


trigonal  axis,  which  is  common  to  the  five  types  of  the  system,  is 
the  twinning  axis,  and  the  face  of  the  octahedron  111  or  the  plane 
parallel  to  it  is  the  com- 
position plane.  For  all 
minerals  crystallizing  in 
the  isometric  system  this 
is  a  possible  form  of 
twinning.  Fig.  289  is  a 
drawing  in  which  mn  is 
the  twinning  axis,  and  Fig. 
290  is  a  photograph  of 
spinel  twins  from  Frank- 
lin, New  Jersey. 

Penetration  twins  with 
the  same  axis  as  the  twin- 
ning axis  occur  in  galena 
and  fluorite,  Fig.  291, 
twins  of  fluorite. 

The  second  law  is  where 
the  twinning  axis  is  per- 
pendicular to  the  face  of  the  rhombic  dodecahedron  face,  110,  and 
is  a  possible  twinning  law  in  those  types  only  where  this  axis  is 


FIG.  290.  —  Spinel  Twins  from  Franklin,  New 
Jersey. 


144 


MINERALOGY 


FIG 


291.  —  Penetration    Twins 
Fluorite. 


not  an  axis  of  symmetry,  as  the 
tesseral  central  and  tesseral  polar 
types.  The  supplementary  twins 
of  pyrite  are  of  this  class. 

Twinning  in  the  tetragonal  sys- 
tem. —  The  general  law  hi  this 
system  is  where  the  twinning  axis 
is  perpendicular  to  a  pyramid  face 
and  is  a  possible  mode  of  twinning 
in  all  seven  types  of  the  system. 
Fig.  292  is  a  drawing  of  twins  of 
cassiterite,  in  which  the  twinning 
axis  is  perpendicular  to  the  pyr- 
amid face 
of  the  sec- 
ond order, 

101,  and  the  composition  plane  cc  is  parallel 
to  101.  .  Fig.  293  is  a  photograph  of  a  nat- 
ural twin  from  Zinnwald,  Bohemia.  In  the 
ditetragona)  polar  type,  the  normal  to  the 
sphenoids  1 1 1  is  the  twinning  axis,  with  the 
sphenoidal  face  the  composition  face ;  when 
developed  as  contact  twins,  they  are  similar 
to  the  spinel  twins  of  the  isometric  system. 

In  the  tetragonal  polar  types  a  third  twinning  law  is  possible,  as 
in  these  types  the  lateral  axes  are  not  axes  of  symmetry  and  are 

therefore  possible  twinning  axes. 
These  axre  supplementary  twins, 
and  when  there  is  no  reentrant 
angle  at  the  equatorial  plane 
there  is  nothing  to  indicate  the 
twinned  nature  of  the  crystal, 
and  the  symmetry  is  apparently 
that  of  an  equatorial  type. 
Most  crystals  of  wulfenite  are  of 
this  character. 

Twinning  in  the  hexagonal 
system.  —  Twins  in  the  hexago- 
nal division  are  rare.  A  pos- 
sible law  in  all  the  types  of  the 
system  is  where  the  twinning 


FIQ.  292.  —  Twins  of  Cas- 
siterite. 


FIG. 293. 


-  Cassiterite  Twins  from  Ziinn- 
wald,  Bohemia. 


RELATION  OF  INDIVIDUAL  CRYSTALS  145 

axis  is  perpendicular  to  a  pyramid  face  and  the  pyramid  face 
is  the  composition  plane.  This  law  is  a  very  common  one  in 
the  trigonal  types,  and  is  the  same  as  where  the  twinning  axis 
is  perpendicular  to  a  scalenohedral  or  rhombohedral  face.  In 
calcite  the  common  form  of  twinning  is  where  the  twinning  axis 
is  normal  to'the  rhombohedron  e  (Oil 2),  Fig.  294. 


FIG.  294.  —  Calcite  Twins  in  which  e  (0112)  is  the  Composition  Face. 
Guanajuato,  Mexico. 

In  types  of  alternating  symmetry  and  trigonal  types  the  vertical 
axis  is  a  trigonal  axis,  and  is  therefore  a  possible  twinning  axis  with 
the  base  as  the  composition  plane ;  Fig.  295  is  such  a  twin  of  cal- 
cite. In  the  polar  types  supplementary  twinning,  as  in  the  tetrag- 
onal system,  is  a  possible  law.  There  are  very  few  minerals  of 
these  types;  the  most  common  is  tourmaline,  in  which  there  are  no 
twins ;  and  in  nephelite  the  polar  symmetry  is  shown  by  the  etch 
figures  only,  and  all  crystals  must  be  considered  as  examples  of 
supplementary  twinning. 

In  the  holoaxial  types,  of  which  quartz  is  an  example,  twinning 
by  reflection  is  the  rule,  as  in  the  Brazilian  twins,  where  the  plane 
of  reflection  is  parallel  to  the  prism  face  (1120),  Fig.  296,  and  x 
is  a  reflection  of  x'. 


146 


MINERALOGY 


FIG.  295.  —  Twins  of  Calcite  in  which  the  Twinning  Axisis  c  and  the 
Composition  Plane  is  the  Base.    Guanajuato,  Mexico. 

Twinning  in  the  orthorhombic  system.  —  There  are  three  classes 
of  twins  possible  in  all  three  types  of  the  system :  — 

I.  Where  the  twinning  axis  is  normal 
to  a  pyramid  face.     This  is  in  fact  a  pos- 
sible law  in  all  32  types  of  crystals,  consid- 
ering  the  octahedron   and  its    resulting 
hemihedral  forms  as  pyramids.      In  Fig. 
297  a,  twins  of  staurolite,  in  which  the 
twinning  axis  is  normal  to  the  pyramid 
(232). 

II.  Where  the  twinning  axis  is  normal 
to  a  prism  face.     Fig.  298  is  a  diagram 
of  aragonite,  in  which  the  twinning  axis  is 
normal  to  the  prism  110,  often  repeated, 
and  as  the  prism  angle  is  nearly  120°,  the 

FIG.  296. -Brazilian Twins    twinned  crystals  have  a  pseudohexagonal 
of  Quartz.  symmetry,  Fig.  299. 


RELATION   OF  INDIVIDUAL  CRYSTALS 


147 


III.  Where  the  twinning  axis  is  normal  to  a  dome,  as  is  well 
illustrated  by  the  cross-shaped  twins  of  staurolite,  Fig.  297  b,  in 
which  the  twinning  axis  is  normal  to  the  dome  (032). 


a  b 

FIG.  297.  —  Staurolite  Twins  from  Georgia. 

In  the  didigonal  equatorial  type  all  faces  may  be  twinning  faces, 
except  the  three  pinacoids,  the  normals  to  which  are  the  axes  of 


FIG.  298.  —  Twins  of 
Aragonite,  Twin- 
ning Plane  110. 


FIG.  299. —  Triplets  of  Aragonite  from 
Bastanes,  France. 


symmetry.  In  the  sphenoidal  and  digonal  holoaxial  types,  these 
normals  are  still  digonal  axes  of  symmetry,  and  only  the  above  three 
types  are  possible.  In  the  digonal  polar  type  supplementary 


148 


MINERALOGY 


twins  are  possible,  as  here  the  lateral  axes  are  not  digonal  axes  of 

symmetry  and  may  be  twinning  axes  with  the  base  as  the  com- 
position plane.  These  supplementary 
twins  are  well  illustrated  by  calamine. 

Twinning  in  the  monoclinic  system.  — 
The  only  direction  not  possible  as  an 
axis  of  twinning  is  the  orthoaxis,  and 
twins  are  very  common  in  the  system. 
They  may  be  divided  into  the  three  types 
as  in  the  orthorhombic  system,  with  the 
addition  that  the  two  pinacoids  may 
also  be  twinning  planes.  In  gypsum  and 
augite  the  twinning  plane  and  composi- 
tion plane  is  the  orthopinacoid,  and  the 
twinning  axis  is  the  normal  to  it.  In  the 
Carlsbad  twins  of  orthoclase  the  twinning 

axis  is  the  vertical  axis,  while  the  composition  plane  is  the  clino- 

pinacoid,  parallel  to  it,  Fig.  300. 

Twinning  in  the  triclinic  system.  —  As  there  is  only  a  center  of 

symmetry,  any  plane  is  possible  as  a  twinning  plane  and  twins  are 


FIG.  299  a.  —  Diagram  of  the 
Basal  Plane  of  Aragonite 
Triplets,  showing  the  Rela- 
tion of  the  Individuals. 


FIG.  300.  —  Carlsbad  Twins  of  Orthoclase.    Brice,  New  Mexico. 


numerous  and  complicated,  often  repeated  polysynthetically  as  in 
the  plagioclase  feldspars. 


CHAPTER  VIII 

ON  THE  MEASUREMENT  OF   CRYSTALS    AND   THE  USE   OF  THE 

GONIOMETER 

IT  is  often  necessary  to  measure  the  angles  between  two  crystal 
faces  in  order  to  identify  the  forms  present  on  the  crystal,  partic- 
ularly when  the  specimen  is  much  distorted.  This  may  be  true 
even  with  such  well-developed  and  easily  recognized  forms  as  the 
prism  and  rhombohedron  on  quartz.  The  forms  present  in  com- 
plicated combinations  must  always  be  proven  by  the  measurement 
of  the  angles  between  the  faces.  The  measurement  of  the  angles 
will  also  help  in  the  identification  of  the  mineral  species ;  and  in 
chemical  compounds  their  variation  from  the  theoretical  value  may 
afford  a  means  of  estimating  the  purity  of  the  compound,  as  chemi- 
cally pure  substances  possess  a  constant  and  characteristic  angle  be- 
tween crystal  forms,  though  in  isomorphous  groups  these  angles  are 
very  nearly  equal ;  yet  when  pure  each  member  of  the  group  will 
possess  an  angle  distinctly  its  own. 

The  goniometer  and  the  principles  of  the  reflecting  goniometer 
have  been  described  in  Chapter  I.  . 

For  the  identification  of  crystal  forms  when  the  crystals  are  not 
too  small,  the  Penfield  card  contact  goniometer,  model  B,  Fig. 
11,  is  a  very  convenient  and  sufficiently  accurate  little  instru- 
ment, and  it  has  the  advantage  of  cheapness,  so  that  each  student 
may  be  provided  with  or  possess  one.  It  answers  also  as  a  protrac- 
tor and  scale  in  the  drawing  of  crystals. 

In  using  the  instrument,  the  card  with  the  scale  is  held  at  right 
angles  to  the  edge  and  one  of  the  crystal  faces,  the  angle  between 
which  and  the  adjacent  face  it  is  wished  to  measure.  The  arm  of 
the  instrument  is  then  rotated  until  it  is  in  contact  with  the  second 
crystal  face ;  the  crystal  with  the  goniometer  in  place  is  now  held 
up  to  the  light,  with  the  line  of  vision  parallel  to  the  edge  between 
the  two  faces ;  the  arm  and  card  are7  carefully  adjusted  to  fit  the 
two  faces  and  the  angle  between  them,  so  that  no  light  is  seen  be- 
tween the  instrument  and  the  crystal  faces.  •  The  instrument  must 

149 


150  MINERALOGY 

be  held  perpendicular  to  the  crystal  faces,  as  the  true  angle  is 
obtained  only  in  this  position;  after  satisfactory  adjustment  the 
angle  is  read  from  the  card.  This  model  also  has  the  advantage  of 
giving  both  the  actual  angle  between  the  faces  and  the  supplement 
to  it  or  that  between  the  poles,  which  is  the  angle  usually  recorded 
in  the  description  of  crystals. 

With  smooth  faces  and  as  large  as  a  centimeter  across,  the  angles 
between  them  may  be  measured  with  the  contact  goniometer  to 
within  one  degree,  an  accuracy  sufficient  for  the  identification  of 
forms  and  species.  Fig.  301  is  a  more  expensive  instrument,  in 
which  the  two  arms  are  detached  from  the  scale  and  one  moves 
along  the  other,  which  enables  one  to  measure  crystals  separated 

by  reentrant  angle.  To 
gain  experience  in  the  use 
of  the  instrument  it  is  well 
for  the  student  to  measure 
the  angles  and  identify  the 
faces  on  a  distorted  Her- 
kimer  County  quartz  crys- 
tal of  about  1.5  cm.  in 
diameter. 

When    more    accurate 


FIG.  301. -Contact  Goniometer.  WOI>k    ls    re(luire(l,    as    in 

the    calculations    of    the 

crystalline  constants  or  characters  of  any  mineral  and  in  the 
determination  of  the  indices  of  the  faces  as  well  as  the  identifica- 
tion of  new  forms,  the  reflecting  goniometer  is  used. 

There  are  several  varieties  of  this  instrument.  One,  the  single- 
circle  goniometer,  in  which  the  angle  is  measured  between  the  poles 
of  the  two  faces  in  question.  Another,  the  two-circle  goniometer, 
by  which  the  pole  of  any  face  is  located,  in  reference  to  some  chosen 
face  as  the  base.  The  face  may  be  said  to  be  located  by  its  lati- 
tude being  measured  on  one  circle  and  its  longitude  being  meas- 
ured on  the  second  circle  at  90°  to  the  first,  just  as  a  point  on  the 
earth's  surface  is  fixed.  There  is  also  a  more  complicated  instru- 
ment in  which  three  graduated  circles  are  used.  Of  these  instru- 
ments only  the  single-circle  goniometer  will  be  described. 

The  card  contact  goniometer  may  be  easily  converted  into  a  single- 
to  reflecting  goniometer,  for  use  in  measuring  crystals  too  small 
to  yield  results  sufficiently  accurate  by  the  contact  method  of 
measurement.    If  a  bridge  be  cemented  on  the  arm  over  the  eye- 


THE  MEASUREMENT  OF  CRYSTALS  151 

let  or  axis  of  the  instrument,  the  crystal  to  be  measured  may  be 
fastened  on  this  with  wax  and  measured..  The  small  crystal  is  then 
mounted  with  the  edge  to  be  measured  perpendicular  to  the  card, 
and  the  edge  should  coincide  as  nearly  as  possible  with  the  axis 
of  the  arm ;  then  with  the  edge  of  the  card  placed  on  the  edge 
of  the  table  and  with  the  eye  at  a  distance  of  a  foot  and  a  half, 
the  arm  is  revolved  until  one  of  the  faces  reflects  the  light,  when 
a  reading  is  taken.  The  instrument  is  replaced  with  the  light, 
card,  and  eye  in  the  same  relative  positions,  which  may  be  as- 
sured by  getting  the  reflection  on  the  same  face  without  moving 
the  arm ;  the  arm  is  now  revolved  until  the  second  face  reflects 
the  light,  when  another  reading  is  taken.  The  difference  be- 
tween these  two  readings  will  be  the  angle  between  the  poles  of 
the  two  faces  reflecting  the  light.  Results  obtained  by  this  method 
are  more  accurate  than  those  obtained  by  the  contact  method. 
It  may  be  unnecessary  to  state  that  the  accuracy  of  the  measure- 
ments is  increased  with  the  distance  of  the  light  and  the  eye  from 
the  crystal. 

The  principles  of  the  reflecting  goniometer  have  been  sufficiently 
illustrated  by  the  measurement  of  a  crystal  with  the  card  con- 


FIG.  302.  —  The  Fuess  Single-Circle  Reflecting  Goniometer.     One  Quarter  Natural 

Size. 


152  MINERALOGY 

tact  goniometer  as  just  described  above.  The  accuracy  of  the  re- 
sults obtained  will  depend  upon  the  size  of  the  source  of  light  used; 
on  the  exact  parallelism  of  the  intersecting  edge  between  the  two 
faces  with  the  axis  of  the  instrument ;  and  upon  the  plane  of  reflec- 
tion, the  plane  in  which  the  angle  is  measured  being  at  exactly  90° 
to  the  edge  of  the  crystal  and  the  axis  of  the  instrument. 

The  instrument  is  constructed  with  all  of  these  conditions  in  view 
and  is  provided  with  devices  allowing  of  adjustments  to  these  ends. 
Fig.  302  is  the  usual  form  of  the  Fuess  single-circle  goniometer  one- 
fourth  natural  size.  The  collimator  Z  is  supported  by  the  post  A, 
whi  ch  is  rigidly  fixed  to  the  frame  of  the  instrument.  The  collimator 
is  provided  with  a  lens  at  the  inner  end,  and  at  the  focus  of  this  lens 
at  the  outer  end  the  slit  admitting  light  is  placed.  The  shape  and 
size  of  this  opening  may  be  adjusted  in  the  more  expensive  instru- 
ments to  suit  the  work  at  hand.  The  usual  form  is  that  illustrated 
in  Fig.  303,  and  known  as  the  Websky  signal  slit,  hourglass  in  shape, 
with  its  vertical  plane  of  symmetry  vv'  parallel  to  the  axis  of  the 
instrument  and  fixed  in  this  position. 

The  telescope  B  is  supported  by  a  similar  post  but  attached  to 
the  disk  or  circle  upon  which  the  vernier  of  the  scale  is  marked,  and 
can  be  revolved  about  the  axis  of  the  instrument,  and  rigidly  fixed 
in  any  required  position  by  the  set  screw  C.  The  telescope  is  fitted 
with  an  eyepiece  Q,  which  js  provided  with  cross  hairs  at  right 
angles,  one  of  which  is  fixed  parallel  to  the  axis  of  the  instrument, 
the  other  will  be  at  90°  to  the  axis.  The  eyepiece  is  adjusted  to 
this  position  by  the  collar  which  clamps  on 
the  eyepiece  and  which  fits  in  a  notch  in  the 
drawtube  of  the  telescope,  thus  always  assur- 
ing the  correct  position  of  the  vertical  hair 
when  the  eyepiece  is  withdrawn  and  re- 
turned to  its  position. 

The  two  cross  hairs  should  divide  the 
Websky  signal  orthorhombically  when  the 
telescope  is  set  directly  opposite  the  collima- 
tor'  as  illustrated  in  Fig.  304.  The  telescope 
eyepiece  is  adjusted  to  parallel  rays;  and 
when  it  is  wished  to  view  the  crystal  being  measured,  the  lens 
D  is  placed  before  the  tube  and  focuses  the  rays  on  the  axis  of 
the  instrument.  This  lens  may  be  revolved  out  of  the  field,  when 
the  signal  will  appear  if  a  face  is  in  position. 

The  circular  disk  shown  under  d  is  the  graduated  circle,  which  is 


THE   MEASUREMENT  OF  CRYSTALS  153 

divided  to  half  degrees  and  with  the  vernier  may  be  read  to  min- 
utes. This  circle  is  revolved  by  the  pilot  wheel  f,  which  in  turn 
may  be  clamped  to  the  axis  of  the  instrument  by  the  set-screw  b. 
This  circle  may  be  accurately  turned  to  any  particular  point  by  first 
revolving  it  with  the  pilot  wheel  and  then  setting  the  screw  a  and 
using  the  fine  adjustment,  or  tan- 
gent screw  F. 

The  crystal  to  be  measured  is 
cemented  with  wax  to  a  carrier 
which  fits  in  a  socket  in  the  table 
of  the  instrument  and  is  held  in 
place  by  the  screw  p ;  the  screw  d 
allows  this  table  to  be  elevated  or 
lowered  until  the  edge  to  be  meas- 
ured may  be  seen  in  the  telescope. 

In  order  to  adjust  the  crystal 

there  are  four  movements  neces-  FIG  304 

sary,  each  of  which  is  controlled 

by  a  separate  screw :  two,  r  and  q,  are  screws  at  right  angles 
to  each  other  and  allow  the  carrier  to  be  pushed  back  and  for- 
ward ;  and  the  two  screws  n  and  o  are  connected  with  sections  of 
cylinder  the  axes  of  which  are  at  90° ;  these  allow  the  crystal 
to  be  tilted  in  planes  at  90°.  By  these  four  screws  the  edge  to 
be  measured  may  be  quickly  brought  to  coincide  with  the  axis 
of  the  goniometer  and  therefore  at  right  angles  to  the  plane  of 
reflection,  and  the  edge  after  adjustment  will  also  be  in  focus  when 
the  lens  D  is  before  the  telescope. 

Measurement  of  a  crystal.  —  Let  the  crystal  selected  for  measure- 
ment be  one  of  topaz  from  Thomas  Mountains,  Utah,  as  these 
crystals  are  combinations  of  forms  of  several  zones,  and  usually 
they  possess  bright  smooth  faces.  The  crystal  is  first  cleansed  with 
alcohol  and  ether  and  then  not  touched  with  the  fingers  to  dull  the 
faces  and  spoil  their  reflecting  qualities.  The  various  zones  pres- 
ent are  noted  and  rough  sketches  or  sections  at  right  angles  to 
each  zone  made;  each  face  in  the  zone  sketched  is  represented 
by  a  letter  on  the  drawing  and  the  letter  placed  opposite  the 
reading  taken  of  the  face  when  the  angles  are  measured ;  this 
enables  each  reading  of  the  goniometer  to  be  referred  to  the 
right  face. 

The  crystal  is  now  mounted  on  a  carrier  with  the  edges  of  the 
zone  to  be  first  measured  perpendicular  to  its  surface ;  the  carrier 


154  MINERALOGY* 

is  now  clamped  in  the  instrument.  Let  the  crystal  be  first  mounted 
on  the  flat  basal  cleavage  and  the  prism  zone  the  first  zone  to  be 
measured,  the  edges  of  which  are  all  at  right  angles  to  the  basal 
cleavage.  The  crystal  is  elevated  or  depressed  by  means  of  the  screw 
d  until  it  is  in  the  plane  of  reflection.  With  the  screw  b  set  and 
the  screw  a  loose,  the  telescope  is  revolved  until  its  axis  is  at  about 
110°  with  the  collimator  and  there  set  with  the  screw  C.  The  lens 
D  is  placed  before  the  telescope  and  the  goniometer  light  before 
the  Websky  slit  in  the  collimator;  the  crystal  is  now  adjusted. 
With  the  right  hand  on  the  screw  q  and  the  axis  of  this  screw  at 
90°  to  the  telescope,  and  the  left  hand  on  the  screw  n,  the  crystal 
is  pushed  back  and  forth  with  the  right  hand  until  an  edge  is  seen 
in  the  telescope,  when  with  the  left  hand  this  edge  is  tilted  until  it 
is  parallel  to  the  vertical  hair,  and  it  is  placed  directly  on  the  hair 
with  the  right  hand.  The  crystal  is  now  revolved  with  the  pilot 
wheel  f  90°  to  the  right  and  the  right  hand  is  placed  on  the  screw 
r  and  the  left  on  the  screw  o ;  the  same  edge  is  adjusted  to  coincide 
with  the  vertical  hair  as  before.  After  these  adjustments  have 
been  carried  out  accurately,  the  crystal  when  revolved  will  turn 
on  the  edge  as  an  axis.  This  edge  will  now  lie  in  the  axis  of  the 
instrument  and  will  therefore  be  at  90°  to  the  plane  of  reflection  and 
in  position  to  measure. 

The  crystal  is  now  rotated  until  one  of  the  adjacent  faces  to  the 
edge  adjusted  is  seen  in  the  telescope  to  reflect  the  light  from  the 
collimator,  the  lens  D  is  lifted  and  the  signal  will  appear.  The  sig- 
nal is  revolved  by  means  of  the  pilot  wheel  f  to  the  vertical  hair  ; 
with  the  screw  a  set,  the  hair  is  made  to  exactly  divide  it,  by  means 
of  the  tangent  screw  f ;  if  the  adjustments  have  been  accurate,  the 
two  hairs  will  divide  the  signal  as  illustrated  in  Fig.  304.  If  they 
do  not,  they  are  made  to  do  so  by  slightly  readjusting  the  crystal. 
The  crystal  is  now  revolved  until  the  adjacent  face  reflects  the 
signal ;  and  as  this  is  in  the  same  zone,  it  will  need  but  little  ad- 
justment to  bring  the  signal  into  symmetrical  position.  Before 
taking  any  readings  both  signals  should  come  into  position  with  a 
simple  revolution  of  the  crystal  and  without  any  adjustment  of  the 
screws  connected  with  the  crystal ;  when  this  is  the  case,  the  crys- 
tal is  in  position  to  measure. 

It  is  always  best  to  take  the  first  reading,  in  the  measurement 
of  a  zone,  near  the  zero  on  the  circle,  but  on  the  359°  side  and  to 
revolve  the  crystal  so  as  to  decrease  the  number  of  degrees  in  each 
succeeding  reading;  they  will  then  all  stand  in  the  column,  so 


THE   MEASUREMENT   OF  CRYSTALS 


155 


that  any  one  below  may  be  subtracted  directly  from  any  one 
above  it. 

In  the  prism  zone  on  the  crystals  of  topaz  from  Thomas  Moun- 
tains there  are  usually  two  prisms  and  the  brachypinacoid,  yield- 
ing ten  readings  to  complete  the  360°.  Fig.  305  is  a  plan  of  this 
zone  with  the  faces  lettered  according  to  the  usual  practice.  Let 
the  first  reading  be  taken  of  the  face  m.  With  the  edge  m  A  1  ad- 
justed the  signal  from  m  is  brought  to  the  vertical  hair  as  described, 
then  a  reading  of  the  vernier  is  taken  with  a  lens,  and  the  number 
of  degrees  and  minutes  recorded  in  the  notes  opposite  the  letter 
standing  for  the  face  on  the 
sketch.  All  faces  of  the  zone 
are  measured  in  the  same 
manner,  and  as  their  inter- 
secting edges  are  parallel, 
but  little  adjustment  should 
be  necessary  as  each  edge  in 
order  is  brought  to  the  ver- 
tical hair.  It  is  usual  in  m> 
accurate  work  to  make  three 
readings  of  the  same  angle, 
using  different  parts  of  the 

graduated  circle  each  time,  to  avoid  being  influenced  by  the  same 
number ;  and  the  average  of  these  three  results  is  taken  as  being 
more  nearly  correct  than  any  one.  It  is  also  well  to  note  opposite 
each  reading  the  character  of  the  signal  reflected  by  the  face,  as  to 
whether  it  is  well  defined  and  bright,  or  irregular,  .dull,  diffused,  or 
complex  from  striations,  as  well-defined  and  bright  signals  will 
yield  results  nearer  the  truth  than  any  poor  signal  will  and  all 
readings  from  bright  sharp  signals  are  to  be  given  more  weight  in 
results.  The  following  are  the  results  of  the  readings  in  the  prism 
zone  of  such  a  crystal  of  topaz : 


FIG.  305.  —  Section  of  a  Topaz  Crystal  90°  to 
the  Prism  Zone. 


m 
1 
b 
I' 


READINGS 

=  358°  42' 

=  339°  55' 

=  ^96°  33' 

=  253°  04 


m'     =    234°  24' 


"    =    178°  42' 
=    159°  52' 


ANGLES 

bAm  =62°9';bAl  =  43°22' 
bAm'  =62°9';bAl'  =  43°29' 
1A1'  =86°  51' 

mAm'     =124°  18' 
m'Am"  =55°  42' 
l'Al"       =93°  12' 


156 


MINERALOGY 


READINGS  ANGLES 

b'Am"    =62°  8'; 
b'      =    116°3'  b'.m'"  =  62°  8'  ;  bM 

1'"     =    73°  05'  I'M'"     =  86°  47' 

m'"  =    54°  26'  m'"  A  m"  =  124°  16' 

m'     =    358°  43'  m"'Am    =  55°  43' 


43°  18' 
'"      43°  29' 


The  signals  yielded  by  the  prism  1  are  complex  from  striations 
and  therefore  the  angles  vary  considerably. 

From  the  above  measurements  the  angles  for  the  two  prisms 
are:  for  m,  m"Am'"  +  m'" Am'  =  111°  25' ^  2  =  55°  42.5';  and 
for  1,  1*1'  +  1"A1'"  =  173°  38'-:- 2  =  86°  49'.  As  the  prism 
m  has  been  selected  as  the  unit  prism,  it  will  intersect  the  macro- 
and  brachy-axes  at  unit  lengths,  or  these  lengths  will  be  in  the 
ratio  of  the  unit  on  the  b  axis  to  the  unit  on  the  a  axis.  In  order 
to  determine  this  ratio  with  sufficient  accuracy  for  use  in  the  draw- 
ing of  the  crystal,  lay  off,  Fig.  306,  ob,  equal  to  unity  on  the  ma- 
croaxis,  say  5  cm.,  and  draw  oa,  the  brachyaxis,  at  90°,  then  draw  om, 
making  the  angle  aom  =  1/2  (55°  42.5')  =  27°  51' ;  from  b  draw  ba 

perpendicular  to  om  and 
where  it  cuts  the  a  axis 
will  be  unit  length  from  o, 
as  oa  =  unity  on  &,  which 
by  measurement  =  .52  +,  or 
oa  =  52/100  of  ob. 

Having  the  units  on  the 
axes  a  and  b  the  parameters 
and  indices  of  1  may  now  be 
determined;  in  the  same 
way  from  o  draw  ol,  making 
the  angle  bol  =  1/2  (86°  49') 
=  43°  25,  and  from  b  draw 
bl  at  right  angles  to  ol,  and 
where  it  cuts  the  a  axis  at 
x  is  its  intercept  when  it  cuts  the  b  axis  at  unit  length,  ox 
is  by  measurement  just  twice  oa;  the  parameters  *  of  1  will  be, 
therefore,  2a  :  b:  ooc  and  its  indices  (120).  m  =  a:b:ooc, 

The  faces  b,  b',  since  their  normals  bisect  the  angles  of  these  two 
prisms,  is  a  pinacoid,  and  its  indices  and  parameters  may  be  writ- 
ten at  once  as  oo  a:  b:  oo  c,  (010),  the  brachypinacoid. 


FIG.  306. 


THE  MEASUREMENT   OF  CRYSTALS 


157 


Measurement  of  the  pyramid  zone.  —  The  crystal  is  now  removed 
from  the  holder  and  remounted  with  one  of  the  edges  of  the  pyra- 
mid zone  perpendicular  to  the  flat  surface  of  the  holder.  It  is 
then  clamped  in  the  goniometer  and  an  edge  adjusted  as  before; 
the  edge  first  selected  for  measurement  should  be  that  between  the 
unit  prism  and  the  first  pyramid,  as  all  the  pyramids  are  in  the  zone 
with  the  unit  prism.  The  first  reading  is  taken  from  m  and  con- 
tinued around  the  zone,  taking  the  faces  in  order  through  the  180° 
until  m"  is  reached.  The  following  are  the  results  of  measure- 
ments in  this  pyramid  zone: 


FACES 

READINGS 

ANGLES 

m      = 

185°  22' 

c  Am     = 

90°  01' 

o       = 

159°  15' 

CA0          = 

63°  54' 

u       = 

140°  56' 

CAU          = 

45°  35' 

i        = 

129°  36 

cAi        = 

34°  15' 

c       = 

95°  21' 

i"     = 

61°  07' 

CAi"        = 

34°  14' 

u"    = 

49°  44' 

CAU"     = 

45°  37' 

o"     = 

31°  26' 

CAO"     = 

63°  55' 

m"    = 

5°  23' 

cAm"    = 

89°  58' 

It  is  seen  that  the  face  c  is  90°  from  the  prism  and  is  therefore 
the  base;  its  parameters  and  indices  are  oo  &:  oo  b:  c,  (001). 

The  second  pyramid  zone,  including  the 
prism  faces  m'",  m",  may  be  measured  and 
averaged  with  the  readings  of  the  first. 
From  the  above  results  the  indices  of  the 
pyramids  present  are  determined  graphi- 
cally as  follows.  Some  one  of  the  pyramids 
must  be  chosen  as  the  unit  pyramid  or  the 
fundamental  form  in  order  to  arrive  at  the 
unit  on  the  axis  c ;  for  this  pyramid  u  has 
been  chosen.  To  determine  the  length  of 
c,  draw,  Fig.  307,  oc  the  vertical  axis  and 
om  at  right  angles  to  it,  making  om  =  om 
of  Fig.  306,  as  om  is  the  trace  of  the  zonal 
plane,  at  right  angles  to  the  prism  face. 
It  is  the  plane  in  which  all  the  poles  of  the 

pyramids  lie  and  is  therefore  the  plane  in  which  all  the  angles  have- 
been  measured;     then   in   Fig.   307,  from  'm  draw   mi,  making 


m 


FIG.  307. 


158 


MINERALOGY 


the  angle  omi  =  c  A  i  and  omu  =  c  A  u  and  omc  =  c  A  o ;  where  these 
lines,  mi,  mu,  me,  intersect  the  axis  c  will  be  their  intercepts  on  c 
when  it  is  unity  on  b.  As  u  is  the  unit  pyramid,  ou  will  be  unity 
on  c,  and  by  measurement  and  comparison  to  the  unit  on  b  =  .47+. 
The  axial  ratios  as  determined  are,  &:b:c  =  .52+:  1 :  .47+ ;  as  cal- 
culated they  are,  d :  b :  c  =  .5285  :  1 :  .4769. 

In  comparison,  the  intercept  of  the  pyramid  o  on  the  axis  c, 
oc,  is  just  twice  ou,  and  the  intercept  of  i,  oi,  is  2/3  of  ou.  The 
parameters  and  indices  of  the  three  pyramids  may  now  be  written 
as  follows  : 

u  =  a:b:6,  (111). 

0  =  a:b:2C,  (221). 

1  =  a:b:2/3c,  (223). 

There  are  usually  two  brachydomes  present,  and  these  may  be 
measured  next  by  remounting  the  crystal  with  an  edge  of  this  zone 
perpendicular  to  the  holder  and  adjusting  an  edge  in  the  goniom- 
eter as  before.  Starting  with  the  first  reading  from  one  of 
the  faces  further  from  the  base  the  results  obtained  are  as 
follows : 

FACE       READINGS 

=  179°  20' 
=  160°  42' 
=  117°  1' 
=  73°  23' 
=  55°  41' 


cy  =  62'  20 


ANGLES 

cAy  =  62°  19' 
cAf  =  43°  41' 


c  A  f '  =  43°  38' 
c  Ay' =62°  21' 
cAf  =43°  39' 


FIG.  308. 


The  parameters  and  indices  of  these  two 
forms  are  determined  as  follows  :  as  they  are 
in  the  brachypinacoidal  zone  they  will  be 
parallel  to  the  a  axis.     In  Fig.  308,  make 
ob  equal  to  unity  on  the  macroaxis  and 
draw  the  vertical  axis  at  o,  then  draw 
bf  making  the  angle  obf  =  43°  39' ; 
if  oc  is  unity  on  c,  then  of  is  twice 
oc  or  2  c  and  the  parameters  and 
indices  of  the  form  f  are  oo  a  •  b  • 
2  c,  (021). 

In  the  same  way  draw  by, 
.    making  the  angle  oby  =  62° 
20',  then  oy  is  4  c  and  the 


THE   MEASUREMENT  OF  CRYSTALS  159 

parameters  of  y  are  oo  a  :  b:4c  and  its  indices  are  (041).  The 
parameters  and  indices  of  any  other  form  occurring  on  the  crystal 
may  be  determined  in  a  similar  way,  and  with  the  data  obtained 
the  crystal  may  be  drawn,  by  consulting  the  chapter  on  the 
drawing  of  crystals. 


CHAPTER  IX 
OPTICAL  PROPERTIES   OF   CRYSTALS 

IT  has  been  shown  that  crystal  forms  are  dependent  upon  and 
are  the  result  of  a  definite  arrangement  of  the  molecules.  In  some 
cases  substances  which  differ  chemically  may  crystallize  with  al- 
most the  same  angles  and  forms.  Again,  substances  which,  upon 
chemical  analysis,  as  pyroxene  and  amphibole,  may  yield  the  same 
percentage  result,  crystallize  with  angles  which  are  different.  Such 
substances  may  be  easily  identified  when  comparatively  large 
specimens  and  well-developed  forms  are  at  hand ;  but  when  in  small 
fragments,  even  chemical  analysis  will  fail,  and  yet  each  fragment 
will  possess  the  peculiar  molecular  arrangement  in  which  the  one 
species  will  differ  from  the  other,  and  in  this  case  pyroxene  from 
amphibole. 

It  is  well  known  that  light  in  its  passage  through  any  medium  is 
modified  in  its  velocity,  direction,  and  vibrations.  These  various 
modifications  of  transmitted  light  are  the  effect,  in  part,  of  the 
molecular  arrangement,  and  these  effects  are  constant  and  char- 
acteristic. They  are  therefore  reliable  when  used  in  the  identifi- 
cation of  crystalline  compounds,  and  just  as  much  so  as  chemical 
tests,  while  in  many  cases  they  are  much  less  troublesome  in  their 
application.  According  to  the  accepted  theory,  light  is  propagated 
in  a  medium  which  heretofore  has  been  purely  imaginary,  but  at 
the  present  time  evidence  is  being  brought  forward,  and  from  sev- 
eral sources,  which  would  seem  to  prove  the  actual  existence  of  this 
imaginary  medium,  the  ether.  Light  is  propagated  and  is  the 
effect  of  very  rapid  oscillations  or  electric  polarization  of  the  ether. 
These  oscillations  are  periodical  and  transverse  to  the  ray  of  light 
or  direction  of  transmission.  They  are  exceedingly  rapid  altera- 
tions of  the  electromagnetic  conditions  of  the  ether,  which  vibrate 
back  and  forth,  or  rotate  in  a  plane  at  right  angles  to  the  direction 
of  propagation. 

That  light  is  an  electric  effect  is  substantially  proven  from  its 
analogy  to  the  electric  waves  used  in  the  transmission  of  wireless 

160 


OPTICAL  PROPERTIES  OF   CRYSTALS 


161 


telegraphy.  They  both  travel  with  the  same  velocity  of  185,400 
miles  per  second,  and  may  be  polarized,  reflected,  defracted, 
and  refracted.  The  wireless  waves  are  very  large,  while  those 
waves  which  our  eyes  are  able  to  detect  as  light  are  very 
small.  The  range  of  our  eye  as  a  detector  is  limited  to  those 
waves  which  fall  within  the  colored  spectrum ;  but  that  waves,  both 
smaller,  as  the  ultra-violet  waves,  and  larger,  as  the  infra-red  waves, 
do  exist  we  know  from  other  detectors,  and  our  eyes  are  not  able  to 
recognize  these  waves  as  light.  The  ether  pervades  all  space,  both 
the  interstellar  and  the  intermolecular,  and  penetrates  even  within 
the  atom  itself,  filling  the  space  between  the  electrons  which  com- 
pose it.  The  interatomic  space  is  probably  as  accessible  to  the 
ether  as  the  space  within  a  stack  of  bird  cages  is  to  the  air,  and  yet 
light  travels  faster  in  a  vacuum  than  in  space  filled  by  a  gas  or  a 
transparent  solid.  It  is  through  this  modification  of  the  velocity 
and  vibration  of  the  light  wave,  as  it  passes  through  a  substance, 
that  the  optical  properties  of  any  particular  crystal  become  appar- 
ent. Light  may  be  considered  as  transmitted  through  a  given 
medium  by  means  of  waves  set  up  in  the  ether.  The  periodic 
changes  which  constitute  these  waves  take  place  at  right  angles 
to  the  line  of  propagation,  and  in  this  respect  they  are  known  as 
transverse  waves  vibrating  back  and  forth  in  all  planes  across  the 
line  of  direction  of  transmission.  The  ray  is  a  term  conveniently 
used  to  denote  the  direction  along  which  the  wave  advances. 

As  an  illustration  of  the  terminology  of  wave  and  wave  motion  it 
is  best  to  select,  as  an  example,  one  in  which  there  is  possibly  no 
imagination  required,  as  is  the  case  of  the  wave  motion  on  the  sur- 
face of  water. 


FIG.  309. 

In  Fig.  309,  the  position  of  any  particle  of  water,  as  a,  on  the  sur- 
face will  determine  the  wave  surface  at  that  point ;  as  a  falls  to- 
ward the  arrow,  the  water  surface  falls  and  the  wave  passes  on  until 
a  reaches  a  maximum  depression  x,  when  the  valley  of  the  wave  x 
is  formed ;  then  a  rises  until  it  reaches  a  maximum  position  above 


162  MINERALOGY 

the  arrow  at  y,  when  at  that  instant  another  crest  is  passing  the 
point  y.  Each  time  that  a  completes  the  path  between  x  and  y, 
and  returns  to  its  original  position,  moving  in  the  same  direction, 
an  entire  crest  and  trough  have  passed,  or  one  wave  length,  denoted 
by  A.  The  wave  length  is  the  distance  oo,  measured  between 
the  paths  of  the  two  particles  aa,  occupying  the  same  position  in 
regard  to  the  arrow  and  traveling  in  the  same  direction;  such 
particles  are  said  to  be  of  like  phase.  The  period  is  the  time  that 
is  taken  by  any  particle  to  complete  the  swing  back  and  forth  and 
to  return  to  its  original  position  and  condition.  The  amplitude 
is  the  distance  oy  =  ox  from  the  median  line  to  the  highest  point 
in  its  path.  It  is  also  to  be  noted  that  the  particle  a  moving 
back  and  forth  along  the  path  yx  is  not  carried  forward  along 
the  arrow,  but  like  a  block  of  wood  rises  and  falls  on  the  waves. 
A  wave  front  is  formed  by  the  particles  or  points  which  are  in- 
fluenced simultaneously  as  each  wave  passes  them;  they  are  all 
in  the  same  phase  and  form  a  surface  or  line  at  right  angles  to 
the  direction  of  transmission,  at  any  particular  point.  When  the 
wave  surface  is  a  curved  surface,  the  plane  tangent  at  any  particular 
point  will  be  at  right  angles  to  the  ray  or  the  direction  of  transmis- 
sion at  that  point.  All  the  above  terms  are  equally  applicable  to 
the  light  wave,  but  just  what  the  change  of  conditions  along  the 
path  xy  is,  is  still  somewhat  in  doubt. 

The  intensity  of  light  is  proportional  to  the  square  of  the  amplitude, 
and  the  color  will  depend  upon  the  length  or  more  correctly  on  the 
periodicity  or  number  of  vibrations  per  second  of  the  wave.  Deep 
violet  light  at  one  end  of  the  spectrum  has  a  wave  length  of  .000396 
mm.,  while  dark  red  at  the  other  end  has  a  wave  length  of  .000795 
mm.,  or  about  double  that  of  violet,  and  the  yellow  sodium  light 
is  about  halfway  between  these  two,  or  .00059  mm.  Vibrations 
larger  or  smaller  than  these  the  eye  is  unable  to  detect  as  light  ; 
but  that  heat  rays  do  exist  above  and  actinic  rays  below  is  easily 
demonstrated  by  detectors  other  than  the  eye. 

Light  waves  of  all  lengths  travel  in  a  vacuum  with  the  same 
velocity,  but  they  differ  in  their  period,  since  short  waves,  as  violet, 
must  vibrate  twice  as  quickly  as  the  red  waves,  which  are  double 
their  length.  Upon  entering  a  transparent  medium  the  velocity 
of  light  of  all  wave  lengths  is  modified ;  the  extent  of  this  modifica- 
tion will  depend  both  upon  the  medium  through  which  the  light  is 
traveling  and  upon  the  wave  length  or  color  of  the  light ;  so  that 
lights  of  different  wave  lengths  will  vary  in  their  velocities  upon 


OPTICAL  PROPERTIES   OF  CRYSTALS  163 

passing  from  one  medium  into  another.  Substances  differ  greatly 
in  the  way  they  transmit  light ;  one  class,  known  as  isotropic  sub- 
stances, transmits  light  equally  or  with  the  same  velocity  in  all  direc- 
tions. If  a  point  within  such  a  transparent  isotropic  substance  be 
imagined  as  the  source  of  light,  the  light  waves  will  travel  in  all 
directions  from  this  point  with  the  same  velocity,  and  if  it  were 
possible  to  stop  the  wave  at  any  instant,  say  after  an  inch  had  been 
traversed  from  the  point  of  emission,  the  extreme  wave  front  would 
be  a  sphere  of  an  inch  radius.  Every  point  of  the  surface  would  be 
one  inch  from  the  source  of  light,  and  each  ray  would  have  traveled 
exactly  the  same  distance,  whatever  the  direction. 

Isotropic  substances  include  all  gases,  most  liquids,  amorphous 
solids,  as  glass,  and  crystals  of  the  isometric  system.  Solids,  how- 
ever, when  under  stress  or  strain  and  which  under  normal  conditions 
are  isotropic  may  show  anomalies  and  apparently  belong  to  the 
second  class,  or  anisotropic  substances ;  in  which  the  wave  front  is 
not  a  sphere  and  the  velocity  of  the  transmitted  ray  will  vary  with 
the  direction. 

In  anisotropic  substances  the  velocity  of  light  will  vary  with  the 
direction  in  which  the  light  is  traveling,  but  in  parallel  directions 
within  the  same  medium  the  velocity  will  be  the  same. 

The  anisotropic  class  includes 'the  tetragonal,  hexagonal,  ortho- 
rhombic,  monoclinic,  and  triclinic  systems  of  crystals,  and  also  iso- 
metric and  isomorphous  solids  when  under  stress  or  strain,  as  well 
as  those  liquid  crystals  which  show  double  refraction. 

The  wave  front  in  anisotropic  substances  is  not  a  sphere,  but 
its  form  will  depend  upon  the  substance. 

When  light  strikes  the  surface  of  a  transparent  substance,  as 
glass,  it  is  modified  in  several  ways:  (1)  some  is  reflected;  (2) 
some  is  refracted ;  (3)  some  is  polarized ;  (4)  some  is  absorbed  or 
lost  as  light,  as  it  is  transformed  to  energy  of  another  kind.  All 
four  effects  will  depend  upon  and  will  vary  with  the  nature  of  the 
surface,  the  angle  of  inclination  of  the  ray,  and  the  substance. 

Reflection.  —  If  from  the  head  of  the  arrow,  in  Fig.  310,  a  ray  of 
light  is  traveling  in  the  direction  ao,  it  will  strike  the  surface  ss' 
at  the  point  o.  The  reflected  portion  will  travel  along  and  in  the 
direction  of  oa'  with  a  velocity  which  is  unchanged.  The  two  direc- 
tions ao  and  a'o  will  be  symmetrical  in  regard  to  the  normal  no 
at  the  point  of  incidence,  and  will  lie  in  the  same  plane.  The  angle 
aon  is  the  angle  of  incidence  =  i  =  the  angle  noa',  the  angle  of  re- 
flection. The  ray  from  b,  traveling  in  the  same  direction  and  with 


164 


MINERALOGY 


an  equal  velocity  as  that  from  a,  will  strike  the  surface  at  point  P 
and  will  be  reflected  in  the  direction  of  Pb'.  When  the  wave  front 
is  at  a',  the  ray  from  b  will  be  at  the  point  V,  for  the  path  apa'  is 
equal  to  the  path  bPb',  and  the  arrow  will  appear  as  at  a'b',  but  in 
a  reverse  position.  If  the  surface  of  reflection  is  a  truly  plane  sur- 
face, the  image  of  the  arrow  at  a'b'  will  be  of  the  same  size  and 

shape. 

Refraction.  —  When  the  ray,  in  Fig.  310,  from  a  strikes  the 
surface  at  o,  a  portion  of  the  light,  depending  upon  the  angle  of 
incidence  and  the  character  of  the  substance,  is  transmitted  or  pene- 
trates the  second  transparent  medium.  The  velocity  and  direction 
of  the  entering  light  will  be  changed ;  the  amount  of  charge  will 


depend  upon  the  medium  and  the  wave  length  of  the  entering 
light.  Suppose  the  upper  medium  to  be  air  and  the  lower  medium 
to  be  water ;  light  travels  approximately  three  quarters  as  fast  in 
water  as  in  air.  The  ray  ao,  in  Fig.  310,  will  be  on  the  point  of 
entering  the  water  at  o,  when  the  ray  from  b  is  at  the  point  b" ; 
while  b  is  traveling  the  distance  b"P,  a  will  have  traveled  three 
quarters  of  this  distance  in  water.  To  find  the  wave  front  of  the 
refracted  rays :  draw  bP  parallel  to  ao,  then  with  o  as  a  center  and 
radius  equal  to  three  quarters  of  b"P  draw  the  circle  dT ;  from  P 
draw  PT  tangent  to  the  circle  dT  and  PT  will  be  the  direction  of  the 
wave  front  of  the  refracted  rays.  When  b  reaches  bi,  a  will  be  at  ai 
and  the  arrow  will  appear  at  aibi,  enlarged,  but  not  reversed,  as  is 
the  case  with  the  reflected  rays.  A  man  standing  on  the  bank  will 


OPTICAL  PROPERTIES  OF  CRYSTALS  165 

appear,  to  a  fish  in  the  water,  one  and  one  half  times  taller  than 
he  really  is ;  while  the  fish  will  appear  smaller,  as  the  rays  follow 
the  same  paths  in  the  reverse  direction.  The  angle  aio^  =  r  = 
the  angle  of  refraction. 

In  passing  from  a  rare  medium  to  one  which  is  more  dense,  the 
ray  is  bent  toward  a  perpendicul  ar ;  and  in  passing  from  a  dense 
medium  to  one  which  is  less  dense,  the  ray  is  bent  from  the  perpen- 
dicular. 

The  angle  of  refraction  will  vary  with  the  angle  of  incidence, 

but  there  is  always  a  relation,  as  the  value  of  — —  is  a  constant. 

smr 

In  Fig.  310,  in  the  two  right-angled  triangles  ob"P  and  oTP,  the 
side  oP,  or  hypothenuse,  is  common  to  both  triangles.  The  angle 
b"oP  =  noa  =  i,  the  angle  of  incidence,  and  TPo  =  Toni  =  r  = 
the  angle  of  refraction.  Pb"  =  v,  the  velocity  in  air,  and,  oT  =  vi, 
the  velocity  in  water  ;  then 

Pb"  =  oP  X  sin  b"oP  =  oP  X  sin  noa 
=  oP  sin  i,  or  v  =  oP  X  sin  i  ; 

.oT  =  oP  X  sin  TPo  =  oP  X  sin  Ton 
=  oP  X  sin  r,  or  v'  =  oP  X  sin  r, 

v       sin  i 
or 


As  the  velocity  of  light  of  the  same  wave  length  is,  in  water,  always 

y 

the  same,  no  matter  what  the  direction,  and  likewise  for  air,  — , 

,   sin  i 

and  -     -  are  constants, 
sin  r 

The  ratio  - —  =  n,  the  index  of  refraction  of  the  water.     When 
sinr 

air  is  taken  as  the  unit  of  comparison,  and  the  velocity  of  light  in 
air  is  one,  n,  the  index  of  refraction  of  water,  is  1.333. 

An  isotropic  substance  has  a  constant  index  of  refraction,  what- 
ever the  direction  of  the  path  of  the  transmitted  ray  may  be,  and 
for  water  the  index  is  1.333.  The  indices  of  refraction  of  a  few  other 
liquids  and  solids  at  room  temperature  and  for  yellow  light  are  as 
follows : 

Ether 1.356 

Turpentine 1.472 

Benzene  .     .     1-502 


166  MINERALOGY 

Oil  of  cedar     .     .     .     .     .     ...........  1-520 

Oil  of  cloves    .....     .......     •    V    •     •  L54° 

Canada  balsam    ......     .     ;   >       .     .     .     .     *     .  1.548 

Carbon  bisulphide    .     .     .  ...     .     ..-..".*.    «  -.-.  1.627 

Methylene  iodide      .     .     .    '-..'.:..    .     .     :    .     .     .     .     ,    .  1.742 

Fluorite      ....     .     .    .    V  v.    ^    I  .  :    .     .     ..;..;>  1.423 

Potash  alum    .....    •;    V;:.  :.     .>    .     .     ..     .     .  -.  1-456 

Crown  glass     .     .     .     .     .     •     •     •     •     •  ..  >     ••-•.•     *     •     •  1-515 

Rocksalt    .     ....    •••..'•'.•    .«.'•'.'.*.-    .     -     •     •     •     •     •  1-544 

Garnet.     .     ...     .     .     .     .    ..  ^  -  .     ...     .'.     .  1.807 

Spinel  (Chrome)  .     .     .     »     .     .     -     .     ......     .  2.096 

Diamond    .     .     .     .     .     .     .     .     .     .     .  •..';>""  •     •     •     .     .  2.467 

Proustite     ....     ......     ....'.,...     .  3.08 

The  index  of  refraction  of  any  substance  is  different  for  light  of 
different  wave  lengths,  and  also  varies  slightly  with  the  temperature; 
as  regards  the  wave  length,  it  is  inversely  as  X2,  or  for  light  of  long 
wave  length,  as  red,  n  the  index  of  refraction  is  less  and  the  angle 
of  refraction  r  would  be  greater  than  in  case  of  violet  light,  with  a 
short  wave,  or  the  violet  ray  would  be  bent  more  in  entering  the 
water,  Fig.  310,  than  the  red  ray.  The  violet  ray  would  lie  nearer 
the  normal  than  the  red  ray.  This  division  of  white  light  into 
colors  is  known  as  dispersion. 

The  relative  velocity  of  light  in  any  substance  is  the  reciprocal  of 

the  index  of  refraction,  as,  n  =  ^7  ;  where  v,  the  velocity  in  air,  is 


Violet  light,  which  has  a  larger  index  of  refraction  in  water,  will 
travel  more  slowly  than  red. 

Critical  angle.  —  In  the  equation,  n  =  -.  —  •  ,  sin  i  may  have  any 

value  between  zero  and  one  ;  when  all  values  are  considered,  there 
are  two  special  cases,  those  of  the  limiting  values,  o  and  i.  In  the 
case  when  sin  i  =  i,  or  the  angle  of  incidence  is  90°,  Fig.  311,  then 

sin  i  i 

n  =  sin~r  °r  S*n  r  =  ~  '  substituting  the  value  of  n  in  case  of  water; 

sin  r  =   -     —  ,  or  the  angle  r  is  48°  36'. 
^  -333 

When  light  is  traveling  along  the  surface  SS',  that  entering  at  any 
point  o  will  take  the  direction  oai,  in  which  the  angle  aioni  is  48°  36'. 


OPTICAL   PROPERTIES   OF   CRYSTALS 


167 


This  is  the  maximum  value  of  r  for  water  and  air,  and  is  termed  the 
critical  angle,  or  the  angle  of  total  reflection;  for  if  a  ray,  as  oan, 
should  reach  the  surface  at  the  point  o,  in  which  the  angle  ano^  is 
greater  than  48°  36',  the  critical  angle  for  water  and  air,  there 
is  no  possible  value  for  sin  i  and  no  light  could  pass  out  of  the  water 
into  the  air,  but  all  is 
reflected  back  along 
the  direction  of  oam. 
Viewed  from  am  un- 
der suitable  condi- 
tions, there  will  be  a 
light  field  outside  of 
the  critical  angle  of  48° 
36 '  and  a  dark  field  in- 
side of  this  angle,  as  in- 
dicated by  the  circle 
in  the  figure.  The  di- 
viding line  between 
these  two  fields  will 
measure  the  critical 
angle. 

By  measuring  the  critical  angle  of  any  substance,  its  index  of 

refraction  is  easily  determined,  as  n  = j-^ — — ; — - . 

sin  (of  the  critical  angle) 

The  determination  of  the  index  of  refraction  by  the  total  refrac- 
tometer  is  based  upon  this  principle. 

The  minimum  value  of  sin  i  is  o ;  then  n  =  —   -  becomes  zero, 

smr 

or  there  is  no  refraction,  and  light  passing  in  the  direction  of  the 
normal  to  the  surface  is  not  refracted. 

All  isotropic  substances  have  but  one  index  of  refraction,  for  the 
reason  that  light  is  transmitted  with  the  same  velocity  in  all  direc- 
tions ;  the  wave  front  is  a  sphere.  In  anisotropic  substances 
there  are  two  and  even  three  indices  of  refraction,  and  the  velocity 
of  light  varies  with  the  path  followed  through  the  crystal.  The 
wave  front  is  no  longer  a  sphere,  as  in  isotropic  substances,  but  its 
shape  and  curvature  will  depend  upon  the  substance. 

The  wave  fronts  in  anisotropic  substances  are  surfaces  all  of 
which  agree  in  being  symmetrical  to  three  planes  of  symmetry  at 
right  angles,  as  the  axial  planes  of  the  orthorhombic  system.  These 
three  planes  intersect  each  other  in  three  straight  lines  at  right 


168 


MINERALOGY 


angles  to  each  other.  Each  of  these  lines  represents  a  direction 
parallel  to  which  there  is  a  maximum  or  minimum  index  of  refrac- 
tion or  velocity,  for  transmitted  light.  The  relative  length  of  these 
axes  will  also  represent  the  relative  speed  of  transmission,  remem- 
bering that  the  velocity  is  the  reciprocal  of  the  index  of  refraction. 
Double  refraction.  —  A  ray  of  light  upon  entering  an  anisotropic 
crystal  or  substance,  in  general,  travels  with  two  different  velocities 
within  the  crystal,  or  it  is  broken  into  two  rays,  each  of  which  pos- 
sesses its  own  index  of  refraction.  In  other  words,  one  is  a  slow  ray, 
the  other  is  a  fast  ray.  The  difference  between  the.  values  of  the 
indices  of  refraction  of  the  two  rays  is  a  measure  of  the  birefrin- 
gency  of  the  substance  in  that  direction,  for  the  birefringency  or 
double  refraction  varies  with  the  direction  of  transmission. 

For  calcite  one  index  of  refraction  =  1.658  and  the  other  = 
1.486 ;  as  these  are  the  maximum  values,  or  represent  the  maximum 
difference  between  the  two  indices,  their  difference,  or  .172,  would 
be  the  double  refraction  of  calcite ;  which  is  very  high  or  strong. 
In  most  minerals  it  is  represented  by  a  small  figure  in  the  second 
decimal  place,  or  even  in  the  third,  as  that  for  quartz  is  .009  and 
that  for  Orthoclase  is  .007.  Since  calcite  is  an  example  of  birefrin- 
gency in  an  exaggerated  degree,  and  it  is  transparent  and  easily 
obtained,  it  is  an  extremely  good  mineral  with  which  to  demon- 
strate this  peculiar  property  of  crystalline  substances. 

The  usual  cleavage  piece  of  calcite  is  a  rhomb  in  shape.  If  such 
a  cleavage  piece  of  calcite  be  placed  over  a  pinhole  in  an  opaque 
paper  and  then  held  up  to  the  light,  two  pinholes  will  appear, 

Fig.  312 ;  one  will  be  seen 
above  and  nearer  than  the 
other;  this  is  due  to  the 
difference  of  the  velocities 
of  the  two  rays.  The  dis- 
tance between  the  two 
images  will  depend  upon 
the  thickness  of  the  cal- 
cite. When  the  rhomb  is 
revolved,  one  image  e,  Fig. 
313,  will  appear  to  revolve 
around  the  other,  or  that  ray  is  refracted  to  a  greater  extent 
than  is  the  other  ray.  In  fact,  when  the  ray  of  light  enters  the 
calcite  at  right  angles  to  the  surface  and  the  eye  is  in  the  direc- 
tion of  this  ray,  when  the  rhomb  is  revolved  one  image  is  sta- 


FIG.  312. 


OPTICAL   PROPERTIES   OF   CRYSTALS  169 

tionary,  and  this  is  what  would  be  expected  if  the  crystal  were  an 
isotropic  substance,  as  there  is  no  refraction  when  the  ray  falls 
normal  to  the  surfaces.  This  ray  follows  the  ordinary  law  and  is 
therefore  termed  the  ordinary  ray.  Its  index  of  refraction  is 
written  co.  In  the  case  of  calcite  the  index  measured  with  monochro- 
matic sodium  light  (yellow)  is  written,  coy  =  1.658.  The  second 
ray  follows  another  law  which  is  entirely  different  from  that  of  the 
ordinary  ray,  and  its  velocity  and 
therefore  its  index  of  refraction 
(written  c)  will  vary  with  the  di- 
rection ;  this  ray  is  known  as  the 
extraordinary  ray.  The  index  of 
refraction  taken  at  its  maximum 
difference  from  that  of  the  or- 
dinary ray  and  for  sodium  light 
is  written  €y  =  1.486. 

When  the  index  of  refraction  of 
the  extraordinary  ray  is  smaller 
than  that  of  the  ordinary  ray,  or 
the  extraordinary  ray  is  the  fast 

ray,  o»€,  the  crystal  is  said  to  be  optically  negative,  written  (  — ) 
as  in  calcite. 

In  quartz,  where  <oy  =  1.544  and  €y  =  1.553,  €>co,  it  is  optically 
(-f),  and  the  extraordinary  ray  is  the  slow  ray. 

All  crystals  of  the  tetragonal  and  hexagonal  systems  have  two 
indices  of  refraction ;  one,  that  for  the  ordinary  ray,  is  constant  for 
all  directions  in  the  crystal,  as  in  isotropic  substances ;  the  other, 
that  for  the  extraordinary  ray,  varies  with  the  direction  in  the  crys- 
tal, from  the  value  of  the  index  of  refraction  for  the  ordinary  ray  as 
one  limiting  value,  to  a  maximum  or  minimum  as  the  other  limit, 
according  to  the  (  — )  or  (+)  character  of  the  crystal. 

Wave  surfaces  in  hexagonal  and  tetragonal  crystals.  —  In  Fig. 
314,  if  any  point  within  a  hexagonal  or  tetragonal  crystal,  as  o, 
be  illuminated,  and  act  as  the  source  of  light  for  the  smallest  frac- 
tion of  a  second,  that  portion  illuminated  will  be  bounded  by  the 
wave  front.  Its  distance  from  the  source  of  light  o,  in  any  direc- 
tion, will  depend  upon  the  velocity  with  which  the  ray  travels 
through  the  crystal  in  any  given  direction.  At  the  end  of  any  short 
period  of  illumination  the  ordinary  ray  to  has  traveled  the  distance 
ox;  as  the  ray  travels  with  the  same  velocity  in  any  and  all  direc- 
tions, the  circle  with  o  as  a  center  and  a  radius  ox  will  represent  the 


170 


MINERALOGY 


-c 

FIG.  314.  —  Negative. 


section  of  the  spherical  wave  front,  in  the  plane  of  the  paper.  The 
extraordinary  ray  travels  with  a  velocity  which  varies  with  the 
direction;  the  minimum  value  of  which,  let  it  be  supposed,  is  in 

the  direction  of  the  c 
axis  and  is  equal  in 
this  direction  to  that 
of  the  ordinary  ray  ox. 
They  will  both  arrive 
at  p  and  p'  on  the  c 
axis  simultaneously,  or 
for  light  traveling 
through  the  crystal 
parallel  to  the  c  axis 
there  is  only  one  in- 
dex of  refraction. 
Crystals  of  the  tetrag- 
onal and  hexagonal 
systems  are  isotropic 
in  the  direction  of  their  c  axis  only;  such  crystals  are  optically 
uniaxial.  The  direction  in  which  the  extraordinary  ray  travels 
with  a  maximum  velocity  is  at  right  angles  to  the  c  axis  or  paral- 
lel to  it  according  to  the 
optical  sign.  Let  it  be  sup- 
posed the  crystal  is  calcite 
(-),  the  maximum  velocity 
will  therefore  be  in  the  plane 
of  the  lateral  axes,  or  the  basal 
plane;  this  is  true  for  any 
direction  in  this  plane  from  o. 
Let  this  maximum  value  be 
represented  by  oa. 

The  cross  section  of  the 
wave  front  parallel  to  the 
basal  plane  is  a  circle.  Inter- 
mediate values  between  po 
and  oa,  as  in  the  direction  of 
od  or  of,  when  plotted  on  the  plane  of  the  paper,  form  an  ellipse, 
which  is  similar  for  all  plane  sections  containing  the  c  axis 
Ihe  whQle  wave  front  of  the  extraordinary  ray  is  an  ellipsoid  of 
revolution,  the  axis  of  which  is  the  c  axis,  or  is  parallel  to  the  optic 
Ihpsoid  of  revolution  or  wave  front  of  the  extraordinary 


FIG.  315.  — Positive. 


OPTICAL   PROPERTIES   OF   CRYSTALS 


171 


ray  is  tangent  to  the  sphere  or  wave  front  of  the  ordinary  ray  at  two 
points  p  and  p',  where  the  crystallographical  axis  c  cuts  them.  The 
sphere  in  this  case,  that  of  calcite,  an  optically  negative  crystal, 
is  entirely  inclosed  by  the  oblate  ellipsoid.  In  the  case  of  quartz, 
an  optically  positive  crystal,  the  wave  front  of  the  extraordinary 
ray  is  represented  by  a  prolate  ellipsoid  of  revolution,  which  is  in- 
closed within  the  circle  or  sphere,  as  represented  in  Fig.  315. 

Optically  biaxial  crystals.  —  The  wave  front  in  crystals  of  the 
orthorhombic,  monoclinic,  and  triclinic  systems  is  not  an  ellipsoid 
of  revolution,  but  a  combination  of  two  wave  surfaces,  one  within 
the  other,  continuous  at  four  depressions,  Fig.  316,  or  symmetrical 
points-,  the  position  of  which  depends  upon  the  relative  values  of 
the  three  indices  of  refraction.  This  fourth  dimensional  surface  is, 
however,  symmetrical  to  three  planes  of  symmetry  intersecting 
each  other  at  right  angles,  in  three  straight  lines,  analogous  to  the 
axes  and  planes  of  the  orthorhombic  system.  The  three  lines  of 
intersection  always  represent  directions  within  the  crystal  parallel 
to  which  there  is  a  maximum  or  minimum  velocity  of  light,  as  all 
such  crystals  have  three  indices  of  refraction.  They  are  repre- 


sented by  a,  p,  and  7.     The  mean  index  of  refraction  is 

and  y  —  a  will  always  represent  the  greatest  double  refraction,  as 
"Y  is  the  greatest  and  a  the  smallest  index. 


FIG.  316. 


FIG.  317. 


Sections  of  the  wave  front  in  the  three  planes  of  symmetry  are 
represented  in  Figs.  316,  317,  318.  It  will  be  noted  that  in  each  case 
there  is  a  circle  and  an  ellipse,  or  for  each  of  these  sections  there  are 


172 


MINERALOGY 


two  rays,  one  of  which  has  a  constant  index  of  refraction  within  the 
plane  and  is  therefore  an  ordinary  ray ;  the  other,  represented  by  the 
ellipse,  is  variable  in  its  velocity,  and  is  an  extraordinary  ray.  In  Fig. 
317  the  radius  of  the  large  circle  represents  the  maximum  velocity 

I,  and  the  inner  ellipse  the  variable  ray  with  its  two  limiting  values 
a 

^  and  - ;  Fig.  318  represents  the  conditions  in  the  plane  of  sym- 
metry at  right  angles  to  a,  in  which  the  inner  circle  is  the  minimum 
velocity  -  and  in  which  *y  is  the  ordinary  ray.  The  diameters  of  the 

ellipse  represent  the  variable  rays ;  in  Fig.  316  the  conditions -in  the 
third  plane  of  symmetry  are  represented,  or  the  intermediate  value 

I  represents  the  velocity  of  the  constant  ray,  the  circle,  and  the  el- 
lipse represents  the  maximum  velocity  -  and  the  minimum  velocity 
-  in  the  crystal ;  in  this  section  the  ellipse  and  circle  cut  each  other 

at  the  four  points  P,  P',  P",  P'",  which  represent  the  de- 
pressions in  the  wave  surface  and  are  the  four  points  at  which 

the  inner  and  outer  surfaces  are  con- 
tinuous. 

The  two  directions  PP"  and  p'p'" 
are  the  two  optic  axes.  Parallel  to 
these  two  directions  there  is  no  double 
refraction,  and  the  section  of  the  wave 
surface  perpendicular  to  the  optic  axis 
in  each  case  is  a  circle,  as  was  also  the 
condition  in  uniaxial  crystals.  Strictly 
these  two  lines  PP"  and  P'P'"  are  the 
secondary  optic  axes,  but  the  true  optic 
axes  are  so  near  them  as  not  to  be 
separable  from  them  in  practice.  In 
any  section  of  the  wave  front  other  than 
in  the  three  planes  of  symmetry  and 
to  an  optic  axis,  light  will  travel  with  two  velocities  or 
rays,  neither  of  which  will  be  an  ordinary  ray,  but  both  will  vary 
m  speed  with  the  direction  of  transmission. 

t  is  seen,  Fig.  316,  that  the  two  optic  axes,  the  direction  of  the 
greatest  velocity,  and  at  right  angles  to  it  the  direction  of  the  least 


FIG.  318. 


OPTICAL  PROPERTIES   OF   CRYSTALS 


173 


velocity,  are  all  contained  in  the  one  plane ;  this  plane  is  known  as  the 
plane  of  the  optic  axes  or  axial  plane,  abbreviated  to  (Ax.  PL).  The 
line  bisecting  the  smaller  angle  between  the  optic  axes  is  the  acute 
bisectrix  (BxJ  or  first  median  line ;  the  line  bisecting  the  larger 
angle  is  the  obtuse  bisectrix  (Bx0).  The  internal  angle  between  the 
acute  bisectrix  and  the  optic  axis  is  represented  by  V  and  2  V  =  pop;, 
the  angle  between  the  optic  axes,  always  less  than  90°  and  measured 
within  the  crystal.  When  measured  in  air  the  angle  is  designated 
2  E ;  2  E  owing  to  refraction  is  always  larger  than  2  V  and  is  often 
180°  from  total  reflection.  The  value  of  2  V  varies  with  different 
substances  and  will  depend  upon  the  indices  of  refraction.  When 
the  three  indices  are  known,  the  angle  2  V  may  be  calculated  from 
the  formula: 


tan  V  = 


As  the  indices  of  refraction  vary  with  the  wave  length  or  color  of 
light,  it  will  be  seen  that  2  V  for  violet  light  will  differ  from  the  value 
of  2  V  for  red  light ;  their  dif- 
ference will  measure  the  disper- 
sion of  the  optic  axes. 

The  optical  sign  of  biaxial 
crystals.  —  The  intermediate  in- 
dex of  refraction  p  in  different 
biaxial  crystals  may  vary  from 
a  as  a  minimum  to  -y  as  a  maxi- 
mum limit. 

In  Fig.  319,  as  the  value  of  p 
decreases  the  circle  ycy'  will  ap- 
proach the  circle  yxy'  and  the 
radius  oc  will  approach  ox.  (The 
figure  is  drawn  with  the  three 
axes  ox,  oy,  and  oz  proportional 
to  the  indices  of  refraction.) 
The  four  points  marked  c  will 
draw  nearer  to  x,  while  the  optic  axes,  perpendicular  to  these  circular 
cross  sections,  will  also  draw  nearer  each  other,  constantly  decreasing 
the  angle  2  V,  pop' ;  when  p  =  a,  c  reaches  x  and  p,  p'  reaches  Z  ; 
in  which  case  the  angle  between  the  optic  axes  is  zero  and  the  cross 


FIG.  319. 


174 


MINERALOGY 


section  is  a  circle,  and  the  ellipsoid  is  one  of  revolution,  with  z  as 
the  axis  of  revolution,  analogous  to  the  prolate  ellipsoid  in  quartz. 
In  such  cases,  where  z  is  the  acute  bisectrix  and  the  value  of  p  is 
nearer  to  a  than  to  y,  the  crystal  is  said  to  be  optically  (+).  On  the 
other  hand,  when  the  point  c  moves  up  toward  z,  the  value  of  p  will 
increase,  and  the  angle  2  V  will  increase  constantly  until  it  is  greater 
than  90°,  when  the  line  oz  will  be  the  obtuse  bisectrix  and  ox  the 
acute  bisectrix ;  when  c  reaches  z,  the  ellipse  will  be  an  oblate  ellip- 
soid of  revolution,  analogous  to  that  of  calcite,  and  the  crystal  is 
said  to  be  optically  negative  (  — ). 

The  three  axes  of  the  ellipsoid  are  usually  written  X  =  a  =  a, 

Y  =  b  =  P,  Z  =  c  =  "Y. 

The  relations  of  the  axes  of  the  ellipsoid  to  the  crystallographical 
axes  in  the  orthorhombic,  monoclinic,  and  triclinic  systems  vary 

with  the  possible  conditions,  de- 
pending upon  the  symmetry  of 
the  system  and  the  relation  of  the 
axial  plane  of  the  ellipsoid  to  the 
planes  of  symmetry  in  the  system. 
In  the  orthorhombic  system, 
where  the  three  crystallographical 
axes  are  at  right  angles  to  each 
other,  these  correspond  in  direc- 
tion to  the  axes  of  the  ellipsoid, 
and  the  position  of  the  planes  of 
symmetry  of  the  ellipsoid  is  fixed 
parallel  to  the  planes  of  symmetry 
of  the  system.  The  axes  X,  Y,  or 
Z  may  correspond  with  any  one  of 
the  crystallographic  axes,  but  for 
any  one  species  this  relation  is  definite,  as  is  shown  in  Fig.  320, 
a  diagram  of  the  optical  conditions  in  the  mineral  aragonite,  where 
the  plane  of  the  optic  axis  is  parallel  to  the  macropinacoid  (Ax.  PI. 
=  100).  The  acute  bisectrix  is  X  =  6,  the  crystal  is  therefore  (  — ) ; 
b  =  Z,  a  =  Y;  2V=  18°  11'. 

In  the  monoclinic  system,  the  plane  of  symmetry  of  the  system  is 
parallel  with  one  of  the  planes  of  the  ellipsoid  and  the  orthoaxis  b 
is  parallel  to  one  of  the  axes  of  the  ellipsoid,  this  axis  is  therefore 
fixed ;  the  other  two  must  lie  in  the  plane  of  symmetry  of  the  sys- 
tem; but  their  relation  to  the  a  or  c  crystallographical  axes  will 
vary  with  the  mineral  species,  and  their  relation  is  characteristic 


FIG.   320.  —  Diagram   of    the    Optical 
Properties  of  Aragonite. 


OPTICAL  PROPERTIES   OF   CRYSTALS 


175 


of  the  species.  The  axial  plane  may  hold  one  of  two  positions : 
(1)  parallel  to  the  plane  of  symmetry  of  the  system ;  and  (2)  at 
right  angles  to  it. 

Figure  321  represents  the 
optical  conditions  in  the 
mineral  wollastonite.  The 
axial  plane  is  parallel  to  010 
(Ax.  PI.  =  010),  with  X  as 
the  acute  bisectrix  (Bxa  =  X) , 
optically  (  — ).  The  angle 
between  the  acute  bisectrix 
and  the  axis  c  is  32°  12'  in 
the  acute  angle  p,  or  ex- 
pressed (BxaAc  =  32°  12'  be- 
hind) ;  2  V  =  40°. 

In  the  triclinic  system, 
where,  at  most,  there  is  only 
a  center  of  symmetry,  there  is  no  relation  between  the  optical 
ellipsoid  and  the  crystallographical  axes,  but  usually  the  plane  of 
the  optic  axes  is  fixed  in  any  given  mineral  species.  In  the 
description  of  the  optical  properties  of  the  triclinic  minerals  the 
plane  of  the  optic  axes  is  located  by  measuring  the  angle  between 
its  trace  and  some  convenient  edge,  or  by  any  convenient  method. 
In  the  case  of  axinite,  the  acute  bisectrix  is  normal  to  111.  The 
trace  of  the  plane  of  the  optic  axes  on  111  makes  an  angle  of  40° 
with  the  edge  111/110,  and  24°  40'  with  the  edge  Ill/Ill. 

POLARIZED  LIGHT 

In  ordinary  light  the  vibrations  are  not  restricted  to  any  one 
plane,  as  the  plane  of  the  paper,  in  Fig.  322,  but  take  place  in  all 

possible  planes  inter- 
secting in  the  ray  as 
an  axis,  thus  the  vibra- 
tions of  the  ordinary 
beam  of  light  are  very 
complex.  When  such 
a  complex  ray  strikes 
the  polished  surface  of 
a  transparent  sub- 
stance, a  .portion  of 
both  the  reflected  and 


176 


MINERALOGY 


the  refracted  ray  is  modified  and  the  vibrations  of  the  modified 
portion  are  restricted  to  one  plane.  The  amount  of  this  modified 
light  will  depend  upon  the  angle  of  incidence,  the  character  of 
the  surface,  and  the  substance.  Light  in  which  the  vibrations  take 
place  in  one  plane  only  is  termed  polarized  light,  or  plane  polarized 
light ;  when  the  vibrations  are  in  circular  orbits,  circular  polarized ; 
and  when  they  are  in  elliptical  orbits,  elliptically  polarized. 

Both  the  reflected  and  refracted  ray  are  completely  polarized 
when  the  angle  between  them  is  90°,  or,  as  Brewster's  law  expresses 
it,  tan  (angle  of  polarization)  =  n  (the  index  of  refraction).  In 
case  of  rock  salt,  n  =  1.544,  or  the  angle  of  polarization  would  be 
57°  5' ;  when,  in  Fig.  322,  the  angle  noR  =  57°  5'  in  case  of  rock  salt 
and  air,  the  angle  R'oRi  would  be  90°  and  both  the  reflected  ray 
oR'  and  the  reflected  ray  oRi  are  completely  plane  polarized.  The 
vibrations  in  the  reflected  ray  take  place  at  right  angles  to  the 
plane  of  the  paper  and  the  ray  is  said  to  be  polarized  in  the  plane 
of  the  paper,  parallel  to  the  plane  of  incidence  RoR'.  In  the  re- 
fracted ray,  the  vibrations  take  place  parallel  to  the  plane  of  the 
paper,  and  it  is  said  to  be  polarized  in  the  plane  perpendicular  to  the 
plane  of  the  paper,  and  at  right  angles  to  the  plane  of  incidence. 
The  two  rays  after  polarization  are  vibrating  in  planes  at  right 

angles.     This  is  the  condition  in 
all  isotropic  substances. 

In  anisotropic  substances,  in 
case  of  refracted  light,  both  the 
ordinary  and  extraordinary  rays 
are  completely  polarized,  their  vi- 
bration planes  are  at  right  angles 
and  rigidly  fixed  by  the  molecular 
arrangement  of  the  crystal. 

In  a  cleavage  piece  of  calcite, 
Fig.  323,  when  the  four  sides  of  the 
rhombic  faces  are  approximately  equal,  the  ordinary  ray  o  upon 
emerging  will  be  vibrating  parallel  to  aa',  its  plane  of  polarization 
will  be  parallel  to  the  short  diagonal  cc' ;  the  extraordinary  ray  e 
will  vibrate  parallel  to  cc',  its  plane  of  polarization  will  be  parallel  to 
the  long  diagonal  aa',  and,  furthermore,  it  is  impossible  for  light  to 
emerge  from  the  calcite^the  vibrations  of  which  do  not  conform  to 
either  of  these  two  directions.  The  two  vibration  planes  and  planes 
of  polarization  are  rigidly  fixed  by  the  crystalline  structure  of  the 
calcite. 


OPTICAL  PROPERTIES   OF   CRYSTALS 


177 


If  in  any  case,  or  by  any  means,  one  ray,  either  the  o  or  e  ray, 
could  be  absorbed,  light  passing  out  of  the  fragment  would  be  vi- 
brating in  one  plane  only.  It  is  a  property  of  plates  of  tourma- 
line that  when  cut  parallel  to  the  c  axis  they  absorb  one  ray,  the 
ordinary,  and  the  extraordinary  ray,  vibrating  parallel  to  the  c  axis 
only,  is  transmitted.  All  light  transmitted  by  such  a  section  of 
tourmaline  is  vibrating  in  one  plane,  that  parallel  to  the  c  axis. 
Such  a  section  of  tourmaline,  or  any  other  device,  used  to  produce 
polarized  light  is  termed  a  polarizer,  Fig.  324. 

When  the  light,  as  transmitted  by  the  polarizer,  is  viewed 
through  another  similar  section  of  tourmaline,  it  will  be  observed 


-a 


-  c 

FIG.  324.  — Tourmaline  Polarizer. 


at  once  that  the  intensity  depends  upon  the  relation  of  the  two 
sections  of  tourmaline.  When  the  two  c  axes  of  the  sections  are  at 
90°,  as  in  Fig.  325,  no  light  will  be  transmitted  by  the  second  section 
in  this  crossed  position,  and  the  condition  will  be  that  of  darkness ; 
the  amount  of  light  transmitted  by  the  second  section,  termed  the 
analyzer,  constantly  increases  from  zero  in  the  crossed  position  to  a 
maximum,  when  the  6  axes  of  the  polarizer  and  analyzer  are 
parallel.  The  analyzer  allows  no  light  to  pass,  the  vibrations  of 
which  are  at  right  angles  to  its  vibration  plane ;  as  all  light  pass- 
ing the  polarizer  is  thus  vibrating,  no  light  can  pass,  when  the 
two  sections  are  in  the  crossed  position,  and  darkness  is  the  result. 
As  the  analyzer  is  rotated,  the  amount  of  light  passing  increases  to 


178 


MINERALOGY 


a  maximum  when  the  two  vibration  planes  are  parallel,  and  all  the 
light  passing  the  polarizer  is  transmitted  by  the  analyzer. 

Intermediate  positions  are  explained  as  in  Fig.  326  ;  let  PP'  be  the 
vibration  plane  of  the  polarizer  and  AA'  that  of  the  analyzer,  be  the 

amplitude  and  plane  of  vibration  of 
light  passing  the  polarizer;  according 
to  the  parallelogram  law  in  mechanics 
this  wave  may  be  divided  into  two 
waves  vibrating  at  right  angles  to 
each  other,  one  ce  parallel  to  the 
vibration  plane  of  the  analyzer  and 
with  an  amplitude  ce,  the  other  vi- 
brating at  right  angles  to  it  and  with 
an  amplitude  eb ;  the  ray  represented 
by  ce  passes  the  analyzer,  while  eb 
at  right  angles  to  it  is  extinguished. 
The  amplitude  of  the  transmitted 
ray  bf,  which  illuminates  the  field  of  the  analyzer,  increases  from 
zero  in  the  crossed  position,  where  all  light  is  absorbed,  to  cb  in 
the  parallel  position  when  all  the  light  is  transmitted. 

Two  such  sections  when  mounted  in  a  holder  are  known  as  the 
tourmaline  tongs,  and  may  be  used  to  test  the  double  refraction 
and  the  vibration  planes  of  light  in  any  mineral  section  placed 
between  them. 

If  the  light  reflected  from  a  polished  table  top  is  viewed  through 


one  of  the.  tourmaline  sections,  as  an  analyzer,  on  revolving  the 
section  the  intensity  of  the  transmitted  light  will  be  greatest  when 
the  long  or  c  axis  of  the  tourmaline  section  is  parallel  to  the  table 


OPTICAL  PROPERTIES  OF  CRYSTALS 


179 


fre 


FIG.  329.  —  Tourmaline  Analyzer. 


top,  and  least  when  at  right  angles  to  it,  showing  that  some  of  the 
reflected  light  is  polarized  and  that  the  plane  of  vibration  of  the 
polarized  reflected  light  is  parallel  to  the  table  top  and  at  right 
angles  to  the  plane  of  incidence. 

In  testing  the  vibration  planes  of  the  two  rays  transmitted  by  the 
calcite  rhomb,  when  the  vibration  plane  of  the  tourmaline  section 
is  parallel  to  the  long  diagonal  of  the  rhombic  face  of  the  calcite, 
Fig.  327,  only  the  ordinary  ray 
will  appear ;  its  vibration  plane 
must  therefore  be  parallel  to  this 
diameter.  Upon  revolving  the 
tourmaline  section,  both  rays  ap- 
pear and  are  equal  in  intensity 
after  a  revolution  of  45°,  Fig.  328. 
Upon  revolving  the  tourmaline 
90°  only  one  ray  will  appear,  the 
extraordinary  ray,  the  vibration 
plane  of  which  must  therefore  be 
parallel  to  the  short  diagonal  of 
the  calcite  face,  Fig.  329.  The 
two  rays  are  polarized  and  their  vibration  planes  are  at  right 
angles ;  this  is  true  of  all  anisotropic  substances. 

The  nicol  prism.  —  As  polarized  light  is  necessary  in  the  study 
of  the  optical  properties  of  minerals,  and  to  avoid  the  natural 
color  of  tourmaline  sections,  Nicol  in  1828  devised  the  instrument 
now  used  as  the  source  of  polarized  light  in  most  optical  instru- 
ments and  known  as  the  nicol  prism  or  nicol. 

It  is  constructed  of  clear  colorless  calcite  in  such  a  manner  that 
one  ray,  the  ordinary  ray,  is  internally  totally  reflected  and  ab- 
sorbed, while  only  the  extraordinary  ray  emerges,  thus  yielding 
plane  polarized  light,  all  of  which  is  vibrating  in  one  known  plane. 

Fig.  330  is  a  section  through  the  short  diagonal  of  a  nicol  prism, 
illustrating  its  construction.  A  clear,  colorless  cleavage  piece  of 
calcite,  three  times  as  long  as  broad,  is  cut  along  the  plane  PP'  per- 
pendicular to  the  plane  of  the  diagram,  from  the  obtuse  angle  at  P 
to  that  of  P'.  The  angle  PP'e  should  be  22° ;  the  end  surfaces  are 
then  cut  down  until  the  angles  dP'P  and  ePP'  are  right  angles.  The 
two  polished  halves  are  cemented  together  in  their  original  -posi- 
tion with  Canada  balsam,  a  film  of  which  will  separate  the  two 
halves  and  lie  along  the  plane  PP'.  The  cemented  calcite  is  then 
set  in  cork,  the  walls  of  which  next  the  calcite  have  been  blackened 


180 


MINERALOGY 


to  absorb  any  light  that  may  fall  on  them  after  being  reflected  to 
the  sides  of  the  calcite.  A  ray  of  ordinary  white  light  on  entering 
a  nicol,  as  at  R,  is  divided  into  an  ordinary  and  an  extraordinary 
ray,  having  different  indices  of  refraction  and  traveling  different 
paths  through  the  calcite;  their  angle  of 
total  reflection  will  therefore  differ.  The 
ordinary  ray,  with  an  index  of  refraction  of 
1.658  between  air  and  calcite,  is  refracted 
more  than  the  extraordinary  ray  with  an 
index  of  refraction  of  1.486 ;  this  ray  will 
meet  the  film  of  Canada  balsam  at  an 
angle  greater  than  69°,  which  is  the  ap- 
proximate critical  angle  of  the  ordinary 
ray  between  Canada  balsam  and  calcite, 
since  o>  =  1.6583  divided  by  the  index  of 
refraction  of  Canada  balsam,  1.548,  = 
1.0712,  the  index  of  refraction  of  the  ordi- 
nary ray  as  between  Canada  balsam  and 
calcite.  As  between  these  two  media,  sin 


s 


(of  the  critical  angle)  = 


critical  an- 


1.0712' 

gle  =  68°  59'.  All  ordinary  rays  meet- 
ing the  film  of  balsam  at  an  angle  greater 
than  68°  59'  will  be  totally  reflected  in  the 
direction  as  indicated,  and  absorbed  by 
the  blackened  walls  of  the  cork  mounting. 
The  index  of  refraction  of  the  extraordi- 
nary ray  varies  with  the  direction  through 
the  crystal,  but  in  this  particular  direction 
it  is  but  little  different  from  that  of  the 
balsam,  1.548;  its  path  on  entering  the 
calcite  is  deviated  much  less  than  the 
ordinary  ray,  and  on  meeting  the  film  of 
balsam  is  but  little  effected,  passing  through 
with  little  or  no  refraction,  and  emerging 
at  the  opposite  end  of  the  nicol  as  plane 

polarized  light,  with  a  vibration  plane  parallel  to  the  short  di- 
agonal cc  of  the  rhombic  section,  and  polarized  in  the  plane 
parallel  to  the  long  diagonal  PP'.  Other  styles  of  polarizing 
prisms  have  been  devised,  either  to  economize  space  or  calcite, 
as  suitable  calcite  is  very  scarce  and  expensive,  since  the  Iceland 


OPTICAL  PROPERTIES   OF  CRYSTALS 


181 


supply  has  been  exhausted.  They  all  agree,  however,  in  the 
principle  of  totally  reflecting  either  the  ordinary  or  extraordinary 
ray  out  of  the  field. 

As  usually  mounted,  the  polarizing  nicol  is  under  the  microscope 
stage,  with  its  plane  of  polarization  crossing  the  field,  from  0°  to 
180°  on  the  scale,  while  the  analyzer  is  mounted  in  the  tube  of  the 
microscope  in  such  a  way  that  it  may  be  pushed  in  or  out  of  the  line 
of  vision  as  required ;  its  plane  of  polarization  is  at  right  angles  to 
that  of  the  polarizer,  or  in  the  crossed  position. 

Interference  of  polarized  light.  —  Whether  we  speak  of  light  as 
due  to  waves,  or  to  the  periodic  vibrations  or  change  in  con- 
ditions, or  whether  light  is  due  to  an  electromagnetic  disturbance 
of  the  ether,  it  remains  nevertheless  true,  that  one  disturbance  is 


FIG.  331. 


influenced  by  another  and  may  be  added  to  or  subtracted  from  the 
other,  according  to  the  phase  of  each.  If  two  waves  of  the  same 
length  are  vibrating  in  the  same  plane  and  phase,  as  the  two  waves 
a  and  b  in  Fig.  331,  but  of  different  amplitudes  or  intensities,  the 
result  is  an  entirely  new  wave  c  with  an  amplitude  oc,  or  a  wave 
with  an  amplitude  of  a  +  b,  and  the  illumination  of  the  new  wave  is 
equal  to  that  of  the  other  two  combined.  When  the  waves  are  in 
opposite  phases,  the  result  is  the  difference  of  the  two  amplitudes, 
the  new  wave  a,  with  an  amplitude  oa  equal  to  oc  —  oa,  and  the 
illumination  is  decreased.  Should  the  amplitudes  be  equal  and 
the  one  wave  be  a  half  phase  or  wave  length  behind  the  other,  their 
difference  would  be  zero  and  darkness  would  result,  or  one  wave  is 
said  to  interfere  with  the  other. 

When  light  of  the  same  wave  length  or  color  and  of  the  same 
intensity,  i.e.,  derived  from  the  same  source,  is  polarized  in  two 
rays,  these  two  rays  will  interfere  if  brought  to  vibrate  in  the  same 
plane.  The  result  of  this  interference  will  depend  upon  the  con- 
ditions of  vibration,  or  how  much  one  wave  has  been  retarded,  or 
is  vibrating  behind  the  other. 


182 


MINERALOGY 


In  Fig.  332,  the  ray  R  is  reflected  at  the  point  o  in  the  direction 
of  oR'.  Some  of  the  light,  however,  enters  the  medium  P  and  is 
refracted  in  the  direction  oRi  and  at  R]  is  reflected  in  the  direction 
of  RIO'  and  passes  out  of  the  medium  P,  in  the  direction  o'R". 
The  light,  or  ray,  o'R"  is  made  up  of  two  rays,  one  which  has  trav- 
eled the  path  oRio'  in  the 
medium  P,  and  a  reflected 
ray  iV,  which  will  prob- 
ably be  vibrating  in  a 
different  phase  and  there- 
fore in  position  to  inter- 
fere. If  the  refracted  ray 
is  retarded  1/2  X  due  to 
the  differences  of  paths 
and  velocities  of  the  two 
rays  while  the  refracted 
ray  is  passing  through  the 
medium  P,  then  the  eye 
at  R"  will  perceive  no 

light,  or  darkness  will  result  when  monochromatic  light  is  being 
used. 

When  the  medium  P  is  of  uniform  thickness  the  relative  paths 
for  each  ray  will  be  the  same  for  all  points  and  the  surface  will  be 
uniformly  lighted.  If  the  paths  within  the  medium  P  can  be  pro- 
gressively varied,  or  if  the  section  P  is  wedge-shaped,  as  indicated 
by  the  dotted  line, 
then  the  difference 
of  phase  between 
the  reflected  and 
refracted  rays  will 
increase  with  the 
thickness  of  the 
wedge,  as  the  in- 
ternal path  fol- 
lowed by  the  ray  FIG.  333.  — Diagram  of  the  Quartz  Wedge. 

R  will  be  much  shorter  than  the  path  followed  by  the  ray 
2,  and  on  emerging  at  o'  and  o'"  will  be  retarded  proportionally. 
At  the  edge  of  the  wedge,  Fig.  333,  where  the  thickness  is  zero, 
there  will  be  no  interference  or  diminution  in  the  illumination. 
As  the  wedge  thickens,  the  refracted  ray  will  be  retarded  more 
and  more  behind  the  reflected  ray,  with  a  decrease  of  the  illumi- 


OPTICAL  PROPERTIES   OF   CRYSTALS  183 

nation  until  the  refracted  ray  is  exactly  1/2  a  wave  length 
behind  the  reflected  ray,  or  1/2  X,  then  darkness  will  be  the 
result.  From  this  point  the  illumination  will  increase  until  the 
refracted  ray  is  a  whole  wave  length  behind  the  reflected  ray,  when 
there  will  be  a  maximum  illumination.  There  will  therefore  be 
bands  of  light,  representing  a  maximum  light  at  each  whole  wave 
length  that  one  ray  is  retarded  behind  the  other,  as  at  1,  2,  3,  and 
there  will  be  a  band  of  minimum  illumination  at  points,  as  1/2, 
3/2,  5/2,  at  which  one  ray  is  retarded  an  odd  number  of  1/2  wave 
lengths  behind  the  other.  This  condition  of  alternate  bands  of 
light  and  darkness  obtains  only  when  monochromatic  light  is  used ; 
when  white  light  is  used,  which  is  composed  of  waves  of  all  lengths 
or  colors,  and  which  differ  in  their  velocities  in  passing  through 
the  wedge,  their  dark  and  light  areas  on  the  wedge  will  not  corre- 
spond, and  the  area  which  will  be  dark  for  yellow  will  be  light  for 
red,  with  a  result  that  the  surface  viewed  with  reflected  light  will 
show  color  bands  (these  bands  may  be  seen  on  the  quartz  wedge 
when  held  at  the  proper  angle).  Beginning  at  the  thin  edge  of 
the  wedge,  all  the  interference  colors  will  have  appeared  in 
order,  when  the  retardation  has  reached  one  wave  length,  or  X ; 
then  they  are  repeated  in  the  same  order,  when  2  X  is  reached, 
and  again  to  3  X. 

These  color  effects  due  to  the  interference  of  light  are  well  illus- 
trated by  the  play  of  colors  on  soap  bubbles ;  in  the  iridescent  films 
of  carbonates,  oxides,  or  oil  on  the  surface  of  water ;  in  the  cleav- 
age fractures  of  such  a  clear  mineral  as  calcite,  and  in  the  small 
internal  and  irregular  fractures  of  the  opal. 

Order  of  colors.  —  In  the  series  of  colors  caused  by  the  inter- 
ference of  light,  those  which  appear  first,  on  the  thin  end  of  the 
wedge,  or  are  caused  by  a  retardation  of  one  wave  length  or  less, 
are  termed  the  colors  of  the  first  order;  those  from  X  to  2X,  the 
second  order ;  and  those  from  2  X  to  3  X,  the  third  order ;  etc.  Above 
the  fourth  and  fifth  orders  the  individual  colors  are  not  well  defined 
and  return  to  the  high  order  gray.  The  lower  orders  of  colors  are 
each  characteristic  in  intensity  and  tone,  and  with  experience  may 
easily  be  distinguished ;  as,  for  instance,  red  of  the  first  order,  from 
red  of  the  second  or  third  orders ;  since  the  order  of  color  yielded 
by  sections  of  approximately  the  same  thickness  of  the  various 
double  refracting  minerals  is  a  measure  of  their  double  refraction, 
it  is  most  important  that  one  should  be  able  to  recognize  the  colors 
of  various  orders.  The  most  important  of  these  are : 


184 


MIXER ALO(  IV 


FIRST  ORDKK 

SECOND  ORDKK 

THIRD  ORDER 

Grays 

Purple 

Light  blue 

Straw  yellow 

Deep  blue 

Bright  green 

Deep  red 

Light  green 

Yellowish  green 

Light  yellow 

Faint  red 

Bright  red 

FOURTH  ORDER 

Indistinct 


These  colors  may  be  compared  by  use  of  the  quartz  wedge, 
Fig.  334;  those  produced  by  the  thin  edge  are  of  the  first  order, 
starting  with  gray  of  the  first  order. 

Uniaxial  crystals.  —  It  has  been  pointed  out  that  light  in  pass- 
ing through  a  uniaxial  crystal  is  divided  into  two  rays  polarized 

and  vibrating  at  right  angles  to 
each  other,  one  of  which  travels 
with  the  same  velocity  whatever 
the  direction,  while  the  velocity 
of  the  other  varies  with  the  direc- 
tion. 

If  an  ellipsoid,  Fig.  335,  be  con- 
structed in  which  the  three  axes 
are  drawn  proportional  to  the 
three  indices  of  refraction,  which 
are  proportional  to  the  reciprocals 
of  the  three  velocities,  this  ellip- 
soid, in  the  case  of  uniaxial  crystals, 
will  be  one  of  revolution,  every 
plane  section  of  which  will  be  an 
ellipse;  there  is,  however,  one  di- 
rection, that  perpendicular  to  the 

optic  axis,  in  which  the  plane  sections  are  circles,  and  light  is 
transmitted  in  the  direction  of  the  optic  axis  without  double  re- 
fraction. The  radii  vectores  of  the  elliptical  section  will  be  a 
measure  of  the  indices  of  refraction  of  the  two  possible  rays 
passing  through  the  crystal  in  a  path  at  right  angles  to  the 
section,  and  their  directions  will  also  indicate  their  planes  of 
polarization. 

Let  Fig.  336  be  a  section  containing  the  axis  of  rotation  of  such 
an  ellipsoid,  in  which  ox  represents  the  smaller  index  of  refrac- 
tion and  oz  the  larger.  Light  traveling  in  the  direction  of  zz'  is 
transmitted  with  a  velocity  proportional  to  oz  and  an  index  of  re- 
fraction of  ox;  similarly  a  ray  in  the  direction  of  xx'  is  transmitted 


\ 

Gray. 
Straw  yellow. 
Red  of  the  first  order. 

Purple. 
Blue. 
Green. 
Yellow. 
Red  of  the  second  ord< 

Blue. 
Green. 
Yellow. 
Red  of  the  third  order. 

1-1 

FIG.  334.  —  Quartz  Wedge. 


OPTICAL   PROPERTIES   OF   CRYSTALS 


185 


FIG.  335. 


with  a  velocity  of  ox  and  an  index  of  refraction  of  oz.     Such  an 

ellipsoid  is  known  as  the  indicatrix  of  Fletcher. 

Any  ray  whatever,  as  the  ray  entering  the  crystal  at  R,  Fig.  335, 

will  in  general  be  transmitted  as 

two  rays.     The  indices  of  refrac- 
tion will   be  represented  by  the 

radii    vectores    of   the    elliptical 

section  of  the  indicatrix,  passing 

through  the  point  o  and  perpen- 
dicular  to   the   direction   of  the 

ray,  as  the  ellipse  bab',  which  in 

uniaxial    crystals    contains    one 

diameter    aa',    representing,   the 

ordinary   ray;    this   diameter  is 

constant  in   all   sections  of   the 

indicatrix    passing    through    the 

point  o.     The  two  planes  Rcob 

and  Raa',  at  right  angles  to  the 

elliptical   section  aba'   and  con- 
taining the  two  diameters  bb'  and 

aa',  are  the  planes  of  vibration   of   the  two  rays ;   of  these  the 

extraordinary  ray  vibrates  in  the  plane  containing  the  optic  axis 

cc'  and  the  direction  of  the  ray  Ro,  and  termed  the  principal  optic 

section.  The  extraor- 
dinary ray  vibrates 
in  the  principal  optic 
section  and  is  polarized 
in  the  plane  ROaa'  at 
right  angles  to  it. 
There  is  one  direction 
in  which  the  two  diam- 
eters of  the  elliptical 
section  are  equal,  that 
at  90°  to  the  optic  axis, 
or  the  ellipse  becomes  a 
circle  and  the  two  rays 
are  transmitted  with 

the  same  velocity  and  with  no  fixed  plane  of  vibration;    they 

are  not  polarized. 

Angle  of  extinction.  —  When  a  section  of  a  uniaxial  crystal,  or  in 

fact  any  double  refracting  substance  with  plane  parallel  faces,  is 


186 


MINERALOGY 


examined  between  crossed  nicols,  it  will  be  found,  on  rotation  of  the 
section  between  the  nicols,  that  the  light  will  be  entirely  extin- 
guished, or  decrease  to  a  minimum  illumination,  at  every  90°,  and 
the  section  will  be  dark.  From  the  point  of  darkness  the  illumi- 
nation increases  constantly  upon  further  revolving  the  section, 
until  a  maximum  is  reached  at  a  point  45°  from  the  point  of  dark- 
ness, and  then  decreases  to  a  minimum  after  a  revolution  of  the 
section  through  another  angle  of  90°  ;  these  conditions  are  repeated 
four  times  in  the  complete  revolution  of  360°. 

Interference  of  polarized  light  in  passing  mineral  sections.— 
Let  Fig.  337  be  such  a  section;  then  light  entering  the  section 
will  be  transmitted  as  two  rays  vibrating  in  planes  at  right  angles 

to  each  other.  Let  ee'oV 
represent  the  elliptical  sec- 
tion of  the  indicatrix; 
the  two  rays  will  leave  the 
section  vibrating  in  the 
planes  ee'  and  oo' ;  also  let 
PP'  and  AA'  be  the  vibra- 
tion planes  of  the  polar- 
izer and  analyzer.  If  RO 
represents  the  amplitude 
and  the  direction  of  the 
vibrations  of  the  plane 
polarized  ray  passing  the 
polarizer,  then  on  entering 
the  section  this  ray  will  be 
resolved  into  two  rays,  ooi,  vibrating  parallel  to  o',  and  oe,  vibrat- 
ing parallel  to  ee'.  When  the  ray  oof  enters  the  analyzer  one  com- 
ponent oon  vibrating  parallel  to  the  vibration  plane  AA'  of  the 
analyzer  passes,  and  passes  without  diminution,  while  the  other 
component,  vibrating  parallel  oiOn  at  right  angles  to  AA'  having 
no  component  in  the  plane  AA'  is  extinguished  by  the  analyzer.  The 
two  rays  oon  and  oen,  vibrating  parallel  to  AA'  and  therefore  in 
position  to  pass  the  analyzer,  are  also  in  position  to  interfere, 
and  the  resultant  light  depends  upon  this  interference.  When 
white  light  is  used,  the  resulting  interference  color  will  depend 
upon  the  double  refraction  of  the  substance;  upon  the  direction 
of  the  section  in  the  crystal;  and  upon  the  thickness  of  the 
section. 

When  monochromatic  light  is  used  and  one  ray  is  retarded  behind 


FIG.  337. 


OPTICAL   PROPERTIES   OF  CRYSTALS  187 

the  other  one  wave  length  in  passing  the  section,  or  any  multiple 
of  whole  wave  lengths,  oon  will  be  opposed  to  the  vibrations  of  oen 
and  there  will  be  darkness  during  a  complete  revolution  of  the 
section.  The  conditions  are  the  same  as  in  the  quartz  wedge,  but 
here  the  half  wave  of  the  nicol  is  added. 

As  the  vibration  planes  of  every  mineral  section  are  absolutely 
fixed,  they  may  be  determined,  and  if  necessary  their  traces  marked, 
on  the  section ;  if  the  section  is  revolved  until  there  is  a  minimum 
amount  of  light  or  darkness,  as  viewed  through  the  analyzer,  be- 
tween crossed  nicols,  then  the  traces  of  the  vibration  planes  of  the 
section  will  be  parallel  to  the  vibration  planes  of  the  analyzer  and 
polarizer  or  to  the  cross  hairs  in  the  eyepiece.  One  of  these  planes 
is  the  principal  optic  section  or  contains  the  optic  axis,  which  in  uni- 
axial  crystals  is  parallel  to  the  c  crystallographical  axis.  It  follows 
that  in  all  sections  through  the  crystal  parallel  to  the  prism  zone 
one  of  the  vibration  planes  of  the  section  will  be  parallel  to  pris- 
matic or  pinacoidal  cleavage  cracks  in  the  section,  or  at  right 
angles  to  them.  Darkness  will  occur  on  viewing  the  section  in  the 
microscope  when  one  of  the  cross-hairs  is  parallel  to  the  cleavage 
cracks;  the  section  under  these  conditions  is  said  to  possess 
parallel  or  straight  extinction. 

The  extinction  angle  of  any  section  is  measured  by  the  cross 
hairs  in  the  eyepiece  of  the  microscope.  They  are  set  parallel  to 
the  vibration  planes  of  the  nicols ;  t^hen  when  extinction  occurs  on 
revolving  a  mineral  section  on  the  stage,  the  vibration  planes  of  the 
section  are  parallel  to  the  cross  hairs.  A  reading  is  taken  from  the 
graduated  circle  on  the  stage,  then  the  stage  is  turned  until  the  cleav- 
age crack  is  parallel  to  the  hair,  when  another  reading  is  taken ;  the 
difference  between  these  two  readings  will  be  the  extinction  angle  of 
the  section,  All  sections  of  uniaxial  crystals,  not  parallel  to  one  axis 
of  the  ellipsoid,  extinguish  at  angles  other  than  90°.  The  extinc- 
tion angle  will  vary  with  the  inclination  of  the  section,  but  extinc- 
tion is  always  symmetrical,  or  divides  the  angle  between  cleavage 
cracks  equally. 

In  basal  sections  of  uniaxial  crystals  there  are  no  definite  vibra- 
tion planes,  and  the  light  passed  by  the  polarizer  will  pass  through 
the  section  unchanged,  to  be  extinguished  by  the  analyzer,  and  the 
field  will  remain  dark  during  a  complete  revolution,  as  if  there  were 
no  section  at  all  between  the  nicols. 

Determination  of  the  slow  ray.  —  The  slow  ray  may  be  deter- 
mined by  means  of  the  quartz  wedge.  This  is  cut  from  a  crystal 


1SS  MINKKALOCJY 

of  quartz  in  such  a  manner  that  one  flat  side  is  parallel  to  a  plane 
containing  the  optic  axis,  i.e.,  the  vertical  crystallographical  axis; 
the  long  edge  of  the  wedge,  in  most  cases,  is  inclined  at  an  angle  of 
45°  to  the  optic  axis.  Some  wedges  are  cut  with  their  long  edge 
parallel  to  the  optic  axis.  In  all  cases  the  vibration  plane  of  the 
slow  or  extraordinary  ray  is  always  indicated  on  the  wedge  by  an 
arrow  or  mark,  as  in  Fig.  334. 

In  the  tube  of  all  petrographical  microscopes,  just  above  the  ob- 
jective, is  a  slot,  into  which  the  quartz  wedge  slips  back  and  forth, 
in  such  a  position  that  the  vibration  planes  of  the  wedge  are  fixed 
at  45°  to  the  vibration  planes  of  the  nicols. 

A  section  in  which  the  vibration  plane  of  the  slow  ray  is  to  be 
determined  is  placed  on  the  stage  of  the  microscope  and  revolved 
to  extinction,  then  placed  at  45°  from  this  position,  when  the  vi- 
bration planes  of  the  section  will  lie  at  45°  to  the  vibration  planes 
of  the  nicols  and  will  be  parallel  to  those  of  the  quartz  wedge  when 
in  position.  At  this  45°  position  the  section  will  be  evenly  colored. 
Minerals  in  rock  sections  are  evenly  ground  to  approximately 
.03  mm.  in  thickness,  and  when  interference  colors  of  individual 
species  are  given  they  refer  to  sections  of  about  this  thickness.  The- 
color  will  depend  upon  the  thickness  of  the  section,  the  direction 
of  the  section  in  the  crystal,  and  the  double  refraction  of  the  sub- 
stance. As  an  illustration  let  it  be  supposed  that  the  section  yields 
a  red  of  the  first  order.  First  order  red  may  be  obtained  by  using 
a  quartz  wedge  as  a  section  on  the  microscopic  stage,  pushing  it 
under  in  the  45°  position  until  the  first  red  is  obtained.  A  second 
wedge  is  now  pushed  in  the  slot  of  the  microscope  above  the  objec- 
tive ;  as  the  edge  of  the  second  wedge  enters  the  field  of  vision  there 
will  be  a  change  of  color  noted.  Whether  the  change  of  color  goes 
up  the  scale,  from  red  of  the  first  order  to  purple,  blue,  green,  etc., 
of  the  second  order,  or  down  the  scale  to  yellow  and  grays  of  the 
first  order,  will  depend  upon  whether  the  difference  between  the 
vibrations  of  the  slow  ray  and  the  fast  ray  is  still  increased  by  the 
second  quartz  wedge  or  decreased.  If  the  slow  ray  of  the  section 
or  the  first  quartz  wedge  used  as  a  section  is  parallel  to  the  slow 
ray  of  the  second  wedge  (the  direction  of  each  is  marked  on  the 
wedge)  which  is  inserted  in  the  tube  of  the  microscope,  the  color 
change  is  up  the  scale,  or  the  effect  is  that  of  thickening  the  section. 
When  the  vibration  plane  of  the  slow  ray  of  the  section  is  at  right 
angles  to  that  of  the  quartz  wedge,  upon  pushing  the  wedge  in 
slowly  the  colors  will  go  down  the  scale,  from  red  to  yellow  and  gray, 


OPTICAL   PROPERTIES   OF   CRYSTALS  189 

and  finally  a  shadow  will  appear,  or  darkness,  at  which  point  the 
difference  between  the  slow  and  fast  rays  of  the  section  is  exactly 
equal  to  that  of  the  quartz  wedge,  and  the  wedge  is  said  to  com- 
pensate the  section ;  for  this  reason  the  wedge  is  often  termed  a 
compensator.  When  compensation  occurs,  if  the  section  is  re- 
moved from  the  stage,  the  quartz  will  show  the  original  color, 
due  to  the  double  refraction  of  the  mineral  section  before  the 
wedge  was  inserted.  At  the  point  of  compensation,  if  the  wedge 
is  pushed  farther  through,  the  colors  will  rise  in  the  scale  unin- 
terruptedly to  the  end  of  the  wedge. 

When  the  direction  of  the  c  axis  in  the  section  can  be  determined, 
either  from  cleavage  cracks  or  crystalline  edges,  and  the  vibration 
plane  of  the  slow  ray  is  known,  then  the  optical  sign  of  the  section  is 
also  known ;  for  when  the  c  axis  is  parallel  to  the  long  edge  of  the 
quartz  wedge  and  the  slow  ray  parallel  to  the  slow  ray  of  the  quartz 
wedge,  the  optical  sign  is  the  same  as  that  of  quartz  (+) ;  when  the 
slow  ray  is  at  90°  to  the  slow  ray,  as  marked  on  the  wedge,  the  sign 
is  opposite  to  that  of  quartz  ( — ) . 

Pleochroism  is  the  unequal  absorption  of  light  waves  of  different 
lengths.  In  the  case  of  tourmaline,  when  the  section  was  thick 
enough  it  absorbed  all  the  light  vibrating  parallel  to  the  basal 
section  and  therefore  the  section  appeared  dark  for  light  polarized 
and  vibrating  only  in  this  direction.  When  one  color  or  wave  of 
one  length  is  absorbed  more  than  another,  the  color  of  transmitted 
light  will  change  with  the  direction,  or  plane  of  vibration.  Miner- 
als in  which  pleochroism  is  well  marked  will  appear  differently  col- 
ored according  to  the  vibration  plane  of  the  transmitted  light. 
The  absorption  reaches  a  maximum  when  the  vibration  planes  are 
parallel  to  the  planes  of  symmetry  of  the  indicatrix.  In  uniaxial 
crystals  there  can  be  only  two  directions,  parallel  to  the  c  axis,  and 
parallel  to  the  basal  section.  Crystals  of  this  class  can  show  only 
two  maximum  absorption  directions  and  are  said  to  be  dichroic. 
In  biaxial  crystals  there  are  three  maximum  directions  possible, 
and  these  are  said  to  be  trichroic  or  pleochroic. 

To  test  a  section  for  absorption  or  pleochroism,  it  is  placed  on 
the  stage  and  revolved  to  extinction,  then  the  analyzer  is  removed 
and  the  color  of  the  section  noted ;  it  is  then  revolved  90°  and  the 
color  again  noted.  Any  difference  in  color  is  due  to  the  unequal 
absorption  of  the  two  rays  vibrating  in  the  section,  as  in  the  first 
position  one  vibration  plane  of  the  section  is  parallel  to  the  vibra- 
tion plane,  of  the  polarizer  and  transmits  the  light,  while  in  the 


190 


MINERALOGY 


second  position  the  second  vibration  plane  is  parallel  to  the  plane 
of  the  polarizer  and  transmits  the  light. 

If  it  is  darker  when  the  extraordinary  ray  is  passing  than  when 
the  ordinary  is  passing,  and  the  difference  is  not  marked,  then  it  is 
noted,  absorption  or  pleochroism  is  weak,  c  <  o>.  If  there  is  a  change 
of  color,  this  is  also  noted  thus :  €  =  green,  <o  =  bluish ;  such  is  the 
case  for  beryl. 

The  dichroscope  is  an  instrument  by  means  of  which  the  color 
of  the  two  rays  vibrating  in  planes  at  right  angles  to  each  other 
may  be  directly  compared,  Fig.  338.  It  is  constructed  of  a  cleav- 
age piece  of  calcite,  c,  long  enough  to  separate  the  two  images  of  the 
square  orifice  o  when  viewed  from  the  opposite  end,  where  the  lens 
1  magnifies  them.  One  image  is  due  to  the  ordinary  ray  and  vi- 
brates parallel  to  the  long  diagonal  of  the  rhombic  section  of  the 


1 

7  °  / 

( 

FIG.  338.  —  Diagram  showing  the  Construction  of  the  Dichroscope. 

calcite;  the  other  is  due  to  the  extraordinary  ray  and  vibrates 
parallel  to  the  short  diagonal.  The  ends  of  the  calcite  are  squared 
up  with  the  two  wedge-shaped  glasses  G,  G'. 

Possibly  the  most  remarkable  example  of  pleochroism  is  shown 
by  thick  sections  of  the  mineral  iolite,  also  known  as  cordierite 
and  dichroite,  an  orthorhombic  mineral,  in  which  therefore  three 
limiting  absorption  directions  are  possible,  one  parallel  to  each 
axis.  If  a  smooth  surface  of  iolite  parallel  to  the  brachypinacoid  is 
held  over  the  orifice,  o,  of  the  dichroscope,  and  the  transmitted  light 
viewed  through  the  lens,  one  ray  will  vibrate  parallel  to  the  c  axis 
and  the  other  parallel  to  the  brachyaxis.  The  first  will  be  yellow, 
the  second  gray  — a  very  marked  contrast.  If  the  dichroscope  be 
revolved  and  the  section  held  stationary,  then  after  a  revolution  of 
90°,  the  two  images  will  reverse  in  color.  If  a  section  parallel  to 
the  base  be  used,  one  image  caused  by  the  vibrations  parallel  to  the 
brachyaxis  is  gray,  the  same  as  it  was  in  the  first  section,  but  the 
image  caused  by  the  vibrations  parallel  to  the  macroaxis  will  be 
blue.  In  the  description  of  a  mineral  these  conditions  are  expressed 
for  iolite  as  X  =  yellow,  Y  =  gray,  Z  =  blue,  absorption  Y  <  Z  <  X. 


OPTICAL   PROPERTIES   OF  CRYSTALS 


191 


Interference  figures  in  uniaxial  crystals.  —  In  a  section  parallel  to 
the  base,  it  has  been  shown  that  when  the  light  ray  is  passing  parallel 
to  the  c  axis,  or  optic  axis,  the  section  is  dark  between  crossed  nicols 
and  remains  so  during  a  complete  revolution.  Sections  in  any  other 
direction  when  viewed  with  parallel  rays  are  of  uniform  color,  pro- 
vided the  section  is  of  uniform  thickness.  This  color  effect,  caused 


FIG.  339. 

by  interference,  is  quite  different  when  viewed  with  converging 
light.  The  path  of  each  ray  varies  with  the  inclination  of  the  ray 
to  the  section  and  will  be  proportionally  retarded.  The  difference 
between  the  slow  and  fast  ray  will  increase  with  the  length  of  the 
path,  yielding  different  interference  colors  when  white  light  is 
used. 

The  petrographic  microscope  is  fitted  with  a  strongly  condensing 
lens,  which  is  capable  of  being  easily  slipped  in  the  line  of  vision, 
directly  under  the  stage,  where  it  is  almost  in  contact  with  the 


192 


MINERALOGY 


mineral  section.  When  in  position  this  lens  yields  a  cone  of  light, 
all  rays  of  which  will  not  be  transmitted  parallel  to  the  optic  axis 
on  passing  a  section,  cut  parallel  to  the  base,  but  only  those  in  the 
axis  of  the  cone  of  light,  Fig.  339,  oc.  This  ray  will  be  perpendicular 
to  the  section,  or  parallel  to  the  optic  axis,  and  will  be  transmitted 
by  the  section  without  double  refraction ;  its  vibration  plane  is 
that  of  the  polarizer  PP',  and  it  is  therefore  extinguished  by  the 
analyzer,  and  the  spot  c  representing  the  optic  axis  will  be  dark. 
All  other  rays  entering  the  section,  the  paths  of  which  are  not 
parallel  to  oc,  are  doubly  refracted.  One  ray  is  retarded  behind 
the  other,  the  amount  of  retardation  depending  upon  the  double 
refraction  of  the  section  and  the  length  of  the  path  that  the  rays 
travel  through  the  section.  As  the  paths  differ  and  increase  in 
length  with  the  change  of  the  inclination  of  the  rays  to  the  section 
SS',  the  path  followed  as  bbi  is  longer  than  cci.  The  differences 
of  phase  of  the  two  rays  will  increase  from  the  center  c,  where  it  is 
zero,  equally  in  all  directions  on  the  surface  of  the  mineral  section. 
There  will  be  a  point  X  where  this  difference  of  phase  will  be  one 
wave  length ;  the  cross  section  of  all  such  waves  at  right  angles 
to  oc  will  form  a  circle,  as  indicated  on  the  diagram.  These  two 
waves  emerging  at  X  when  viewed  with  the  analyzer  will  be  brought 
to  vibrate  in  one  plane  and  in  a  position  to  interfere ;  as  the  ana- 
lyzer adds  one  half  wave  length  to  the  difference,  the  effect  will 
be  at  the  point  X  and  on  the  circle  with  a  radius  X  c,  a  difference  of 

phase  of  3/2  X,  or  of  darkness  if 
monochromatic  light  is  used. 

Somewhat  further  away  from 
c  than  X  there  will  be  a  circle 
where  the  difference  in  phase 
will  be  2  X  as  appearing  in  the 
analyzer,  and  a  concentric  re- 
gion of  maximum  light  will 
mark  the  region,  followed  by 
a  concentric  ring  of  darkness, 
etc.  The  concentric  rings  of 
light  and  darkness  will  be  nar- 
row or  close  together  as  the 

angle  of  the  inclination  to  the  ray  grows  smaller  or  the  ray  is  more 
inclined.  In  viewing  the  interference  figure  in  the  microscope,  two 
dark  areas,  parallel  to  the  vibration  planes  of  the  nicols,  will  be 
noted  as  crossing  the  center  or  optic  axis  at  right  angles  and  divid- 


FIG.  340. 


OPTICAL   PROPERTIES   OF   CRYSTALS  193 

ing  the  concentric  circles  of  light  and  darkness  into  four  equal  quad- 
rants. These  are  caused  by  the  nicols  extinguishing  the  compo- 
nents vibrating  perpendicular  to  their  planes  of  vibration. 

Thus  in  Fig.  340 .  any  ray  whatever,  as  at  R,  upon  emerging 
from  the  section  is  divided  into  two  rays :  One,  the  extraordinary 
ray  vibrating  in  the  principal  optic  section,  which  contains  the 
ray  R  and  the  optic  axis  c ;  the  trace  of  this  plane  on  the  section 
is  Re.  The  ordinary  ray  vibrates  in  a  plane  at  right  angles  to  this, 
as  oo' ;  each  of  these  rays  will  have  components  vibrating  parallel 
to  AA'  and  will  pass  the  analyzer ;  the  point  R  on  the  section 
will  therefore  appear  illuminated  and  with  a  maximum  illumina- 
tion when  the  angle  RCA  is  45° ,  as  at  this  angle  the  component 
passing  the  analyzer  is  the 
largest.  For  any  point  on 
AA'  there  is  only  one  com- 
ponent possible,  that  parallel 
to  PP',  and  this  is  extinguished 
by  the  analyzer ;  the  line  AA' 
is  dark.  The  same  conditions 
hold  also  for  the  line  PP',  which 
is  dark.  These  two  dark  re- 
gions, parallel  to  PP'  and  AA', 
cross  each  other  through  the 
optic  axis,  Fig.  341. 

When  either  the  analyzer  or 

polarizer  is  revolved  Until  their      FlQ  341  _  interference  Figure  of  Calcite. 

vibration  planes  are  in  paral- 
lel position,  the  interference  figure  is  crossed  by  a  single  band  of 
darkness,  parallel  to  the  plane  of  vibration  of  the  nicols. 

When  monochromatic  light  is  used,  the  interference  figure,  is  illu- 
minated with  concentric  circles  of  the  color  used,  as  yellow,  red, 
or  blue;  but  when  white  light  is  used,  the  concentric  rings  are 
colored  with  the  interference  colors,  as  was  the  case  with  the 
quartz  wedge.  The  color  nearest  the  optic  axis  is  violet,  then 
blue,  yellow,  and  on  through  the  colors  of  the  first  order  to  red, 
after  which  they  are  repeated  as  the  second  order  colors,  then  the 
third  order  colors,  and  on  according  to  the  orders.  The  dark  cross 
remains  the  same  as  in  monochromatic  light.  When  the  section 
is  cut  exactly  at  90°  to  the  optic  axis,  the  central  dark  spot, 
where  the  optic  axis  emerges,  will  be  in  the  center  of  the  field 
of  the  microscope,  and  the  colored  rings  are  also  symmetrically 


194 


MINERALOGY 


FIG.  342.  —  Interference  Figure  of  Brucite. 


placed  in  the  field;    but  when   the   section  is    slightly  inclined, 
the  optic    axis  will  not  appear  in  the    center  of  the  field,  but 

to  one  side;  and  when  the 
section  is  revolved  the  point 
where  the  shadows  cross  will 
describe  a  circle  around  the 
axis  of  the  microscope,  the  di- 
ameter of  which  will  depend 
upon  the  inclination  of  the 
section.  When  the  inclination 
is  large,  the  optic  axis  may 
fall  without  the  field  of  the 
microscope  and  only  a  portion 
or  segment  of  the  colored  cir- 
cles will  show;  but  still  the 
radii  of  these  quadrants  will 
point  to  the  optic  axis.  The 
dark  shadows  of  the  cross  will  move  across  the  field,  on  revolving 
the  section,  holding  parallel  positions. 

Use  of  the  one  quarter  wave  mica  plate  in  determining  the  opti- 
cal sign  of  a  section.  —  The  quarter-wave  mica  plate  is  a  cleavage 
piece  of  mica  of  such  a  thickness  as  to  yield  an  interference  color 
between  crossed  nicols  of  gray  or  light  blue-gray.  That  is,  on 
emerging,  one  ray  is  retarded  1/4  X  behind  the  other.  Mica  is 
monoclinic  with  the  acute  bisectrix  nearly  parallel  to  the  vertical 
axis ;  the  two  rays  emerging  in  such  a  section  are  -y  or  the  slow 
ray  and  (3  or  the  intermediate  ray.  The  cleavage  piece  is  mounted 
with  the  vibration  plane  of  y  parallel  to  the  long  edge  of  the  slide, 
or  the  direction  of  the  vibrations  of  the  slow  ray  is  marked  on  the 
slide  as  was  the  case  in  the  quartz  wedge.  The  mounted  section 
slips  in  the  slot  in  the  tube  of  the  microscope,  between  the  ob- 
jective and  the  analyzer,  with  its  planes  of  vibration  at  45°  to 
those  of  the  nicols. 

Fig.  343  is  a  diagram  of  the  interference  figure  to  be  tested  with 
the  1/4  X  mica  plate  inserted,  with  the  slow  ray  X  vibrating  parallel 
to  the  arrow  and  across  the  quadrants  1  and  3.  The  faster  ray  will 
vibrate  across  the  quadrants  4  and  2  at  right  angles  to  the  slow 
ray.  Let  the  section  under  observation  be  one  of  quartz,  cut 
perpendicular  to  the  optic  axis,  an  optically  (+)  mineral.  In 
this  case  the  extraordinary  ray  is  the  slow  ray ;  its  vibrations 
at  any  point  in  the  section  are  directed  toward  the  optic  axis  c 


OPTICAL  PROPERTIES   OF  CRYSTALS  195 

or  the  center  of  the  diagram.  In  the  quadrants  1  and  3  it  will 
therefore  vibrate  parallel,  or  nearly  so,  in  all  positions  to  the 
arrow  or  the  slow  ray  of  the  mica  plate.  When  it  emerges  from  the 
section,  it  will  have  been  retarded  and  is  vibrating  in  phase  behind 
the  other  ray,  vibrating  at  right  angles  to  the  arrow.  In  passing 
the  mica  plate  the  slow  ray  is  still  more  retarded  and  the  difference 
between  the  phases  of  the  two  rays  is  increased  by  1/4  X,  that  due  to 
the  mica  plate.  This  is  exactly  the  same  as  if  the  section  was 
increased  in  thickness,  in  the  quadrants  1  and  3,  an  amount  suffi- 
cient to  produce  a  difference  of  phase  in  the  two  rays  of  one  quarter 
wave  length,  and  all  color  bands  in  quadrants  1  and  2  will  move 
toward  the  center,  a  distance 
due  to  one  quarter  wave 
length. 

In  quadrants  2  and  4,  the 
extraordinary  ray  is  now  vi- 
brating at  right  angles  to  the 
arrow,  or  the  slow  ray  of  the 
mica  plate,  and  the  two  rays 
on  emerging  from  the  mica 
plate  are  reduced  in  phasal  dif- 
ference by  one  quarter  wave 
length.  The  effect  is  exactly 
the  same  as  if  the  section 
had  been  decreased  in  thick- 
ness, in  the  two  quadrants  2  and  4,  an  amount  that  would  equal  a 
difference  of  phase  in  the  two  rays  of  one  quarter  wave  length. 
All  color  bands,  therefore,  in  these  two  quadrants  will  be  pushed 
out  from  the  center  a  distance  equal  to  a  phasal  difference  of  one 
quarter  wave  length.  The  interference  figure  will  be  displaced 
along  the  planes  of  the  nicols,  and  the  colored  circles  will  be 
interrupted  or  carried  toward  the  center  in  quadrants  1  and 
2  a  distance  equal  to  one  quarter  wave  length,  and  in  3  and  4 
pushed  out  a  distance  equal  to  one  quarter  wave  length.  The 
displacement  between  two  adjacent  quadrants  is  one  half  wave 
length.  In  quadrants  2  and  4  the  optic  axis  is  displaced  and  now 
two  dark  spots  appear,  one  in  each  quadrant,  caused  by  the 
compensation  of  the  section  by  the  mica  plate.  In  positive  crys- 
tals the  extraordinary  ray  is  the  slow  ray,  and  these  two  dark  spots 
will  appear  in  the  quadrants  not  crossed  by  the  arrow,  or  a  line 
connecting  the  two  dark  spots  crosses  the  arrow  at  right  angles, 


196  MINERALOGY 

i.e.  makes  a  +  sign.  The  crystal  is  then  optically  (+)  and  the 
extraordinary  ray  is  the  slow  ray,  e  >  <o.  When  the  two  dark  spots 
are  in  the  quadrants  crossed  by  the  arrow,  i.e.  are  on  the  arrow, 
and  the  line  connecting  them  is  parallel  to  the  arrow,  the  crystal 
section  is  optically  ( — )  and  the  extraordinary  ray  is  the  fast  ray 
and  (o  >  €. 

Circular  polarization.  —  When  the  interference  figure  of  a  sec- 
tion of  quartz  of  3.5  mm.  in  thickness  cut  at  90°  to  the  optic  axis 
is  viewed  in  the  microscope,  it  will  be  noted  that  the  central  por- 
tion is  not  dark,  as  would  be  expected,  but  colored,  and  the  dark 
cross  is  not  continuous  through  the  central  portion  of  the  field, 
Fig.  344.  This  is  caused  by  a  peculiar  property  of  most  crystals 

belonging  to  the  holoaxial  types, 
of  rotating  the  plane  of  polarized 
light.  When  the  light  from  the 
polarizer  enters  such  a  section, 
cut  at  90°  to  the  optic  axis,  it 
is  broken  up  into  two  rays  cir- 
cularly polarized  in  opposite  di- 
rections and  one  traveling  faster 
than  the  other.  The  two  rays 
on  emerging  from  the  section 
unite  to  form  plane  polarized 
light,  but  as  one  ray  was  faster 
than  the  other  the  plane  of  the 

FIG.  344. —  Interference  Figure  of  Quartz.  ,,.  ,  i      •       i 

resulting  plane  polarized  ray  is 

not  the  same  as  that  of  the  ray  on  entering  the  section,  but  it  has 
been  rotated  through  an  angle,  the  size  of  which  will  depend  upon 
the  thickness  of  the  section,  the  specific  rotating  power  of  the 
substance  and  the  color  of  light  used.  The  plane  will  be  rotated 
to  the  right  (clockwise)  in  right-handed  crystals  and  to  the  left 
(anticlockwise)  in  left-handed  crystals. 

If  monochromatic  light  is  used  to  make  this  observation,  in  the 
ordinary  section  the  central  portion  of  the  field  is  dark,  as  the  light 
passing  is  still  vibrating  in  the  plane  of  ^he  polarizer  and  is  ex- 
tinguished by  the  analyzer ;  but  in  case  of  quartz  the  central  por- 
tion of  the  field  is  illuminated,  as  the  plane  of  polarization  has  been 
rotated  through  an  angle  and  the  analyzer  no  longer  extinguishes 
it.  In  order  to  do  so,  the  analyzer  must  be  rotated  through  the 
same  angle,  clockwise  in  right-handed  crystals  and  anticlockwise 
in  left-handed  crystals,  through  an  angle,  other  things  being 


OPTICAL  PROPERTIES  OF  CRYSTALS 


197 


FIG.  345.  —  Airy's  Spirals. 


equal,  depending  upon  the  wave  length  or  color  of  the  light.     In 

the  case  of  white  light  a  right-handed  crystal,  when  the  analyzer  is 

rotated  t  clockwise,  and   to  the 

right,  the  central  portion  of  the 

field  passes  from  red  to  orange, 

yellow,  green,   blue,   violet,    or 

will  go  down  the  scale  of  colors. 

In  a  left-handed  section,  this 

order  of  colors  is  yielded  by  a 

rotation  of  the  analyzer  to  the 

left  or  anticlockwise. 

When  a  right-handed  section 
is  superimposed  on  a  left- 
handed  section,  a  very  peculiar 
interference  figure  is  yielded, 
Fig.  345,  known  as  Airy's 
spirals.  These  spirals  are  often 

yielded  by  sections  of  natural  crystals,  and  are  due  to  the 
twinning  of  right-  and  left-handed  forms.  The  rotating  power 
of  a  crystal  decreases  as  the  inclination  of  the  section  to  the 

optic  axis  increases,  until  in  a 
parallel  position  it  is  nil. 

The  indicatrix  of  biaxial  crys- 
tals is  an  ellipsoid,  but  not  an 
ellipsoid  of  revolution.  The 
two  rays  are  both  variable  rays, 
except  in  the  planes  of  sym- 
metry. It  is  in  these  three 
planes  of  symmetry  that  one 
ray  is  an  ordinary  ray  or  has 
a  constant  index  of  refraction 
and  the  wave  front  would  be 
a  circle,  that  of  the  other  an 
ellipse.  Figure  346  is  a  dia- 
gram of  the  indicatrix  con- 
structed with  its  three  axes 
proportional  to  the  three  indices 
of  refraction,  OX  =  a,  OY  =  3, 
OZ  =  -y.  Then  a  ray  passing  through  the  crystal  in  the  direc- 
tion of  OY  will  be  divided  into  two  rays,  one  vibrating  in  OY 
with  an  index  of  refraction  OY ;  the  other  variable,  and  its  index 


198  MINERALOGY 

of  refraction  will  lie  between  a  and  -y  as  limiting  values.     Similarly 
along  the  two  directions  OX  and  OZ. 

In  general  all  plane  sections  of  the  indicatrix  passing  through  O 
are  ellipses,  except  in  two  directions  when  they  are  circular.  Light 
passing  perpendicular  to  all  elliptical  sections,  as  at  any  point  p  in 
the  direction  pO,  will  be  transmitted  as  two  plane  polarized  rays 
vibrating  in  planes  at  90°  to  each  other,  the  traces  of  which  are 
the  major  and  minor  diameters  of  the  elliptical  section  of  the  indi- 
catrix, cut  by  the  plane  at  90°  to  the  direction  of  the  entering  ray. 
The  extremities  of  these  two  diameters  are  the  conjugate  points, 
from  which  normals  to  the  surface  determine  the  velocity,  the  di- 
rection of  transmission,  and  vibration  planes  of  the  two  rays.  The 
major  and  minor  diameters  of  the  elliptical  section  perpendicular 
to  the  direction  of  the  ray  are  the  traces  of  the  planes  of  vibrations  of 
the  resulting  rays  in  the  crystal.  These  two  diameters  always  bisect 
the  angle  included  between  the  traces  on  the  same  plane,  of  the 
planes  containing  the  ray  and  the  optic  axes,  as  the  two  planes  pOA 
and  POA'.  The  directions  AA"  and  A' A"'  are  the  optic  axes,  and 
sections  of  the  indicatrix  at  right  angles  to  these  two  directions  are 
circular,  as  yey'e'  and  ycyV,  and  there  is  no  double  refraction,  as 
there  is  no  definite  plane  of  vibration,  as  many  or  all  planes  are 
possible ;  and  light  is  transmitted  along  A' A'"  and  AA"  without 
polarization  or  double  refraction.  Each  section  of  a  biaxial 
crystal,  not  perpendicular  to  an  optic  axis,  will  transmit  two  rays 
vibrating  in  planes  and  polarized  in  planes  at  90°,  and  when  the 
section  is  revolved  on  the  stage  between  cross  nicols,  light  will  be 
extinguished  four  times  in  360°,  as  in  uniaxial  crystals. 

In  the  orthorhombic  system,  where  the  planes  of  symmetry  of 
the  indicatrix  are  parallel  to  the  crystallographical  axes,  the  pina- 
coidal  zones  will  show  straight  or  parallel  extinction;  any  other 
section  will  show  symmetrical  extinction  as  in  uniaxial  crystals. 
There  are  three  possible  positions  for  the  plane  of  the  optic  axes, 
parallel  to  each  of  the  three  pinacoids  in  turn.  In  the  monoclinic 
system,  where  one  plane  of  symmetry  of  the  indicatrix  must  coin- 
cide with  the  plane  of  symmetry  of  the  system,  one  axis  of  the 
indicatrix  will  be  fixed  parallel  to  the  orthoaxis  of  the  crystal. 
There  will  be  parallel  extinction  in  one  zone  only,  that  in  which 
the  orthoaxis  is  the  zonal  axis.  In  all  other  directions  there  will  be 
an  extinction  angle,  reaching  a  maximum  in  the  plane  of  symmetry. 

In  the  monoclinic  system,  the  extinction  angles  in  particular 
zones  or  on  fixed  planes  are  characteristic,  particularly  that  of  the 


OPTICAL  PROPERTIES   OF   CRYSTALS 


199 


plane  of  symmetry ;  and  these  angles  are  of  great  service  in  the 
identification  of  mineral  species  in  rock  sections. 

The  plane  of  the  optic  axis  when  parallel  to  the  plane  of  symmetry 
of  the  system  is  fixed,  but  the  acute  bisectrix  may  revolve  in  that 
plane  around  the  orthoaxis,  and  the  angle  it  makes  with  the  vertical 
axis  c  is  the  measure  of  the  angle  of  extinction  in  the  plane  of  sym- 
metry and  is  characteristic  of  mineral  species,  but  varies  with  the 
composition  of  the  specimen.  Fig.  347  is  a  diagram  of  the  plane  of 
symmetry  of  the  amphiboles  representing  the  extinction  angles  of 
the  common  varieties.  The  angle  is  measured  in  the  clinopina- 
coidal  section  in  reference  to  the  crystalline  outline  or  the  prismatic 
and  orthopinacoidal  cleavage  cracks. 
Again  the  acute  bisectrix  may  be  the 
orthoaxis,  at  right  angles  to  the  plane 
of  symmetry,  when  the  plane  of  the 
optic  axis  may  revolve  around  the 
acute  bisectrix  as  an  axis;  in  this 
case  the  extinction  angle  is  measured 
from  the  obtuse,  bisectrix,  which  will 
lie  in  the  plane  of  symmetry. 

In  the  triclinic  system,  any  plane 
may  be  the  plane  of  the  optic  axes,  and 
there  is  no  relation  between  the  in- 
dicatrix  or  optical  symmetry  and  the 
crystallographical  axes,  except  in  in- 
dividual species,  where  the  angles  of 
extinction  are  usually  given  in  reference 
to  some  well-marked  cleavage  plane, 
or  the  acute  bisectrix  is  oriented  by 

giving  the  angles  it  forms  with  the  normals  of  common  crystal  faces 
of  the  species. 

In  measuring  the  angle  of  extinction,  at  times  it  is  quite  im- 
possible to  determine  exactly  the  point  at  which  there  is  no  double 
refraction  or  the  least  illumination.  To  the  unaided  eye  this  area 
may  seem  to  extend  over  several  degrees.  At  such  times  a  sensi- 
tive plate  is  used,  one  by  means  of  which  the  slightest  double 
refraction  may  be  detected.  This  sensitive  plate  is  made  from  a 
cleavage  piece  of  selenite,  of  such  a  thickness  that,  when  mounted 
and  slipped  in  the  tube  of  the  microscope  in  the  same  position 
as  the  quartz  wedge,  with  its  vibration  planes  at  45°  to  those  of 
the  nicols,  will  illuminate  the  field  of  the  microscope  evenly  with 


FIG.  347.  —  Diagram  of  the  Ex- 
tinction Angles  of  the  Amphi- 
bole. 


200 


MINERALOGY 


a  red  of  the  first  order.  In  measuring  the  angle  of  extinction  the 
crystal  section  is  revolved  until  this  even  tone  of  red  is  not  affected, 
when  there  will  be  no  double  refraction  due  to  the  section  and  the 
vibration  planes  of  the  section  will  be  parallel  to  the  planes  of  the 
nicols,  and  the  section  will  be  in  the  position  of  extinction.  If 
there  is  the  slightest  double  refraction,  the  red  of  the  first  order  will 
change  to  blue  or  yellow,  or  the  tone  of  red  will  be  changed  as  the 
case  may  be,  and  the  vibration  planes  of  the  section  will  not  be 
exactly  parallel  to  those  of  the  nicols. 

Interference  figures  in  biaxial  crystals.  —  Let  Fig.  348  represent  a 
section  SS'  cut  perpendicular  to  the  acute  bisectrix,  in  which  OCi 


FIG.  348. 

and  O'Ci  are  the  two  optic  axes.  Here  also,  as  in  uniaxial  crystals, 
light  transmitted  in  these  directions  is  not  doubly  refracted,  and 
leaves  the  section  vibrating  in  the  same  plane  as  it  did  on  entering 
that  of  PP',  the  vibration  plane  of  the  polarizer,  and  therefore  when 
viewed  with  the  analyzer  in  position,  the  two  points  O  and  O' 
marking  the  position  of  the  optic  axes  in  the  section  will  be  dark. 
If  the  section  is  cut  exactly  perpendicular  to  the  acute  bisectrix  the 


OPTICAL  PROPERTIES   OF  CRYSTALS 


201 


FIG.  349.  —  Interference  Figure  of  Ara- 
gonite  with  the  Plane  of  the  Optic  Axea 
Parallel  to  the  Plane  of  one  of  the 
Nicols. 


two  optic  axes  will  emerge  an  equal  distance  on  either  side  of  the 
axis  of  the  microscope,  and  the  interference  figure  as  a  whole  will 
lie  symmetrically  placed  in  the  field  of  the  microscope.  The  line 
drawn  through  the  two  points 
O  and  O'  will  be  the  trace  of 
the  plane  of  the  optic  axes  on 
the  plane  of  the  section. 

The  optic  axes  will  be  the 
axes  of  cones  of  rays  which  pass 
through  the  section;  the  in- 
clination or  path  of  each  will 
vary  with  the  distance  OO'. 
At  some  distance  from  O  and 
O',  depending  upon  the  double 
refraction  and  thickness  of  the 
section  and  the  inclination  of 
the  ray,  there  will  emerge  two 
rays  with  a  phasal  difference  of 
a  whole  wave  length.  These 
two  rays  will  be  made  to  vibrate  in  the  same  plane  in  passing 
the  analyzer  and  will  interfere.  The  point  where  one  ray  is 
retarded  behind  the  other  one  wave  length  will  appear  dark  if 
monochromatic  light  is  used,  as  the  analyzer  adds  a  phasal 

difference  of  1/2  X.  As 
the  section  of  this  cone  of 
rays,  in  the  plane  of  the 
mineral  section,  is  ellipse- 
like,  elliptical  shadows  or 
dark  areas  will  appear 
around  each  optic  axis  as 
indicated.  Alternating 
concentric  areas  of  light 
and  darkness  will  appear 
as  indicated  in  the  dia- 
gram, according  to  the 
phasal  difference  of  the 
emerging  rays.  When 
white  light  is  used,  the  concentric  areas  will  be  colored  as  in 
the  interference  figure  of  uniaxial  crystals.  In  order  to  determine 
what  portion  of  the  field  will  be  dark  in  crossed  nicols,  due  to  the 
light  extinguished  by  the  nicols,  it  is  necessary  to  determine  the 


8 


202 


MINERALOGY 


direction  of  the  vibration  planes  of  the  two  rays  emerging  at 
any  particular  point.     In  the  plan  or  upper  part  of  the  diagram, 

let  any  point  whatever,  asR,  be 
taken,  there  will  be  two  rays 
emerge,  vibrating  in  planes  at 
right  angles.  If  from  the  point 
R  the  lines  RO  and  RO'  be 
drawn,  they  will  be  the  traces 
on  the  plane  of  the  section  of 
the  planes  containing  the  ray 
R  and  the  optic  axes  O  and 
O'.  The  angle  ORO'  is  bi- 
sected by  the  trace  of  the 
vibration  planes  on  the  plane 
of  the  section,  of  one  ray 

FIG.  351.  — Interference    Figure    of    Ara-      emerging  at  R,  that  of  the  6X- 

gonite  with  the  Plane  of  the  Optic  Axes     traordinary  ray,  as  eei ;    the 

at  45°  with  the  Planes  of  the  Nicols.  .,       , .  ,  e 

vibration  plane  of  the  other 

ray  will  be  at  right  angles  to  this  plane,  as  ff .  The  two  rays 
emerging  at  R,  one  vibrates  parallel  to  ee',  the  other  parallel 
to  ff7,  both  have  components 
parallel  to  PP'  and  AA',  the 
vibration  planes  of  the  nicols, 
and  the  point  R  in  the  field 
will  be  illuminated.  When  all 
points  in  the  field  are  tested,  it 
will  be  found  that  when  the 
plane  of  the  optic  axis  is  paral- 
lel to  either  vibration  plane  of 
the  nicols,  the  dark  area  will 
be  in  the  form  of  a  cross,  as 
represented  in  the  photograph, 

Fig.  349.     Let  the  stage  of  the  ^ 

microscope  with  the  section  be    Frr    w  "*~ 

rpvnhrorl     AKO  '       ±u       j-  G'   352' ~~  Interference    Figure    of    Ara- 

revolved    45      as    m    the    dia-  gonite  with  the  Plane  of  the  Optic  Axes 

gram,     Fig.    350.       O,    O'    are  revolved    slightly    out    of    the    Parallel 

the  optic  axes  and  OO'  is  the  Position,  showing  the  Formation  of  the 

trace   of  plane   of  the   optic 

axes,  now  at  45°  to  the  planes  of  the  nicols.  The  dark  areas  in 
this  position  will  be  quite  different  from  that  illustrated  in 
Fig.  349.  If  any  point  R  be  taken  as  before,  and  the  vibration 


OPTICAL  PROPERTIES   OF  CRYSTALS 


203 


planes  of  the  two  emerging  rays  found  as  before,  ff '  and  ee'  will 
both  have  components  in  the  direction  of  the  vibration  planes  of  the 
nicols,  and  the  point  R,  dark  before,  will  now  be  illuminated.    In  the 
same  way,  if  all  points  in  the 
field  are  tested  and  the  dark 
areas  plotted,   the  dark   areas 
would  have  the  form  of  hyper- 
bolas, as  shown  in  the  photo- 
graph, Fig.  351,  with  the  optic 
axes  at  the  poles  and  the  plane 
of  the  optic  axes  bisecting  the 
curved  shadow.     The  acute  bi- 
sectrix is  located  on  the  con- 
vex side  of  the  curved  shadow 
midway  between  the  two. 
The     movement     of     these 

shadows     should     be     Carefully     FIG.  353.  —  Interference  Figure  of  Topaz. 

observed  on  revolving  the  sec- 
tion, as  their  paths  and  curves  help,  very  materially,  to  locate 
the  'acute  bisectrix  and  the  direction  of  the  plane  of  the  optic  axes 
when  but  a  small  portion  of  the  interference  figure  is  within  the 

field  of  the  microscope  or 
when  the  section  is  inclined 
to  the  acute  bisectrix,  causing 
the  figure  to  lie  eccentric  in 
the  field  of  view.  Fig.  352 
is  a  photograph  of  the  inter- 
ference figure  of  aragonite,  re- 
volved just  a  little,  showing 
how  the  cross  breaks  up  into 
the  two  hyperbolas.  Often 
the  angle  between  the  optic 
axes  is  so  large  that  the  optic 
axes  emerge  out  of  the  field 
of  the  microscope ;  but  when 
the  section  is  perpendicular 
to  the  acute  bisectrix,  the 
symmetrical,  as  illustrated  in 


FIG.  354.  —  Interference  Figure  of  Barite, 
section  nearly  perpendicular  to  the  Optic 
Axis. 


interference    figure   will   still    be 
Fig.  353. 

When  the  section  is  cut  perpendicular  to  an  optic  axis,  the  curve 
or  color  areas  are  circles  around  the  optic  axis,  as  in  uniaxial  crys- 


204 


MINERALOGY 


tals,  or  possibly  a  little  elongated  at  the  margin,  as  indicated  in  Fig. 
354,  in  the  direction  of  the  other  optic  axis.  On  revolving  such  a 
section  the  curved  shadow  revolves  around  the  optic  axis  as  a  center, 
counter  to  the  revolution  of  the  section  and  always  with  its  convex 
side  toward  the  other  optic  axis.  The  trace  of.  the  plane  of  the 
optic  axis  will  pass  through  the  pole  of  the  curve  or  the  optic  axis, 
bisecting  the  shadow  symmetrically. 

The  optical  sign  of  biaxial  crystals.  —  The  positive  or  negative 
character  of  a  crystal  may  be  determined  from  its  interference  figure. 
The  section  is  placed  between  crossed  nicols,  with  the  plane  of  the 

optic  axis  at  45°  to 
the  vibration  planes 
of  the  nicols;  the 
quartz  wedge  is  then 
inserted,  with  the  vi- 
bration plane  of  the 
slow  ray  of  the  wedge 
parallel  to  the  axial 
plane  of  the  section, 
as  indicated  in  the 
diagram,  Fig.  355. 
When  the  slow  ray 
of  the  section  vibrates 
in  a  plane  parallel  to 
the  slow  ray  of  the 
wedge,  the  circles 
around  the  optic  axis 

will  contract  from  the  center  of  the  figure  as  they  disappear 
at  the  optic  axes.  Other  color  bands  will  contract  along 
the  long  axis  of  the  wedge,  until  they  meet  at  the  acute  bi- 
sectrix, when  they  break,  forming  two  circles,  one  around  each 
optic  axis;  all  the  color  bands  will  continue  to  contract  in  this 
manner  as  the  wedge  is  advanced.  The  direction  of  this  con- 
traction is  indicated  by  the  arrows  in  the  diagram.  The  effect 
is  that  of  thickening  the  section,  and  the  sign  of  the  section 
is  the  same  as  that  of  quartz,  or  positive  (+).  The  heads  of 
the  arrows,  indicating  the  direction  of  contraction,  make  a  posi- 
tive sign  with  the  long  axis  of  the  wedge  as  usually  mounted. 
When  the  motion  of  the  color  bands  is  the  reverse,  or  they  expand 
from  the  optic  axes,  the  section  has  been  thinned  by  the  advance  of 
the  wedge,  and  the  section  is  the  reverse  of  that  of  .the  quartz,  or 


OPTICAL  PROPERTIES   OF  CRYSTALS  205 

negative  (  — ).  The  arrows  in  the  diagram  are  reversed.  Care 
should  always  be  taken  that  the  same  relative  positions  of  the  wedge 
and  the  axial  plane  exist ;  for  if  the  slow  ray  of  the  wedge  is  intro- 
duced at  right  angles  to  the  plane  of  the  optic  axes,  all  motions  of 
the  color  bands  are  reversed  and  the  sign  may  be  taken  opposite  to 
what  it  really  is.  Also  in  sections  of  negative  crystals  after  the 
point  of  compensation  has  been  reached  and  the  wedge  is  still  ad- 
vanced, the  effect  is  as  if  thickening  the  section,  or  a  positive  crystal. 

Measurement  of  the  angle  between  the  optic  axes.  I.  —  The  ap- 
proximate value  of  2  E  may  be  obtained  by  measuring  the  distance 
between  the  two  poles  of  the  hyperbolas  in  the  interference  figure 
of  a  section  perpendicular  to  the  acute  bisectrix  and  in  the  45° 
position  with  a  micrometer  eyepiece.  Placing  this  value  at  2  d, 
then  sin  E  =  d/C,  where  C  is  a  constant  for  the  combination  of 
lenses  used,  and  may  be  determined  by  a  section  in  which  the  angle 
between  the  optic  axes  is  known,  as  aragonite. 

II. — After  the  three  indices  of  refraction  have  been  determined, 
the  angle  2  V  may  be  calculated  from  the  formula, 


cosV  = 


As  the  value  of  2  V  is  influenced  considerably  by  variations  in  the 
fourth  decimal  place  of  the  value  of  the  indices  of  refraction,  this 
method  is  not  as  accurate  as  the  direct  determinations  of  the  angle. 

III.  —  In  the  direct  determination  a  section  of  the  crystal  is 
required,  cut  perpendicular  to  the  acute  bisectrix.  The  section  is 
mounted  in  the  axial  angle  goniometer,  with  the  plane  of  the  optic 
axis  at  45°  to  the  vibration  planes  of  the  nicols.  One  hyperbola  is 
brought  tangent  to  the  hair  of  the  eyepiece  and  a  reading  taken  ; 
then  the  second  hyperbola  is  brought  tangent  to  the  hair  and  a  sec- 
ond reading  taken  ;  the  difference  between  the  two  readings  is  the 
value  of  2  E,  the  axial  angle  measured  in  air  ;  2  V,  the  true  axial  angle, 
may  be  calculated  when  the  median  index  of  refraction  p  is  known, 

sini       smE 


sin  r       sin  V 


2  E  is  always  greater  than  2  V  ;  and  when  2  V  is  large,  the  ray  along 
the  optic  axes  is  often  totally  reflected  at  the  surface  of  the  section, 


206 


MINERALOGY 


and  the  angle  in  air  would  be  180°.  It  is  then  necessary  to  immerse 
the  section  in  a  strongly  refracting  liquid,  which  decreases  the 
apparent  angle.  2  H  is  the  term  used  when  measured  in  a  strongly 
refracting  liquid,  as  oil  or  any  of  those  liquids  given  under  the 

determination  of  the  index  of  re- 
fraction, page  215.     Then  sinV  = 


—  sin  H,  where  n  is  the  index  of  re- 

P 

fraction  of  the  liquid  in  which  the 

section  is  immersed. 

Dispersion  of  the  optic  axes.  — 
When  light  of  different  wave  lengths 
is  used  in  the  measurement  of  the 
axial  angle,  the  value  will  differ 
with  different  colors  and  change 
progressively  from  one  end  of  the 
spectrum  to  the  other.  This  change 

of  the  axial  angle  for  light  of  different  wave  lengths  is  termed  the 
dispersion  of  the  optic  axes,  Fig.  357.  Whether  the  angle  is 
greater  for  red  light,  p,  than  for  violet  light,  v,  or  the  reverse, 
will  depend  upon  the  relative  values  of  the  three  indices  of  re- 
fraction for  these  individual 
wave  lengths.  When  the 
angle  is  greater  for  violet 
light  than  for  red,  it  is  ex- 
pressed u>p,  and  the  reverse, 


FIG.  356. 


This  can  usually  be  deter- 
mined by  a  close  inspection 
of  the  interference  figure 
yielded  by  white  light.  If 
the  angles  for  all  colors  were 
the  same,  i.e.  no  dispersion 
of  the  optic  axes,  the  hyper- 
bolas for  each  color  would  lie 

in     the     same     position     and       FIG.  357.  — Dispersion  of  the  Optic  Axes. 

those  for  all  colors  would  be 

superimposed;  but  when  they  differ  for  different  wave  lengths, 
as  when  that  for  red  is  greater  than  that  for  violet,  the  hy- 
perbola for  red  is  farther  away  from  the  acute  bisectrix  than 
that  for  violet ;  and  when  white  light  is  used,  the  red  wave  will  be 


OPTICAL  PROPERTIES  OF    CRYSTALS 


207 


suppressed  along  the  concave  side  of  the  hyperbolas,  the  side  far- 
thest away  from  the  acute  bisectrix,  and  the  color  appearing  will  be 
white  light  minus  red,  or  blue.  On  the  convex  side  of  the  hyper- 
bolas will  be  white  light  minus  violet,  or  red. 

The  convex  side  is  red  when  p>u,  and  the  convex  side  is  blue 
when  v  >  p.  In  the  orthorhombic  system  the  dispersion  of  the  optic 
axes  takes  place  in  one  of  the  planes  of  symmetry  of  the  crystal, 
and  the  acute  bisectrix  holds  the  same  position  for  light  of  all 
colors,  except  when  2  E  is  nearly  90°.  2  E  for  red  may  be  less 
than  90°  with  the  vertical  axis  c  the  acute  bisectrix,  and  greater 
than  90°  for  violet  when  one  of  the  lateral  axes  would  be  the  acute 
bisectrix  for  the  violet  wave.  Again  the  plane  of  the  optic  axis 
may  change  from  one  pinacoidal  plane  to  another  with  the  wave 
length,  as  is  the  case  in  brookite,  in  which  the  plane  of  the  optic 
axes  for  waves  including  red  to  yellow  is  parallel  to  the  base, 
with  the  brachyaxis  as  the  acute  bisectrix ;  for  waves  shorter 
than  yellow  the  plane  of  the  optic  axis  is  parallel  to  oio,  or  macro- 
pinacoid,  with  the  brachyaxis  still  the  acute  bisectrix.  In  such 
a  case  the  plane  of  the  optic  axis  is  said  to  be  crossed,  or  the  dis- 
persion is  crossed,  and  there  must  be  a  wave  length  of  light  a  little 
shorter  than  yellow  for 
which  the  angle  between 
the  optic  axes  is  90°,  or 
for  which  the  mineral 
brookite  would  be  uni- 
axial. 

Dispersion  in  the  mono- 
clinic  system.  —  In  the 
monoclinic  system,  where 
but  one  of  the  axes  of  the 
indicatrix  is  fixed  in  rela- 
tion to  the  crystallograph- 
ical  axes,  that  in  the  di- 
rection of  the  orthoaxis, 
three  kinds  of  dispersion 
are  possible,  as  the  three 
axes  of  the  indicatrix,  a,  p, 

•y,  each  in  turn  may  be  parallel  to  the  orthoaxis.  1.  When  p 
is  parallel  to  the  orthoaxis,  the  plane  of  the  optic  axes  will 
lie  in  the  plane  of  symmetry  of  the  crystal  and  will  be  fixed  at 
90°  to  the  orthoaxis,  but  may  revolve  around  it  as  an  axis. 


FIG.   358.  —  Inclined   Dispersion  of    the   Optic 
Axes. 


208 


MINERALOGY 


It  is  not  necessary  that  the  angle  between  the  acute  bisectrix 
and  the  vertical  axis  should  be  constant,  and  it  is  constant 
only  for  individual  species  when  chemically  pure.  The  value  of 
this  angle  will  change  with  the  composition  of  the  mineral,  and 
with  the  wave  length  of  light  for  the  same  composition.  In 
common  hornblende  this  angle  is  19q  53'  in  the  obtuse  angle  p. 
Expressed,  BxaAC  =  19°  53'  in  front;  this  is  also  a  measure  of 
the  extinction  angle,  which  is  inclined.  If  monochromatic  light 
of  different  wave  lengths  is  used,  it  will  be  found  that  this  angle 
will  vary  with  the  color  of  light  used.  The  interference  figure  as 
a  whole  is  displaced,  and  that  of  one.  color  will  not  be  superimposed 
on  that  of  another,  yet  the  trace  of  the  plane  of  the  optic  axes 

will  divide  them  all 
symmetrically.  This 
is  termed  inclined  dis- 
persion, Fig.  358. 

2.  When  the  ortho- 
axis  is  the  obtuse  bi- 
sectrix, the  plane  of 
the  optic  axis  may  ro- 
tate around  it  as  an 
axis  of  revolution;  and 
the  interference  figure 
for  each  wave  length 
of  light  which  is  in 
the  section  perpendic- 
ular to  the  acute  bi- 
sectrix may  be  dis- 
placed sidewise  through  an  arc  measured  in  the  plane  of  symmetry. 
The  trace  of  the  plane  of  the  optic  axes  will  also  be  displaced 
through  this  same  angle  for  each  wave  length.  The  traces  of 
the  planes  for  each  color  will  lie  parallel  on  the  section,  but  the 
planes  will  all  intersect  in  the  obtuse  bisectrix  or  orthoaxis,  which 
is  fixed,  Fig.  359.  This  is  termed  horizontal  dispersion. 

3.  When  the  acute  bisectrix  coincides  with  the  orthoaxis. 
Now  the  interference  figure  will  lie  in  the  clinopinacoidal  section, 
and  will  revolve  around  the  acute  bisectrix  as  a  center.  The 
traces  of  the  plane  of  the  optic  axes  for  light  of  the  various  colors 
will  all  pass  through  the  fixed  point,  the  center  or  acute  bisec- 
trix, Fig.  360.  This  is  termed  crossed  dispersion. 

Dispersion  in  the   triclinic   system.  —  In  the  triclinic  system, 


FIG.  359.  —  Horizontal  Dispersion. 


OPTICAL  PROPERTIES   OF  CRYSTALS 


209 


FIG.  360.  —  Crossed  Dispersion. 


where  there  is  no  fixed  direction  to  which  any  of  the  axes  of  the 
indicatrix  must  conform,  it  is  possible  for  all  varieties  of  disper- 
sion described  in  the  other  systems  to  take  place  at  one  and  the 
same  time,  and  the  interfer- 
ence figure  may  be  entirely 
without  symmetry. 

Methods  of  determining 
the  indices  of  refraction.  — 
The  index  of  refraction  of 
any  substance  is  a  physical 
constant,  characteristic  of 
the  substance.  It  not  only 
serves  as  a  means  of  identi- 
fication, but  also  as  a  meas- 
ure of  purity.  The  value 
of  the  index  of  refraction 
varies  with  the  temperature, 
but  this  variation  in  case  of 
solids,  at  ordinary  temper- 
atures, is  small  and  within 
the  limits  of  error,  therefore  negligible.  The  index  of  refraction, 
when  all  precautions  and  when  great  care  are  taken,  together  with 
an  average  of  several  observations,  may  be  determined  within  .0002. 
The  value  of  the  index  of  refraction  for  minerals  will  lie  between 

1.3,  that  of  ice,  and 
3.08,  that  of  pyrar- 
gyrite. 

I.  The  most  ac- 
curate method  is  that 
in  which  the  angle  of 
deviation  of  the  re- 
fracted ray  is  ac- 
tually measured,  as 
transmitted  through 
a  prism  of  60°,  or 
one  not  varying  more 
than  5°  from  60°.  As  the  angle  of  deviation  will  vary  with  the 
inclination  of  the  ray,  the  angle  of  least  deviation  is  found  and 
measured  as  follows. 

In  Fig.  361  abc  is  the  prism,  with  the  angle  at  a  nearly  60°.     This 
angle  is  accurately  measured  with  the  goniometer.     The  ray  of 


FIG.  361. 


210  MIXKRALOGY 

light  RO  enters  at  O,  is  refracted  to  O',  and  is  again  transmitted,  on 
leaving  the  prism,  in  the  direction  of  O'R'.  The  angle  of  deviation 
due  to  the  prism  is  R'de  =  R'O'e'  =  8.  This  will  be  at  a  mini- 
mum, or  is  the  least  deviated,  when  the  ray  passes  through  the 
prism  symmetrically  as  drawn  in  the  figure. 

sin  * ;  i  =  ROn.     The  angle  bac  =  a;    dab  =  1/2  a  and  axO 


sin  r 


90° 


aOl  =  lO'a  =  90°.  VxOl  =  xaO  =  nOP  =  1/2  a  =  r,  the  angle  of 
refraction. 

Draw  O  V  parallel  to  Re,  then  from  the  symmetry  of  the  figure  the 
angle  e'O'R'  =  R'de  =  B  =  e'O'P  +  P'O'R',  POR  =  dOx  =  e'O'P'; 
also  dox  =  P'O'R'  =  POR  =  1/2  8,  but  NOP  =  xOl  =  1/2  a. 

i  =  NOR  =  1/2  a '  +  1/2  8  and  n  =  ^J  =  sin  1/2  (a  +  8)  ?  where 

sin  r  sin  1/2  a 

the  angle  a  is  carefully  measured  with  the  goniometer. 

The  angle  of  least  deviation  is  found  as  follows :  The  telescope  of 
the  goniometer  is  set  exactly  opposite  the  collimator  and  the  direct 
ray  through  the  Websky  slit  is  observed  and  adjusted  to  the  cross 
hairs,  when  a  reading  is  taken  with  everything  clamped. 

The  graduated  circle  and  telescope  remaining  clamped,  the 
crystal  is  mounted  and  adjusted  so  that  the  edge  at  the  angle  a 
is  parallel  to  the  vertical  hair.  It  is  then  pushed  in  with  the  screw, 
between  the  collimator  and  the  telescope,  until  the  image  of  the  slit 
disappears  on  looking  in  the  telescope,  the  base  of  the  prism 
being  to  the  left.  With  'the  graduated  circle  still  clamped,  the 
telescope  is  now  undamped  and  revolved  to  the  left  until  the  image 
or  signal  reappears,  then  the  prism  is  revolved  back  and  forth 
through  a  small  arc,  at  the  same  time  following  the  signal  with  the 
vertical  hair  of  the  telescope.  It  will  soon  be  seen  that  on  revolv- 
ing the  prism  there  is  a  maximum  position  to  the  right  for  the  sig- 
nal, and  having  reached  this  position,  even  though  the  prism  is  still 
revolved  the  same  way,  the  signal  moves  up  to  this  position  then 
reverses  its  motion  or  turns  back.  This  maximum  position  marks 
the  point  where  the  ray  is  symmetrical  to  the  prism  as  drawn  in  the 
diagram,  Fig.  361.  By  moving  both  the  telescope  and  prism  at 
the  same  time,  the  vertical  hair  is  brought  to  this  maximum  posi- 
tion of  the  signal  and  a  reading  taken.  The  difference  between  the 
original  position  of  the  signal  and  this  of  the  least  deviation  will 
be  the  angle  8.  When  white  light  is  used,  there  will  be  a  series  of 
colored  signals,  one  for  each  color,  due  to  dispersion,  but  only  one 


OPTICAL   PROPERTIES   OF   CRYSTALS  211 

image  when  monochromatic  light  is  used.  Having  measured  the 
angle  of  least  deviation,  the  index  of  refraction  is  obtained  from  the 
formula  above.  In  all  isotropic  substances  n  will  be  of  the  same 
value,  whatever  the  relation  of  the  edge  of  the  prism  to  the  crystal 
may  be.  This  is  not  the  case  in  anisotropic  substances,  where 
the  prism  edge  must  be  cut  with  a  definite  relation  to  the  axes  of  the 
indicatrix. 

In  the  tetragonal  and  hexagonal  systems,  the  edge  of  the 
prism  containing  the  angle  a  should  be  cut  parallel  to  the  base, 
and  the  plane  bisecting  the  angle  parallel  to  the  vertical  axis,  or  the 
prism  edge  containing  a  may  be  cut  parallel  to  the  vertical  axis.  In 
either  case,  in  measuring  the  angle  of  least  deviation,  two  signals  will 
appear,  one  caused  by  the  ordinary  ray,  the  other  by  the  extraordi- 
nary ray.  These  two  readings  substituted  in  the  formula  will 
yield  two  indices  of  refraction,  one  that  of  the  ordinary  ray  co,  the 
other  that  of  the  extraordinary  ray  €. 

In  biaxial  crystals,  where  there  are  three  indices  of  refraction 
to  be  determined,  two  prisms  are  necessary.  One  must  be  cut 
with  the  edge  containing  the  angle  a  parallel  to  an  axis  of  the  in- 
dicatrix, and  the  plane  of  symmetry  of  the  indicatrix  containing  this 
edge  of  the  prism  must  also  bisect  the  angle  a.  The  second  prism 
must  be  cut  in  the  same  relation  to  a  second  axis  and  plane  of  the 
indicatrix.  Each  of  these  prisms  will  yield  two  signals  as  in  the 
uniaxial  prism  and  therefore  two  indices  of  refraction.  One  prism 
will  yield  a  and  p,  the  other  p  and  -y-  The  index  repeated  or 
determined  in  both  prisms  will  depend  upon  the  axes  of  the  in- 
dicatrix to  which  the  edges  of  the  prisms  are  parallel. 

II.  Method  of  total  reflection.  —  Fig.  362  is  a  diagrammatic 
section  of  the  Abbe  Total  Reflectometer,  in  which  C  is  a  hemisphere 
of  Jena  flint  glass,  having  an  index  of  refraction  n  =  1.8904,  the 
upper  surface  of  which,  b,  is  polished  to  a  true  plane  passing  through 
the  center  of  the  sphere  and  adjusted  so  as  to  pass  through  the 
axis  of  the  vertical  graduated  circle  from  which  the  readings  are 
taken  and  which  is  not  represented  in  the  diagram.  A  polished 
section  S  is  cut  parallel  to  a  plane  of  symmetry  of  the  indicatrix. 
Often  a  cleavage  surface  will  fill  this  requirement.  A  small  drop  of 
a  highly  refracting  liquid,  usually  methylene  iodide,  n  =  1.742, 
having  first  been  placed  on  the  center  of  the  plane  b,  then  the  pol- 
ished face  of  the  section  S  is  placed  gently  on  the  hemisphere  with 
a  thin  film  of  the  highly  refracting  liquid  separating  the  two  sur- 
faces. M  is  a  mirror  which  reflects  light  in  the  required  direction. 


212 


MINERALOGY 


The  limiting  ray  tO,  striking  the  surface  of  the  section  at  O,  is 
totally  reflected  in  the  direction  Ot'.  The  angle  t'Oe  is  the  critical 
angle.  Any  ray,  as  mO,  within  this  angle  will  at  the  point  O  be  re- 


FIG. 362. 

fracted  and  pass  out  in  the  direction  Ot",  and  some  will  be  reflected 
within  the  angle  t'Oe,  and  the  field  t'Oe  will  be  semi-illuminated. 
Any  ray,  as  8O,  striking  the  plane  surface  s  at  an  angle  greater 

than  the  critical  angle 
eOt',  will  be  totally 
reflected  in  the  di- 
rection of  O8'  and 
the  field  a'Ot'  will 
be  fully  illuminated. 
At  the  boundary 
between  these  two 
fields,  t'O  will  be 
marked  by  a  shadow. 
The  field  of  the  tele- 
scope, represented  by 
the  circle  F,  is  brought 
with  the  cross  hairs 
to  the  shadow,  and 

a  reading  which  gives  the  critical  angle  eOt'  is  taken.  Then 
n  =  N  sin  r,  where  N  is  the  index  of  refraction  of  the  glass  hemi- 
sphere and  r  is  the  critical  angle. 

This  method  has  the  advantage  of  the  possibility  of  determining 
all  three  indices  of  refraction  in  one  and  the  same  "section.     As 


OPTICAL  PROPERTIES  OF  CRYSTALS  213 

one  ray  is  constant,  the  shadow  caused  by  it  will  not  change  on 
completely  rotating  the  hemisphere  with  the  section.  The  other 
ray  will  increase  from  a  minimum  to  a  maximum  according 
to  the  position  of  the  section;  and  these  two  limiting  values 
will  represent  the  critical  angles  for  the  other  two  indices  of 
refraction.  The  telescope  is  fitted  with  a  nicol  so  that  light 
vibrating  only  in  the  plane  required  may  pass  and  illuminate 
the  field. 

The  ray  may  also  be  adjusted  to  enter  the  section  as  represented 
in  Fig.  363,  and  the  field  a'Ot'  will  be  entirely  dark,  and  the  field 
t'Oe  will  be  illuminated  and  the  contrast  between  the  two  fields 
will  be  greater. 

III.  A  convenient  refractometer,  as  constructed  by  Herbert 
Smith  of  the  British  Museum  and  illustrated  in  Fig.  364,  is  so  ar- 


FIG.  364.  —  Refractometer. 

ranged  that  the  index  of  refraction  may  be  read  directly  from  a 
scale  in  the  instrument  to  the  second  decimal  place  and  the  third 
estimated.  The  specimen  is  placed  on  the  highly  refracting  glass 
with  a  film  of  liquid  between,  as  in  the  Abbe  instrument.  The  light 
entering  at  O,  the  shadow  is  thrown  on  the  scale  and  the  reading 
taken.  In  double  refracting  substances  two  shadows  will  be  noted, 
indicating  the  two  indices  of  refraction ;  the  section  is  revolved  until 
these  are  maximum  or  minimum  values.  This  instrument  is  very 
convenient  for  jewelers  in  the  determination  of  the  refraction  and 
identification  of  cut  stones. 

IV.  Cleavage  pieces  of  transparent  minerals  may  be  used  to 
determine  the  index  of  refraction  with  the  microscope.  The  thick- 
ness of  the  section  S,  Fig.  365,  is  measured  with  a  micrometer  cali- 


214 


MINERALOGY 


pers  =  T.  A  slide  with  a  fine  scratch  is  placed  on  the  stage  and 
the  microscope  very  carefully  focused  on  the  scratch  o.  Then  the 
mineral  section  is  placed  over  the  scratch.  The  scratch  will  now 
not  be  in  focus,  but  will  again  come  in  focus  by  turning  the  fine 

adjustment  of  the  microscope,  so 
as  to  lift  the  objective;  o  will 
appear  as  if  at  o';  the  distance 
oo'  which  the  scratch  seems  to 
have  been  lifted  will  depend  upon 
^ —  the  thickness  of  the  section  and 

^/ 

FIG  365  the  index  of  refraction  of  the  min- 

eral.    The  apparent   thickness  of 

the  section  do'  is  determined  by  measuring  the  distance  oo'  with 
the  micrometer  of  the  microscope,  and  subtracting  oo'  from  T. 


Then,n  = 


or 


The  actual  thickness 


,  for  n  = 


sin  i      tan  i 
sin  r      tan  r 


T  — oo'         The  apparent  thickness 
— -f,  as  for  these  small  angles  the  ratio  of  the  tangents  is 

practically  the  same  as  that  of  the  sines.  This  method  is  only  an 
approximate  one  and  thick  sections  must  be  used.  Ordinary  rock 
sections  are  too  thin  for  accuracy. 

V.  A  convenient  method  of  determining  the  index  of  refraction  of 
minerals  in  powder  or  very  small  crystals  is  to  mount  them  on  a 
slide  in  fluids  of  different  indices  of  refraction.  A  list  of  such  fluids 


FIG.  366.  —  Section  of  a  Quartzite,  showing  the  Low  Relief.     Crossed  Nicols. 

which  cover  the  range  of  the  indices  of  refraction  of  most  important 
rock-forming  minerals,  as  given  by  P.  E.  Wright,  are  :- 


OPTICAL  PROPERTIES  OF  CRYSTALS 


215 


1.450-1.475  .     .     Mixtures  of  petroleum  and  turpentine. 

1.480-1.535  .     .     Turpentine  and  ethylene  bromide. 

1.540-1.635  .     .     Clove    oil    and    «-monobromnaphthalene    and 

a-monochlornaphthalene. 

1.660-1.740  .     .     a-monobromnaphthalene  and  methylene  iodide. 
1.745-1.790  .     .     Methylene  iodide  and  sulphur. 

When  the  index  of  refraction  of  the  crystal  fragments  are  the 
same  as  the  fluid  in  which  they  are  mounted,  the  outlines  of  the 
fragments  are  very  indistinct  and  scarcely  visible.  This  is  well 
illustrated  by  the  glass  rod  in  the  Canada  balsam  bottle,  the  indices 
of  which  are  nearly  the  same,  and  the  rod  is  scarcely  visible.  Quartz 
mounted  in  balsam  is  also  indistinct,  as  the  indices  co  =  1.544  and 
€  =  1.553,  while  for  balsam  n  =  1.548,  just  between.  In  a  section 
of  quartzite  where  the 
grains  are  all  the  same 
color,  their  outlines  will 
be  invisible  in  ordinary 
light ;  but  in  crossed  nic- 
ols,  where  they  will  be 
differently  illuminated, 
they  are  quite  distinct, 
Fig.  366.  The  outlines 
of  mineral  fragments  are 
more  marked  or  distinct, 
the  greater  the  difference 
between  the  indices  of 
refraction  of  the  two 
substances.  In  calcite, 
where  €  =  1.486  and  co  = 
1.658,  mounted  in  balsam  the  outlines  are  distinct  and  the  surface 
of  the  section  is  rough,  Fig.  367.  It  is  said  to  have  a  marked  or 
high  relief.  With  the  polarizer  only  in,  if  the  section  is  revolved 
through  90°,  the  relief  for  the  two  rays  will  be  noted  to  be  quite 
different,  as  the  difference  in  one  case  is  .11  and  in  the  other 
.062.  The  relief  then  is  a  measure  of  the  difference  of  the  index 
of  refraction  of  the  mineral  and  the  index  of  refraction  of  the 
medium  in  which  the  section  is  mounted. 

VI.  Becke's  method,  or  the  determination  of  the  relative 
values  of  the  indices  of  refraction  of  two  adjacent  minerals  in  the 
same  section.  Where  the  index  of  refraction  of  one  is  known,  that 


FIG.   367. — Section  of  Calcite  showing   Marked 
Relief  and  Rhombohedral  Cleavage. 


216  MINERALOGY 

of  the  other  may  be  approximately  estimated .  This  method  depends 
upon  the  principle  that  light  passing  from  a  rarer  medium  or  one 
with  a  lower  index  of  refraction  to  one  with  a  higher  index  of  re- 
fraction will  pass  at  all  angles  and  there  will  be  no  critical  angle ; 
but  in  the  reverse  direction  there  will  be  a  critical  angle  and  some  of 
the  light  is  reflected  to  the  side  of  the  higher  index  of  refraction. 
The  illumination  on  the  side  of  the  higher  index  of  refraction  will 
be  brighter,  as  some  of  the  rays  are  totally  reflected  to  that  side. 

As  an  example  the  index  of  refraction  of  quartz  and  orthoclase 
in  a  section  of  granite  may  be  tested  in  this  way.  The  effect  is 
best  seen  with  a  medium  high  objective  and  with  a  converging  light, 
the  light  being  diaphragmed  off  so  as  to  illuminate  only  a  small  area. 
If  the  microscope  is  carefully  focused  on  the  boundary  between 
the  two  minerals,  then  the  objective  slowly  lifted,  a  line  of  light 
parallel  to  the  line  of  contact  will  appear  to  move  in  the  direc- 
tion of  the  mineral  with  the  higher  index  of  refraction,  or  on  the 
side  of  quartz;  the  difference  in  this  case  on  the  average  is  .02.  With 
careful  work  and  experience  a  difference  of  .001  may  be  detected  by 
this  method. 

Study  of  minerals  in  rock  sections.  —  In  the  study  of  a  mineral 
section  or  of  minerals  in  rock  sections,  an  order  of  observation 
should  always  be  followed.  The  section  should  be  carefully 
studied  in  all  parts  with  the  low  power  adjective  in  order  to  de- 
termine the  relative  abundance  of  each  mineral  species,  their  rela- 
tions and  relative  size,  and  favorable  sections  should  be  chosen  for  the 
optical  observations,  then  the  following  order  should  be  followed : 

1 .  Color.  —  When  the  mineral  is  opaque,  it  is  observed  in  reflected 
light,  with  the  mirror  under  the  stage  turned  off.     Minerals  which 
are  opaque  in  thin  sections  are  of  metallic  luster,  as  magnetite, 
pyrite,  pyrrhotite,  or  chromite.     Transparent  sections  are  observed 
in  transmitted  light  and  the  color  noted ;  whether  evenly  colored  or 
irregular,  caused  by  a  difference  in  chemical  composition  or  due  to 
inclusions,  cavities,  etc. 

2.  Form. —  The  outline  of  each  individual  species  is  noted  if 
bounded  by  straight  lines,  i.e.  crystal  faces  well  developed,  the 
individuals  being  euhedral  or  idiomorphic,  in  contrast  to   those 
irregular    in  outline,   which   have  no   well-defined   crystal  faces 
developed  and  are  anhedral  or  allotriomorphic ;  such  irregular  in- 
dividuals usually  act  as  a  matrix,  filling  the  cavities  between  the 
minerals  of  earlier  crystallization,  which  are  often  of  larger  size, 
with  distinct  outlines  or  phenocrysts. 


OPTICAL   PROPERTIES   OF  CRYSTALS  217 

3.  Cleavage   cracks,  partings,  and   fractures  due  to  pressure  or 
strain  should  be  noted  and  the  angles  between  two  well-defined 
cleavage  directions,  where  they  occur,  measured  in  a  number  of 
sections,  in  order  to  obtain  the  true  angle  or  that  measured  at  90° 
to  the  intersection  of  the  two  cleavage  planes. 

4.  Note  the  crystal  outlines  where  well  developed  and  where 
elongated ;  the  direction  of  the  elongation  in  regard  to  the  crys- 
tallographic  axes  is  determined,  also  any  irregularity  of  the  outline, 
as  that  due  to  corrosion,  resolution,  alteration,  or  weathering,  and 
whether  these  changes  are  restricted  to  the  surface  or  have  fol- 
lowed cleavage  cracks  and  fractures ;  also  the  nature  of  the  alter- 
ation product  is  noted. 

5.  Index  of  refraction.  —  Rock  sections  are  usually  mounted  in 
Canada  balsam,  the  index  of  refraction  of  which  is  approximately 
1.539,  but  varies  slightly  with  the  amount  of  solvent  it  contains  or 
with  the  age  of  the  mounted  section,  as  the  balsam  is  constantly 
hardening  with  age.     Specimens  with  an  index  of  refraction  near 
that  of  balsam  will  have  little  or  no  relief ;  their  surfaces  will  appear 
flat  and  smooth.  It  is  well  to  have  several  mineral  sections  mounted 
for  comparison,  the  index  of  each  being  known ;  their  relief  may  be 
compared  with  the  unknown  section  and  the  index  of  refraction 
approximately    determined.     Sodalite,     1.483;     leucite,     1.508; 
orthoclase,   1.523;    quartz,   1.547;    beryl,   1.584;    olivine,   1.670; 
and  rutile,  1.712,  —  are  good  minerals  for  comparison.     A  specimen 
with  an  index  of  refraction  above  1.60  or  below  1.50  will  have  a 
rather  high  relief.     The   cracks,    as  cleavage,   scratches   on   the 
surface  made  in  grinding  the  section,  and  the  outline,  —  all  will 
appear  well  marked  and  distinct.     Whether  the  refraction  is  above 
or  below  1 .549,  that  of  balsam,  can  be  determined  by  Becke's  method. 
Minerals  with  a  high   relief  seem   particularly  rough  when  the 
section  is  shaded  from  reflected  light  by  passing  the  hand  up  and 
down  in  front  of  the  microscope. 

6.  In  crossed  nicols  and  parallel  light.  —  If  the  section  remains 
dark  between  crossed  nicols  when  the  stage  is  revolved,  it  is  either 
amorphous,  as  glass,  isometric,  or  a  double  refracting  mineral  cut 
perpendicular  to  an  optic    axis.     An  anisotropic  section  between 
crossed  nicols  will  yield  an  interference  color  which  will  be  a  measure 
of  its  double  refraction  and  the  section  will  extinguish  every  90°, 
that  is,  each  time  the  vibration  planes  of  the  section  are  parallel  to 
the  cross  hairs  of  the  eyepiece. 

7.  The  angle  of  extinction  is  measured  in  reference  to  the  vertical 


218  MINERALOGY 

axis,  as  determined  in  the  section  by  the  cleavage,  outline,  or  elon- 
gation. Whether  it  is  parallel  or  inclined  is  noted  and  the  angle 
of  extinction  measured,  also  the  twinning  of  the  section  may  be 
seen  in  relation  to  the  extinction  angles. 

8.  Pleochroism.  —  This  is  caused  by  the  light  being  absorbed 
along  one  plane  of  vibration  in  the  section  differently  from  that  along 
the  other.     The  section  is  turned  on  the  microscopic  stage  until 
extinction,  when  the  vibration  planes  of  the  section  will  be  parallel 
to  those  of  the  nicols.     The  analyzer  is  pushed  out  of  the  line  of 
view,  when  the  section  will  be  illuminated  by  the  rays  vibrating 
parallel  to  the  one  plane  of  vibration  in  the  section  only,  that  of  the 
polarizer,  usually  running  from  right  to  left.     The  color  of  the 
section  and  the  degree  of  illumination  are  both  noted,  as  is  also  the 
relief  of  the  section ;  then  the  section  is  revolved  on  the  stage  90°, 
when  the  light  will  be  vibrating  parallel  to  the  second  plane  of  vi- 
bration of  the  section,  and  any  change  in  color,  shade,  or  relief  is 
noted. 

9.  In  crossed  nicols  and  converging  light.  —  All  sections  of  the 
mineral  under  observation  in  the  specimen  are  carefully  examined, 
and  one  selected  as  nearly  perpendicular  as  possible  to  the  optic 
axis,  in  uniaxial  crystals,  and  to  the  acute  bisectrix,  in  biaxial  crys- 
tals, is  chosen.     The  interference  figure  is  observed  and  the  uniaxial 
or  biaxial  character  of  the  crystal  noted,  as  well  as  the  approximate 
axial  angle  in  the  latter. 

10.  When  uniaxial,  the  optic  sign  is  determined  with  the  mica 
plate ;  and  when  biaxial,  with  the  quartz  wedge.     Where  the  inter- 
ference figure  is  well  formed,  the  character  of  the  dispersion  may 
also  be  noted. 


PART   II 

CHAPTER  I 
THE    RELATION    OF    MINERALS    TO    THE   ELEMENTS 

MINERALS,  when  considered  from  a  chemical  standpoint,  are 
either  elements  in  the  uncombined  state  or  are  combinations  of 
elements  which  have  been  brought  together  during  the  past  ages 
and  united  by  chemical  forces  which  differ  in  no  way  from  the 
same  forces  with  which  we  have  become  acquainted  in  the  labora- 
tory. Some  mineral  molecules  are  simple  combinations  of  a  few 
chemical  elements ;  and  the  same  compounds  are  daily  produced 
in  the  laboratory,  in  the  simple  processes  of  chemical  analyses,  as 
in  the  precipitation  of  calcium  carbonate  from  a  solution  of  a  soluble 
calcium  salt  with  an  alkali  carbonate.  If  this  is  carried  out  at 
room  temperatures  and  the  precipitate  is  allowed  to  stand,  it  will 
•crystallize,  forming  calcite.  If  the  same  precipitation  is  carried 
out  at  temperatures  near  the  boiling  point  of  water,  that  is,  on  the 
water  bath,  and  allowed  to  crystallize  in  a  hot  solution,  the  form 
aragonite  will  be  produced.  Here  are  two  minerals,  calcite  and  ara- 
gonite,  one  crystallizing  in  the  hexagonal  system,  the  other  in  the 
orthorhombic  system.  They  are  both  calcium  carbonate,  and 
chemically  they  are  identical ;  but  physically  they  are  different,  and 
as  in  the  precipitation  they  are  formed  under  different  conditions, 
they  are  two  phases  of  the  same  chemical  substance;  moreover, 
calcium  carbonate  directly  after  a  rapid  precipitation  is  amorphous, 
another  phase,  and  becomes  crystalline  only  after  standing.  This 
property  of  chemical  compounds,  of  occurring  in  different  physical 
forms  is  known  as  polymorphism.  Calcium  carbonate  is  thus  tri- 
morphic.  It  may  be  amorphous,  it  may  possess  the  molecule  of 
calcite,  or  it  may  possess  the  molecule  of  aragonite.  Silica,  SiO2, 
is  said  to  possess  six  different  forms  or  phases. 

The  various  forms  of  a  polymorphic  substance  will  never  under 
the  same  conditions  possess  the  same  degree  of  equilibrium,  and 
one  form  will  be  more  stable  than  the  others.  It  is  always  the 

219 


220  MINERALOGY 

tendency  of  one  form,  the  less  stable,  to  pass  over  to  the  other,  the 
more  stable  form,  under  the  fixed  conditions.  Calcium  carbonate 
when  quickly  precipitated  from  solution  separates  as  an  amorphous 
solid,  and  on  standing  passes  over  to  the  more  stable  crystalline 
forms,  calcite  or  aragonite,  according  to  the  temperature  of 
the  solution,  as  calcite  is  the  more  stable  form  in  cold  solutions  and 
aragonite  is  the  more  stable  form  in  hot  solutions.  When  the  change 
of  phase  proceeds  in  one  way  only,  the  compound  is  said  to  be  mono- 
tropic,  but  if  the  change  may  go  back  and  forth  with  the  change  of 
temperature,  or  is  reversible,  the  compounds  or  phases  are  said  to 
be  enantiotropic.  Such  a  case  is  represented  by  sulphur.  Sulphur 
when  fused  and  then  allowed  to  solidify  at  a  temperature  above  96° 
will  form  monoclinic  crystals,  which  are  the  stable  phase  between 
96°  and  120°,  the  fusing  point  of  sulphur.  Below  96°  these  mono- 
clinic  crystals  become  brittle  and  clouded  and  pass  over  to  ortho- 
rhombic  sulphur,  a  phase  more  stable  than  the  monoclinic  form  at 
low  temperatures.  If  the  temperature  is  again  raised  above  96°,  the 
reverse  of  this  will  take  place  and  in  time  the  orthorhombic  sulphur 
will  form  monoclinic  crystals.  The  orthorhombic  phase  is  the 
more  stable  below  96°,  and  the  monoclinic  phase  is  the  more  stable 
above  96°.  There  are  several  other  phases  of  sulphur,  but  at  ordi- 
nary temperatures  these  are  all  less  stable  than  the  orthorhombic 
phase,  and  it  is  for  this  reason  that  all  natural  occurring  sulphur  is 
of  the  orthorhombic  phase.  Graphite  and  diamond  are  different 
phases  of  the  same  element,  carbon.  There  are  numerous  other 
examples  of  dimorphism  in  minerals,  as  sphalerite  and  wurtzite; 
quartz  and  tridymite ;  smaltite  and  safflorite ;  pyrite  and  marcasite. 
Pyroxenes  and  amphiboles  are  probably  dimorphous  conditions  or 
phases  of  the  same  compound,  and  even  trimorphic  cases  occur, 
as  in  the  three  minerals  rutile,  anatase,  and  brookite,  all  three  of 
which  are  different  phases  of  TiO2. 

Source  of  the  elements,  —  Minerals  are  the  source  of  all  the  ele- 
ments ;  it  has  been  through  the  chemical  study  and  the  analyses  of 
minerals  that,  with  few  exceptions,  all  the  elements  have  been  dis- 
covered. Those  elements  which  occur  in  nature  in  the  uncombined 
state  as  minerals  are  few  in  number  and  are  restricted  principally 
to  the  group  known  as  metals ;  as  platinum  and  the  platinum  group, 
silver,  mercury,  gold,  copper,  lead,  bismuth,  iron,  arsenic,  and  anti- 
mony. Some  of  these,  as  iron,  lead,  antimony,  arsenic,  and  bis- 
muth, are  rare  as  native  elements,  though  quite  common  enough  as 
constitutents  of  minerals  when  combined  with  other  elements. 


THE  RELATION  OF  MINERALS  TO  THE  ELEMENTS    221 

The  non-metallic  elements  sulphur  and  carbon  occur  in  the  uncom- 
bined  state  in  nature  in  considerable  quantities. 

Of  the  eighty  established  elements,  oxygen  far  surpasses  any 
other  in  its  wide  distribution,  forming  one  fifth  of  the  atmos- 
phere, eight  ninths  of  the  water,  and  from  45  to  50  per  cent,  of 
the  earth's  crust.  The  results  of  careful  calculations  indicate 
that  the  amount  of  oxygen  is  hardly  equaled  by  all  the  other  ele- 
ments taken  together,  or  oxygen  forms  about  50  per  cent,  of  the  earth 
as  known  by  man.  Oxygen  enters  the  composition  of  a  large  num- 
ber of  minerals  as  an  important  factor.  Of  the  other  elements  there 
are  seven,  silicon,  aluminium,  iron,  calcium,  magnesium,  sodium,  and 
potassium,  in  their  order  of  abundance,  each  of  which  composes  at 
least  2  per  cent,  of  the  earth's  crust,  and  they  are  universally  dis- 
tributed. The  above  eight  elements  compose  at  least  97  per  cent, 
of  the  earth  as  known  to  man.  Metals  to  which  we  have  become 
accustomed,  through  their  use  in  our  daily  life,  thinking  of  them  as 
common  elements,  as  copper,  lead,  zinc,  silver,  or  gold,  occur  only 
in  restricted  localities;  and  owing  to  their  commercial  value 
the  minerals  containing  them  have  been  mined,  with  a  constant 
accumulation  of  the  metal  reduced.  Elements  such  as  titanium, 
thorium,  cerium,  tungsten,  uranium,  and  molybdenum,  which  even 
the  chemist,  in  the  past,  considered  very  rare,  are  at  the  present 
time  becoming  well  known  in  the  commercial  world,  from  recently 
discovered  uses  to  which  their  properties  adapt  them.  These 
rare  elements  are  important  constituents  of  but  comparatively  few 
minerals,  and  these  are  usually  restricted  to  localities  where  many 
of  them  occur  associated  together. 

Such  localities  are  constantly  being  searched  for  by  the  pros- 
pector, urged  on  by  the  constant  demand  and  increase  in  price, 
as  several  of  these  rare  elements  have  been  proven  useful  in  the 
production  of  special  steels,  in  the  manufacture  of  lighting  man- 
tles, and  in  incandescent  lamp  filaments. 

At  the  present  time  salts  of  some  rare  earths  are  being  produced 
by  the  ton  as  by-products,  which  had  chemists  wished  to  secure 
in  pound  lots  only,  it  would  a  few  years  ago  have  been  impossible. 
Of  the  most  important  elements  the  following  list  includes  those 
which  are  found  in  the  earth's  crust  in  amounts  exceeding  .02 
per  cent. : 

Oxygen  49.98       Sodium  2.28      Phosphorus        .09 

Silicon  27.21       Potassium       2.23       Manganese        .07 


222  MINERALOGY 

Aluminium  7.26  Hydrogen  .94  Barium  .05 

Iron  5.08  Titanium  .41  Sulphur  .04 

Calcium  3.51  Carbon  .22  Nitrogen  .02 

Magnesium  2.50  Chlorine  .15  Strontium  .02 

Chromium,  fluorine,  lithium,  and  uranium  are  all  less  than  .02  and 
probably  greater  than  0.01 ;  bromine  and  all  other  elements  are 
each,  in  turn,  less  than  .01. 

In  a  classification  of  the  minerals  their  most  important  charac- 
teristics must  be  considered,  and  the  starting  point  of  a  natural- 
classification  is  without  doubt  one  in  which  the  chemical  composi- 
tion and  physical  properties  are  the  most  important  consideration, 
though  the  latter  to  a  large  extent  are  derived  from  the  elements 
a  mineral  contains.  Those  minerals  which  are  composed  largely 
of  the  same  elements  should  stand  in  any  scheme  of  classification 
near  together,  especially  since  mineral  species  are  mostly  deter- 
mined by  the  chemical  tests  for  the  elements  which  they  contain. 
The  classification  of  minerals  will  follow  closely  the  natural  classi- 
fication of  the  elements  themselves.  It  has  been  shown,  especially 
by  Mendeleef,  that  the  properties  of  the  elements  are  determined 
by  their  atomic  weights,  and  from  a  consideration  of  this  fact  a 
natural  classification  of  the  elements  has  been  adopted  which  places 
the  elements  in  the  order  of  their  atomic  weights. 

In  a  list  of  the  elements  placed  in  the  order  of  their  atomic 
weights,  starting  with  lithium,  the  first  element  which  possesses  a 
well-developed  chemical  character,  the  atomic  weight  increases  un- 
til sodium  is  reached,  with  an  atomic  weight  of  23.  Sodium  is  an 
element  very  similar  to  lithium  in  its  chemical  properties  and  very 
different  from  fluorine,  with  an  atomic  weight  of  19,  directly  pre- 
ceding it.  Sodium  is  then  written  in  the  column  with  lithium  and 
a  second  horizontal  line  or  group  is  started. 

In  each  case,  elements  having  similar  or  related  chemical  prop- 
erties will  fall  in  the  same  column,  and  these  properties  will  be 
repeated  periodically.  This  whole  scheme  is  known  as  the  periodic 
classification  of  the  elements. 

Elements  falling  under  each  other  in  the  same  column  of  the  table 
of  elements  are  of  the  same  valence  and  are  capable  of  replacing 
each  other  in  mineral  molecules  to  a  large  extent,  though  this  prop- 
erty may  not  extend  from  top  to  bottom  of  the  column  in  all 
groups.  In  the  first  column  or  group,  lithium,  potassium,  rubid- 
ium, and  caesium  fall  directly  under  each  other.  These  are  all  alkali 


THE   RELATION   OF  MINERALS  TO  THE  ELEMENTS    223 


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224  MINERALOGY 

metals,  strong  bases,  and  only  take  the  part  of  a  base  when  com- 
bining with  other  metals  or  acids  to  form  salts.  The  solubility  of 
their  salts  is  graded  and  decreases  from  the  top  to  bottom,  or  from 
lithium  to  caesium.  Similar  salts  crystallize  in  the  same  system 
and  usually  with  the  same  symmetry,  and  with  angles  and  ele- 
ments very  closely  related.  While  the  alkali  metals  form  a  very 
closely  related  group  of  elements,  possibly  more  closely  related  than 
some  other  groups  of  the  table,  it  is  true,  however,  that  those  ele- 
ments falling  under  each  other  in  the  vertical  columns  are  very 
closely  related  in  their  chemical  properties.  In  a  considera- 
tion of  those  elements  which  fall  in  the  continuous  line,  as  from 
lithium  to  fluorine,  there  is  a  continuous  gradation  from  the  very 
basic,  on  the  one  hand,  metals  which  are  alwaj^s  found  taking  the 
part  of  a  base  in  the  formation  of  salts,  to  those  which,  on  the  other 
hand,  as  fluorine,  are  always  found  taking  the  part  of  an  acid 
when  forming  salts.  Between  these  two  extremes  there  are  ele- 
ments which  may  take  the  part  either  of  an  acid  or  a  base ;  this  is 
particularly  true  of  those  elements,  like  aluminium,  which  are 
found  near  the  middle,  between  the  two  extremes. 

It  has  long  been  recognized  that  all  salts  were  composed  of  two 
distinct  components,  the  acid  radicle,  or  acid  anhydride,  and  the 
base,  or  oxide  of  the  metal.  This  was  Berzelius's  conception  of  a 
salt,  and  the  formulae  of  minerals  were  formerly  written  with  this 
conception  in  view,  as  barite,  BaO,  S03.  Here  BaO  is  the  oxide 
of  the  base,  barium,  and  SO3  is  the  oxide  of  the  acid-forming  ele- 
ment, sulphur.  Indeed  at  the  present  time  the  analyses  of  rocks 
and  minerals  are  still  reported  in  this  form  as  BaO  =  65.7  and 
SO3  =  34.3  per  cent. 

By  the  study  of  the  behavior  of  salts,  or  more  especially  elec- 
trolytes, in  solution  and  under  the  influence  of  an  electric  current, 
it  has  been  demonstrated  that  salts  do  not  break  down  into  the  two 
components  of  a  basic  oxide,  as  BaO,  on  the  one  hand,  and  acid 
oxide,  as  S03,  on  the  other,  but  they  are  dissociated  into  ions,  inde- 
pendent components,  one  of  which  is  formed  by  the  metal  Ba  or  a 
complex  like  NH4,  taking  the  place  of  a  metal,  charged  with  a  posi- 
tive charge  of  electricity  and  traveling  toward  the  negative  pole ; 
these  are  termed  cations.  The  anion  is  that  component  which 
carries  a  negative  charge,  in  this  case  SO4,  and  travels  toward  the 
positive  pole  oranode.  Barium  sulphate  thus  breaks  up  in  the  two 
ions  Ba  and  SO4. 


THE  RELATION   OF  MINERALS  TO  THE  ELEMENTS    225 

In  the  classification  of  minerals  the  acid  radicle  has  been  chosen  as 
the  most  important  of  the  two  components,  and  all  minerals  occur- 
ring as  sulphates,  or  the  salts  of  sulphuric  acid,  have  been  placed  in 
one  group. 

Water  in  minerals.  —  Some  minerals,  especially  those  which 
are  formed  at  a  low  temperature,  or  which  separate  from  a  water 
solution,  contain  water,  or  at  least  when  heated  in  a  closed*  tube 
they  yield  water.  This  water  may  be  combined  with  the  mineral 
molecule  in  various  ways,  as  is  indicated  by  the  widely  different 
temperatures  at  which  it  is  driven  off.  Water  which  is  driven  off 
at  low  temperatures  is  considered  to  be  loosely  combined  with  the 
other  elements  of  the  molecule  or  with  the  mineral  molecule.  It 
may  be  directly  combined  with  the  mineral  as  water,  though  there 
is  nothing  to  prove  that  this  is  actually  the  condition  or  fact. 

Copper  sulphate  crystallizes  with  five  molecules  of  water, 
CuS04,  5  H2O.  When  this  salt  is  heated,  four  of  these  molecules  of 
water  are  driven  off  at  100°  C.,  but  to  drive  off  the  fifth  molecule  of 
water  and  to  completely  dehydrate  the  salt  the  temperature  must 
be  raised  to  200°.  Zinc  sulphate  crystallizes  with  seven  molecules  of 
water,  ZnS04,  7  H20,  six  of  which  are  driven  off  at  100°  C.,  but  the 
seventh  will  not  be  driven  off  until  the  temperature  of  240°  C.  is 
reached.  Such  facts  would  indicate  that  that  portion  of  the  water 
contained  in  salts  which  may  be  driven  off  at  low  temperatures  is 
bound  up  in  the  mineral  molecule  in  some  very  simple  way,  and 
the  bond  is  easily  broken.  On  the  other  hand,  some  is  bound  up 
with  the  molecule  very  closely  and  may  be  dislodged,  as  is  the  case 
with  serpentine,  only  at  a  red  heat.  The  water  which  may  be 
driven  off  at  low  temperatures  is  termed  water  of  crystallization, 
and  is  usually  written  or  indicated  in  the  mineral  formula  as  water ; 
thus  gypsum,  CaSO4,  2H2O,  contains  two  molecules  of  water  of 
crystallization,  one  of  which  is  liberated  at  120°  C.,  the  other  at 
200°  C.  When  there  are  several  molecules  of  water  of  crystalliza- 
tion present  in  a  mineral,  they  are  not  all  liberated  at  the  same  tem- 
perature, but  they  are  given  off  by  steps  or  one  at  a  time.  The 
evolution  of  water  on  heating  a  crystal  is  discontinuous.  The 
zeolites  present  an  exception  to  this  rule ;  in  their  case  the  evolu- 
tion is  continuous.  Some  of  this  water  of  crystallization  may  be 
so  slightly  bound  to  the  chemical  molecule,  that  it  evaporates  into 
dry  air,  and  the.-  crystal  falls  down  to  a  powder,  or  whitens  and  is 
changed  in  appearance.  Such  compounds  are  said  to  be  efflores- 
cent. Sodium  carbonate,  NaaCOs,  10  H2O,  in  dry  air  will  lose 
Q 


226  MINERALOGY 

9  H20  and  form  a  white  powder,  Na-jCOa,  H2O.  In  certain  cases 
the  water  of  crystallization  when  driven  off  may  be  reabsorbed  from 
a  damp  air.  Gypsum,  CaSO4,  2  H2O,  when  heated  to  130°  C.  loses 
one  of  the  molecules  of  water  and  when  powdered  forms  plaster  of 
Paris ;  this  will  gradually  pass  back  to  CaSO4,  2  H2O  by  the  absorp- 
tion of  water  from  a  damp  atmosphere,  or  it  will  take  it  up  very 
quickly  when  mixed  with  water,  setting  into  a  hard  mass,  and  at 
the  same  time  evolving  heat  as  the  water  is  combined.  This  heat 
of  combination  will  vary  with  the  molecule  of  water  given  off  and 
is  the  greatest  in  case  of  the  last  molecule  to  be  liberated  from  a 
compound,  or  the  one  which  is  liberated  at  the  highest  temperature. 
The  heat  of  combination  is  a  measure  of  the  bond. 

Water  which  is  driven  off  at  high  temperatures  is  termed  water 
of  constitution,  and  its  relation  to  the  crystalline  molecule  is  usually 
considered  to  be  entirely  different  from  that  of  the  water  of  crystal- 
lization. This  difference  is  not  only  indicated  by  the  heat  of  com- 
bination, but  when  the  compound  is  dissolved,  the  water  of  consti- 
tution is  found  to  occupy  a  much  smaller  volume  than  the  water 
of  crystallization,  which  is  of  exactly  the  same  volume  as  the 
water  of  the  solvent,  or  the  water  in  which  the  salt  is  dissolved. 
Also  the  change  of  volume  in  case  of  fusion  is  much  smaller  with 
the  water  of  constitution. 

Water  of  constitution,  when  written  in  the  mineral  formula, 
to  indicate  this  difference,  is  incorporated  with  the  chemical 
elements,  and  is  not  written  as  water  after  the  formula,  as  is  the 
case  with  the  water  of  crystallization.  Epsomite,  MgSO4,  7  H20, 
will  lose  6  H20  at  100°  C. ;  the  seventh  molecule  is  not  separated 
until  210°  C.  is  reached.  The  last  molecule  must  be  combined 
in  some  other  or  different  way,  and  when  combining  with  mag- 
nesium sulphate  this  seventh  molecule  will  liberate  twice  as  much 
heat.  To  indicate  this  difference  in  bond  of  this  single  molecule 
of  water,  the  formula  of  magnesium  sulphate  may  be  written 
(Mg .  OH)HSO4,  6  H2O.  The  water  of  constitution  may  be  con- 
nected with  the  metal  or  base,  forming  basic  water  or  basic  salts, 
or  it  may  be  connected  with  the  acid,  forming  acid  salts. 

In  all  acids  the  acid  radicle  is  combined  with  hydrogen  as 

TT     _        /~\  ^ 

jj  _  Q  /SO2,  sulphuric  acid.  In  the  formation  of  salts  this 
acid  hydrogen  is  replaced  with  a  metal.  When  all  the  hydrogen  is 
replaced,  -^  ~  O^>SO2,  the  salt  is  said  to  be  normal,  or,  as  rep- 


THE  RELATION  OF  MINERALS  TO  THE  ELEMENTS  227 
resented  by  the  formula,  a  normal  sulphate.  When  only  one  of 
the  hydrogen  atoms  is  replaced,  jj  Z  Q  /  ^2>  an  ac^  sa^  *s  *  orined, 
acid  sodium  sulphate. 

<r\  _  TT 
O  —  H'   magnesium   hy- 

droxide; when  hydroxides  combine  with  acids  to  form  salts, 
all  of  the  hydroxyl  (OH)  may  be  replaced,  and  the  salt  will 
be  normal,  in  that  there  will  be  present  no  basic  water;  as 

{  |  ^SC>2,  normal  magnesium  sulphate  ;    or  again,  only  a  part 


of  the  hydroxyl  may  be  replaced,  iQH/^4'  ^as^c  maSnesmm 
sulphate.  Malachite  is  a  basic  copper  carbonate,  -u  _  Q  __  QU  /COs, 

in  which  one  bond  of  the  copper  is  taken  up  by  hydroxyl  and  the 
other  by  the  acid  radicle.  In  each  group  of  minerals,  as  the  sul- 
phates, there  are  possible  normal  anhydrous  salts  ;  normal  salts  with 
water  of  crystallization,  or  hydrous  salts  ;  basic  salts  ;  and  acid  salts  ; 
and  in  some  of  the  complex  mineral  molecules  there  may  be  hydrated 
and  acid  and  basic  water  present. 

It  is  not  always  possible  to  determine  the  structural  formula 
of  a  mineral,  or  to  tell  just  what  the  exact  relations  of  each  atom 
are.  The  empirical  or  percentage  formula  is  calculated  from  the 
results  of  analysis,  as  this  is  simply  the  reverse  of  the  calculation 
of  the  percentage  of  any  element  from  a  given  formula.  The 
percentage  of  each  oxide  is  divided  by  its  molecular  weight,  which 
will  give  the  ratios  of  the  various  oxides  in  the  formula;  thus 
in  case  of  the  sulphate  of  calcium,  the  mineral  gypsum  : 

RATIO 

CaO  =  32.5  -s-  56.1  =  .579  =  1 
SO3  =  46.6  •*-  80.6  =  .577  =  1 
H2O  =  20.9  *  18.0  =  1.15  =  2 

Dividing  the  ratios  by  the  least  of  their  number,  as  there  must  be 
whole  atoms  in  the  molecule,  we  obtain  for  CaO,  one  ;  for  SO3,  also  one  ; 
and  for  H2O,  two.  The  percentage  formula  will  be  CaO,  SO32H2O, 
or  CaSO4,  2  H2O.  In  the  mineral  and  crystalline  molecule  cer- 
tain groups  of  elements  have  been  found  able  to  replace  each  other, 
and  at  the  same  time  the  form  or  angle  of  the  crystals  will  be  but 
slightly  changed.  Such  groups  of  elements  are  very  closely  related 
to  each  other  in  several  ways.  Their  molecular  volumes  are  very 


228  MINERALOGY 

nearly  alike.  Their  crystals  when  pure  are  nearly  of  the  same 
angle.  Compounds  which  replace  each  other  are  usually  alike 
chemically  in  that  they  are  salts  of  the  same  acid,  as  carbonates, 
sulphates,  or  phosphates ;  but  this  is  not  always  true,  since  sodium 
nitrate,  NaNO3,  crystallizes  in  the  same  forms  and  nearly  the  same 
angles  as  calcite,  CaCO3.  Elements  or  compounds  which  replace 
each  other  in  the  crystalline  molecule  in  all  proportions  are  said  to 
be  isomorphous.  Formerly  they  were  thought  to  be  of  exactly 
the  same  form  and  angles,  but  at  the  present  time  it  is  thought 
that  it  is  only  necessary  for  the  angles  and  molecular  volume  to  be 
of  nearly  the  same  value.  This  is  well  illustrated  in  the  isomor- 
phous group  of  natural  carbonates: 

TAT        MOLECULAR  VOLUME 

Calcite  CaCO3  74°  55'  36.8 

Magnesite  MgCO3  72°  36'  27.8 

Siderite  FeCO3  73°  00'  30.3 

Rhodochrosite  MnCO3  73°  00'  31.9 

Smithsonite  ZnCO3  72°  20'  28.0 

The  molecular  volume  or  volume  of  the  unit  of  the  space  lattice 
is  found  by  dividing  the  molecular  weight  by  the  specific  gravity 
of  the  substance.  If  the  molecular  weights  of  crystalline  sub- 
stances were  the  same  and  they  differed  in  specific  gravity,  then  the 
same  volume  of  the  denser  substance  would  contain  more  molecules 
per  volume  than  the  less  dense  substance,  and  the  molecular  vol- 
ume or  the  relative  size  of  the  space-lattice  units  will  vary  inversely 
as  the  specific  gravity. 

Topic  parameters  represent  the  relative  distances  or  the  ratios 
of  the  distances  between  centers  of  the  simple  structural  units  of 
the  space- point-system,  measured  along  the  three  axial  directions. 
The  topic  parameters  are  functions  of  the  molecular  volumes  and 
the  axial  ratio  of  a  compound. 

The  crystalline  angles  will  vary  directly  with  the  composition; 
in  pure  calcite,  with  a  rhombohedral  angle  of  74°  55',  as  an  end  mem- 
ber of  a  series  in  which  pure  smithsonite,  with  a  rhombohedral  angle 
of  72°  "20',  is  the  other  end  member,  every  molecule  of  zinc  carbonate 
crystallizing  with  the  calcium  carbonate  will  have  the  effect  of 
proportionally  decreasing  the  angles  of  the  calcium  carbonate. 
The  amount  of  decrease  in  the  angles  will  be  directly  proportional 
to  the  percentage  of  zinc  carbonate  present.  If  there  is  no  car- 
bonate present,  other  than  zinc  carbonate  and  calcium  carbonate, 
in  the  crystal,  their  percentage  proportion  could  be  calculated  from 


THE  RELATION  OF  MINERALS  TO  THE  ELEMENTS    229 

a  measurement  of  the  angles.     The  angles  are  then  a  function  of 
the  chemical  composition. 

Mesitite  is  a  naturally  occurring  carbonate,  forming  crystals 
in  which  iron  and  magnesium  carbonates  have  crystallized  in  pro- 
portions of  two  of  magnesium  to  one  of  iron,  and  they  should  each 
have  the  same  proportional  influence  on  the  crystalline  angles : 


r  A  r 


Mesitite,  2  MgC03,  FeC03  2  MgCO3  =    145°  12' 

FeCO3  =      73° 


218°  12'  -f-  3  =  72°  46'; 
the  measured  angle  is  72°  46'. 

The  increase  or  decrease  of  the  topic  parameters  with  the  addi- 
tion of  isomorphous  substances  in  the  molecule  may  not  be  the 
same  in  all  directions.  These  parameters  of  mixed  crystalline 
substances  will  increase  more  rapidly  in  one  direction  with  the 
increase  of  the  percentage  of  certain  elements  than  in  others.  This 
influence  will  have  more  effect  upon  molecules  which  are  compara- 
tively simple  in  their  structure  than  on  those  which  are  complex ; 
and  compounds  which  are  isomorphous  in  complex  mineral  mole- 
cules may  not  be  able  to  replace  each  other  in  such  simple  mole- 
cules as  the  chlorides.  Thus  potassium  and  sodium  are  isomor- 
phous in  many  silicates,  yet  their  simple  chlorides  crystallize  in 
different  types  of  symmetry. 

Owing  to  the  unequal  increase  in  the  topic  parameters,  complex 
isomorphous  groups,  as  the  pyroxenes  and  amphiboles,  may  pass 
through  three  entire  cystallographical  systems. 

Generally  the  physical  properties  of  mixed  crystals,  or  those 
formed  by  isomorphous  groups  replacing  each  other,  will  stand  as  a 
mean  between  the  properties  of  their  constituents ;  and,  in  the  strict 
sense  of  the  term,  compounds  are  isomorphous  when  the  physical 
properties  of  their  mixed  crystals  are  continuous  functions  of  their 
chemical  composition. 

Elements  which  stand  in  the  same  column  in  Mendeleef  s  table 
of  the  elements,  or  those  of  the  same  group,  are  usually  isomor- 
phous, especially  in  complex  mineral  molecules,  and  those  which 
fall  directly  under  each  other  in  the  odd  and  even  groups  are  iso- 
morphous to  a  great  extent  in  simple  molecules. 

In  group  I,  the  alkali  or  univalent  metals,  it  will  be  noted  that 
lithium  is  written  on  the  left,  while  sodium  is  written  on  the  right 
and  not  directly  under  lithium.  Two  columns  of  elements  are 
thus  formed ;  any  metal,  as  potassium,  is  more  closely  related  to  the 


230  MINERALOGY 

metals  of  the  same  column,  as  lithium  and  rubidium,  than  it  is  to 
the  metals  of  the  other  column,  as  sodium.  The  elements  lithium, 
potassium,  rubidium,  and  caesium  are  isomorphous  in  such  simple 
salts  as  the  chlorides,  nitrates,  iodides :  or  sulphates;  but  sodium  is 
isomorphous  with  these  only  in  more  complex  compounds,  as  the 
feldspars,  pyroxenes,  or  amphiboles,  and  complex  silicates  gener- 
ally. 

Copper,  silver,  and  gold  are  isomorphous  in  their  sulphides  and 
as  elements. 

II.  In  the  bivalent  metals  there  are  two  distinct  groups ;    the 
first,  calcium,  strontium,  barium,  and  lead,  are  isomorphous  in  their 
carbonates,  sulphates,  silicates,  and  practically  in  all  minerals. 

The  second  bivalent  group  is  composed  of  calcium,  magnesium, 
manganese,  ferrous  iron,  nickel,  cobalt,  zinc,  and  cadmium.  Thek 
oxides  are  isomorphous  in  the  spinel  group,  carbonates,  arsenates, 
tungstates,  silicates.  Like  sodium  in  the  univalent  groups,  cal- 
cium is  a  connecting  element  in  the  bivalent  group ;  it  is  a  member 
of  both,  forming  a  carbonate  which  is  rhombohedral,  crystallizing 
with  the  second  group,  and  a  second  carbonate  which  is  ortho- 
rhombic,  aragonite,  crystallizing  with  the  first  group.  The  two 
groups  can  replace  each  other  to  some  extent  in  the  complex 
silicates,  as  the  pyroxenes  and  amphiboles. 

III.  The  trivalent  elements,  with  the  exception  of  aluminium 
and  ferric  iron,  are  not  common;  these  "two  replace  each  other  in 
such  simple  molecules  as  the  spinels ;    Cr2O3  is  also  included  here. 
They  are  found  replacing  each    other  throughout  the  silicates, 
where  Mn2O3  and  Ti203  may  be  added,  as  in  the  garnets. 

IV.  In  the  fourth  group,  TiO2,  Sn02,  ZrO2,  SiO2,  and  ThO2are 
found  replacing  each  other  in  the  rutile-cassiterite  group  of  the 
tetragonal  system.     In  silicates  ZrO2,  TiO2,  and  SiO2  are  found  the 
more  often  replacing  each  other. 

V.  In  the  group  of  pentoxides,  phosphorus,  arsenic,  and  vana- 
dium replace  each  other,  as  in  the  apatite  group,  and  added  to 
these  are  antimony  and  bismuth,  which  are  all  isomorphous  in  their 
sulphides  and  thiosulphates. 

VI.  In  the  sixth  group,  sulphur,  selenium,  and  tellurium  are  iso- 
morphous in  the  bivalent  state  only.     In  the  sexvalent  state  the 
sulphates,  molybdates,  chromates,  and  tungstates  are  very  closely 
related. 

VII.  In  the  fluorine  group,  fluorine  itself  stands  apart  from  the 
other  members  of  the  group,  the  three  most  important  of  which 


THE   RELATION   OF  MINERALS   TO  THE  ELEMENTS    231 

are  chlorine,  iodine,  and  bromine,  which  are  completely  isomor- 
phous  in  such  simple  salts  as  those  of  silver.  Fluorine  enters  as  an 
isomorphous  element  with  them  only  when  the  molecule  becomes 
complex,  as  in  the  silicates,  when  hydroxyl  (OH)  may  also  replace 
them,  as  in  topaz. 

From  a  consideration  of  the  above  isomorphous  groups  of  ele- 
ments which  may  replace  each  other  in  the  simple  mineral  mole- 
cule, not  only  will  the  number  of  elements  in  each  isomorphous 
group  increase  with  the  complexity  of  the  mineral  molecules,  but 
in  the  more  complex  silicates  whole  groups  of  elements  replace 
each  other,  In  the  amphi boles  such  groups  as  Na2,  H2,  (A12OF2), 
(Fe2OF2),  (A12O(OH2)2),  (Fe2O(OH2)2)  are  considered  to  be  isomor- 
phous. It  is  readily  appreciated  that  mineral  species  are  with 
rare  exceptions  never  pure  chemical  compounds,  constant  in  their 
percentage  composition,  where  such  replacements  are  possible.  In 
the  attempt  to  deduce  from  the  percentage  analysis  of  any  min- 
eral its  formula,  and  thus  its  relation  to  other  mineral  species,  it  is 
always  necessary  to  group  the  equivalent  elements,  or  those  which 
belong  to  the  same  isomorphous  groups,  under  the  same  head. 

Thus  the  formula  of  garnet  is  written  R3"  R2'"(SiO4)3,  where  R" 
stands  for  all  those  bivalent  elements  or  groups  of  elements  which 
may  replace  each  other  in  the  garnet  molecule ;  R"  usually  is  Ca, 
Mg,  Fe,  Mn ;  and  R'"  is  usually  Al,  Fe,  Cr,  Ti,  and  Mn.  TiO2  may 
also  replace  Si02.  In  the  analysis  of  a  garnet  the  following  per- 
centages were  found ;  the  formula  would  be  calculated  as  follows  : 

MOLECULAR 


PER  CENT. 

WEIGHT      RATIOS 

SiO2     =  41.32  - 
TiO2    =     0.16  - 

-    60  =  .689 
-    80  =  .002 

}  .691  =  3.02  =  3(R02) 

A12O3  =  21.21  - 

-  102  =  .208 

Cr203  =       .91  - 

-  152  =  .006 

.240  =  1.04  =  R203 

Fe2O3  =    4.21  - 

-  160  =  .026 

FeO     =     7.92  - 

-    72  =  .110 

MnO  =       .34  - 
MgO  '=  19.32  - 

-    71  =  .004 
-    40  =  .483 

,  .685  =  3.      =  3(RO) 

CaO    =    4.94— 

-    56  =  .088 

The  general  formula  will  then  be: 

3(R"0)R2'"03(R""02)3  or  R3"R2"(R""O2)3, 

or  by  substituting  the  elements  actually  present  for  R,  the  for- 
mula for  this  garnet  is  (Mg .  Ca .  Fe .  Mn)3(Al .  Cr .  Fe)2((Si .  Ti)  04)3 ; 


232  MINERALOGY 

placing  all  groups  of  isomorphic  elements  within  brackets,  and  the 
most  important  element  in  each  group  first.  It  is  always  under- 
stood that  elements  thus  written  in  a  mineral  formula  may  replace 
each  other,  and  they  are  always  written  in  the  order  of  their  im- 
portance in  that  mineral  molecule.  If  it  is  wished  to  still  further 
simplify  the  above  formula,  it  is  at  once  seen  that  both  titanium 
and  manganese  are  present  in  only  very  small  amounts  and  may  be 
neglected  in  the  formula. 

Classification  of  minerals.  —  Any  natural  classification  of  min- 
erals must  consider  both  the  chemical  and  physical  properties  of 
the  mineral,  but  as  the  physical  properties  depend  to  a  large  extent 
upon  the  elements  present,  the  chemical  relations  of  the  elements  in 
the  mineral  molecule  are  given  more  weight  in  the  scheme  of  classi- 
fication than  the  presence  of  any  one  element.  Minerals  are  therefore 
classified  as  chemical  compounds,  following  the  order  of  the  natural 
classification  of  the  elements,  but  the  acid  radicle  will  determine 
the  group ;  for  example  all  sulphates  are  placed  in  the  same  large 
group.  The  subheads  are  determined  by  the  character  of  the  salt ; 
whether  it  is  anhydrous,  basic,  acid,  or  hydrated,  as  well  as  by  a 
consideration  of  its  crystalline  type.  By  this  method  of  classifica- 
tion those  minerals  which  form  natural  isomorphous  groups  are  kept 
together ;  for  example,  barium,  strontium,  calcium,  and  lead  sul- 
phates are  placed  in  the  same  group  of  anhydrous  sulphates,  as 
they  should  be,  since  they  crystallize  in  the  same  type  and  are 
isomorphous.  If  they  were  classified  from  their  basic  or  metallic 
elements,  they  would  be  widely  separated  in  four  different  groups, 
even  though  they  are  all  found  in  nature  under  the  same  condi- 
tions and  often  several  are  mixed  in  the  same  crystals. 

The  system  of  classification  here  adopted  is  that  of  Dana,  which 
may  not  be  in  all  respects  the  most  desirable,  yet  most  of  the  min- 
eral collections  in  the  American  museums  follow  this  system  in  their 
arrangement.  Not  taking  into  account  the  hydrocarbons,  there 
are  eight  large  groups  in  this  scheme,  as  follows  : 

I.  Native  elements.     Includes  the  elements  which  occur  in  na- 
ture in  an  uncombined  state,  as  gold,  copper,  silver,  sulphur,  dia- 
mond, etc. 

II.  Sulphides,  including  the  arsenides,  antimonides,  and  other 
similar   compounds.     Mostly   salts   of   hydrogen   sulphide,    H2S, 
as  galena,  PbS ;    also  including  the  corresponding  compounds  of 
tellurium  and  selenium. 

III.  Sulpho-compounds,     as     the    sulpharsenites,     sulphanti- 


THE  RELATION  OF  MINERALS   TO  THE  ELEMENTS    233 

/S-H 

monites,  etc.     Derived  from  such  acids  as  As  ^— S  —  H  =  H3AsS3, 

\S-H 

as  proustite,  Ag3AsS3.     The  group  also  includes  basic,  acid,  nor- 
mal, and  meta  groups. 

IV.  Haloids,  or  salts  derived  from  the  haloid  acids,  as  HC1, 
HBr,  HI,  and  HF ;  halite,  NaCl ;  iodyrite,  Agl ;  bromyrite,  AgBr ; 
fluorite,  CaF2. 

V.  Oxides,  or  the  combinations  of  the  elements  with  oxygen. 

a.  The  oxides  of  the  univalent  metals,  as  cuprite,  Cu2O. 

b.  Oxides  of  the  bivalent  metals,  as  zincite,  ZnO. 

c.  Oxides  of  the  trivalent  elements,  as  corundum,  A1203. 

d.  Oxides  of  the  quadrivalent  elements,  as  quartz,  Si02. 

Also  included  in  this  grou£  are  the  hydroxides,  in  which  the 
base  is  combined  with  hydroxyl,  as  brucite,  Mg(OH)2. 

The  aluminites,  ferrites,  and  chromites,  salts  of  such  acids 
as  HA102,  as  spinel,  Mg(AlO2)2,  or  of  H(FeO2),  as  magnetite, 
Fe(FeO2)2,  and  chromite,  Fe(CrO2)2,  are  included  in  this  group, 
though  they  must  be  considered  as  salts  and  not  oxides. 

VI.  Salts  of  the  oxygen  acids. 

a.   Carbonates,  H2CO3,  as   CaCOs,  calcite;    basic  carbonates, 
as  malachite,  (CuOH)2COs,  and  hydrous  carbonates,  as 
trona,  Na2CO3 . 10  H2O. 
6.   Silicates,  titanates,  etc. 

Orthosilicates  are  the  normal  salts  of  the  tetrabasic  acid,  H4Si04, 
as  fayalite,  Fe2Si04. 

Metasilicates.  Orthosilicic  acid  by  a  loss  of  some  of  its  water 
forms  the  dibasic  acid,  metasilicic  acid. 


There  are  many  minerals  which  are  salts  of  this  acid,  as  enstatite, 
MgSiO3,  or  leucite,  KAl(SiO3)2. 

Diorthosilicic  acid  is  derived  from  two  molecules  of  orthosilicic 
acid  by  the  loss  of  one  molecule  of  water. 


o-H   „  n. 

0-H-H20^ 

(HO)3=Si-O-Si=(OH)3;  H6Si207. 

It  is  an  hexabasic   acid,  an  example  of  which  is  barysilite, 
Pb3Si2O7. 


234  MINERALOGY 

Dimetasilicic  acid  is  derived  from  two  molecules  of  an  ortho- 
silicic  acid  by  the  elimination  of  three  molecules  of  water,  or 
from  two  molecules  of  a  metasilicic  acid  by  the  elimination  of  one 
molecule  of  water. 


(  J_TT  TT i~\  l> 

i     -LJ.          XI.          Vy  '    x    /-^  •    x  v"-r   i  •*-*•  i          TT    r-4*    ^^^v 

H2Si20£ 


orthosilicic  acid 

O  =  Siv        ___________  r  ^81=0  :  HjSifQi     dimetasilicic  acid. 

\O;H     H-Oi/ 

metasilicic  acid 

Example,  Petalite,  LiAl(Si205)2. 

Trisilicic  acid  is  an  acid  obtained  from  three  molecules  of  the 
orthoacid  by  the  elimination  of  four  molecules  of  water. 

Orthosilicic  acid 

O  =  Si-O-H 
iH+Qv  si/0-H  | 

;H-O/  NqrHi  o 

T-H~0\Si/  3t.?.i;  H-O-Si-O-H;  H4Si308  =  trisilicic  acid. 
H~~O  /    \  O—  H  | 

9 

H_0-Si-0 


Example,  Orthoclase,  KAlSi3O8. 

In  some  minerals  several  silicic  acids  rnay  be  present  in  the  mole- 
cule, and  a  mineral  may  be  a  mixture  of  two  or  more  of  these  acids 
and  yet  on  analysis  yield  the  same  percentage  results  of  oxygen  and 
silicon.  From  inspection  alone  it  is  impossible  to  determine 
whether  the  analysis  represents  a  metasilicate  or  a  trisilicate  and 
orthosilicate  mixed;  to  this  difficulty  must  also  be  added  the 
chances  of  basic,  acid,  and  hydrated  salts  being  present.  For  the 
same  analysis  several  structural  formulae  of  the  mineral  may 
be  written,  all  of  which  will  represent  the  results  equally  well; 
thus,  andalusite,  Al2SiO5,  may  be  written  as  an  orthosilicate, 
O  =  Al  -  SiO4  =  Al,  or  as  a  metasilicate  (AlO)2SiO3,  both  of 
which  will  yield  the  same  percentage  composition  on  analysis. 

The  probable  structural  formula  is  derived  from  a  study  of  the 
decomposition  products  of  any  mineral,  as  well  as  by  a  thorough 
study  of  its  synthesis  where  that  is  possible.  In  the  case  of  anda- 


THE   RELATION   OF  MINERALS  TO  THE  ELEMENTS    235 

lusite,  it  decomposes,  forming  muscovite.  The  formula  of  mus- 
covite is  that  of  an  orthosilicate,  and  when  written  in  the  form  of  a 
substitution  product  in  the  normal  aluminium  orthosilicate  is  as 
follows : 

Muscovite  Andalusite 

XH  /Al  =  0 

Si04<c-H  /  Si04/~Al  =  O 

,A1  = 
Al  <t Si04=Al  Al 


In  the  formation  of  muscovite  from  andalusite,  hydration  attacks 
the  Al  =  0  group,  forming  gibbsite,  A1(OH)3,  which  at  the  same 
time  may  be  carried  off  in  solution.  The  necessary  potassium  for 
substitution  in  the  formula  of  andalusite  to  form  muscovite  is  fur- 
nished by  the  potassium  carbonate  dissolved  in  the  ground  water. 

2  (Al3(Si04)3(AlO)3)  +  11  H2O  +  K2C03 

=  2  (Al3(SiO4)3H2K  +  6  Al(OH),  +  CO2. 

If  the  formula  as  written  for  muscovite  is  true,  it  would  follow 
that  the  formula  as  written  for  andalusite  would  be  most  probable 
and  it  would  be  an  orthosilicate. 

In  the  case  of  pyroxene  and  amphibole,  from  their  analysis  alone 
both  could  be  expressed  by  the  formula,  R"SiO3,  as  metasilicates, 
in  which  R"  represents  practically  the  same  group  of  isomorphous 
elements.  Their  crystallographical  constants  and  physical  prop- 
erties are,  however,  quite  different,  indicating  entirely  different 
molecular  structures.  Pyroxene  is  more  dense  than  amphibole, 
which  taken  with  the  evidence  of  uralitization,  a  process  in  which 
amphibole  is  formed  from  pyroxene,  with  but  little  chemical  change, 
would  indicate  that  the  molecule  of  pyroxene  is  more  compact  than 
that  of  amphibole  and  uralitization  is  the  breaking  down  of  a  com- 
plex but  compact  molecule  into  less  dense  and  simple  molecules. 
Pyroxene  is  therefore  usually  considered  as  a  metasilicate,  while 
amphibole  may  be  written  as  a  mixed  silicate  composed  of  the 
orthosilicic  acid  and  the  trisilicic  acid,  preserving  the  ratio  of  silicon 
to  oxygen  as  found  in  analyses. 

Pyroxene  Amphibole 

Mg 

II 

n(R"Si03)  Mg/^4)>Mg  =  tremolite. 


Ca 


236  MINERALOGY 

The  alteration  and  relation  of  many  orthosilicates  is  easily 
explained,  and  their  formulae  may  be  written  as  substitutions  in 
the  general  formula  of  an  aluminium  orthosilicate,  in  which  the 
aluminium  is  replaced  by  other  bases. 

/Si04=Al 

Aluminium  orthosilicate  Al\- — SiO4= Al ;  Al4(Si04)3. 

\Si04=Al 


/ 

/  Ca;  Ca3Al2(Si04)3. 

Garnet  =  Al<— -SiO4LQa 

\Si04=Al  • 

H 
K 

Mg 
Biotite  =  Als— Si0/Mg.  HKMg2Al2(Si04)3. 

\SiO4=Al 

/Si04=(AlF2)3 

Topaz  =  Al<~Si04= Al ;  Al,  (A1F2)3  (Si04)3  =  Al  (A1F2)  Si04. 
\SiO4E=Al 

c.  Columbates  and  tantalates. 

Salts  of  columbic  acid,  HCbO3,  as  columbite,  (Fe.Mn)  (CbO3)2, 
an  orthocolumbate  of  iron  and  manganese. 

Tantalates,  salts  of  tantalic  acid,  HTaO3,  as  tantalite,  Fe(Ta03)2. 

d.  Phosphates,  arsenates,  and  vanadates,  etc. 

Salts  of  the  acids,  H3PO4,  H3As04,  H3VO4 ;  as  xenotime,  YPO3 ; 
berzelite,  (Ca.Mg.Mn)3(As04)3 ;  and  pucherite,  BiV03. 

e.  Borates  and  uranates. 

Salts  of  boric  acid,  as  sassolite,  H3B03 ;  also  biborates,  as  borax, 
Na2B4O7. 

/.   Sulphates,  chromates,  and  tellurates. 

Salts  of  sulphuric  acid,  as  barite,  BaSO4.  Salts  of  chromic  acid, 
as  crocoite,  PbCr04. 

g.   Tungstates  and  molybdates. 

Salts  of  tungstic  acid,  H2WO4,  as  wolframite,  (Fe.Mn) W04. 

Salts  of  molybdic  acid,  as  wulfenite,  PbMoO4. 

In  most  cases  the  above  acid  groups  include,  in  addition  to  the 
normal  salts,  hydrated,  acid,  and  basic  salts,  as  well  as  complex 
molecules,  the  exact  structure  of  which  has  not  in  all  cases  been 
determined. 


CHAPTER  II 
THE   ORIGIN   OF  MINERALS 

THE  origin  of  all  minerals  must  have  been  from  solutions  and 
gases,  or  the  result  of  the  interaction  of  these  on  minerals  previously 
separated  from  solution.  It  is  of  very  little  difference  whether  the 
solution  is  one  of  water  or  of  fused  silicates,  homogenous  and  fluid 
only  at  high  temperatures.  In  either  case  the  same  laws  will 
apply,  and  the  same  physical  and  chemical  laws  will  hold  true 
whether  that  solution  is  but  a  thimbleful  of  silicate  fused  in  the 
electric  furnace  of  the  laboratory  or  contained  in  the  enormous 
crucibles  of  nature.  Practically  the  same  conditions  can  be  du- 
plicated in  the  laboratory  with  the  exception  of  time  and  pressure. 
Time  is  the  factor  so  essential  for  the  formation  of  large  and  per- 
fect crystals. 

Most  minerals,  at  least  the  common  species,  have  been  synthe- 
sized or  artificially  produced  in  the  laboratory.  The  exception  is 
a  class  of  minerals  which  it  is  believed  have  been  formed  under  pres- 
sure and  in  the  presence  of  water,  or  they  belong  to  that  class  which 
are  produced  by  contact  metamorphic  conditions,  in  most  cases, 
as  the  micas,  epidote,  topaz,  vesuvianite,  and  amphibole.  The 
artificial  products  lack  water  in  their  constitution,  and  therefore  the 
conditions  under  which  they  were  formed  in  nature  have  not  been 
reproduced  or  duplicated  in  case  of  the  artificial  product. 

Indeed  the  conditions  surrounding  the  crystallization  of  any 
mineral  in  nature  must  be  excessively  complex.  The  conditions 
under  which  individual  minerals  may  form  as  conditioned  by  tem- 
perature, pressure,  and  other  components  in  the  system  have  been 
studied  in  but  few  cases,  and  in  these  the  result  has  been  the  dis- 
covery of  unknown,  unsuspected  phases,  which  have  been  of  great 
assistance  in  the  explanation  of  the  various  isomorphous  series, 
containing  the  same  elements  but  of  different  symmetry. 

In  order  to  arrive  at  some  small  conception  of  the  importance  and 
influence  of  one  compound  or  component  in  solution  on  the  separa- 
tion or  crystallization  of  another  component  of  the  same  system,  it 
is  probably  best  to  illustrate  by  a  system  in  which  the  relations  are 

237 


238  MINERALOGY 

as  simple  as  possible  and  with  one  which  is  familiar  to  all,  as  the 
solution  of  salt  in  water  —  a  two-component  system  easily  tried  and 
from  the  consideration  of  which  the  application  of  the  terms  and 
diagrams  may  be  appreciated. 

If  any  crystalline  substance  is  heated  and  the  application  of  heat 
is  constant  and  continuous,  the  temperature  of  the  crystalline  solid 
increases  constantly ;  in  time  a  temperature  will  be  reached  at  which 
the  solid  passes  to  the  liquid  state  or  the  solid  is  said  to  fuse  or  melt. 
The  molecular  network  or  point-system  of  the  crystalline  state  is 
broken  down.  If  the  temperature  of  the  system  when  the  two 
phases,  solid  and  liquid,  are  in  contact  is  measured,  it  will  be  found 
that  this  temperature  is  a  constant  one,  providing  that  the  com- 
pound is  chemically  pure  and  the  pressure  is  the  same.  Thus  ice 
when  heated  to  0°  C.  melts  and  forms  water ;  this  temperature  of 
fusion  is  the  melting  point.  Water  is  an  exception  to  the  general 
rule,  and  the  melting  point  of  water  is  lowered  with  an  increase  of 
pressure,  the  reverse  of  the  conditions  in  most  other  substances.  The 
transition  of  a  solid  crystalline  substance  to  a  liquid  is  an  abrupt  one, 
and  there  is  no  constant  decrease  of  viscosity,  with  the  rise  of 
temperature,  to  be  noted  in  case  of  the  crystalline  substance  as  is 
the  case  with  the  amorphous  solid.  In  the  amorphous  solid  there 
is  no  constant  sharply  defined  temperature  of  fusion,  no  abrupt 
change  from  solid  to  liquid.  The  viscosity  simply  decreases  with 
the  rise  of  temperature.  The  line  of  separation  between  liquids  and 
solids,  when  considered  from  this  standpoint,  should  be  drawn 
between  the  cyrstalline  substances  and  amorphous  substances 
rather  than  between  solid  amorphous  substances  and  the  ordinary 
liquids. 

The  difference  between  amorphous  solids,  which  are  excessively 
viscous  liquids,  and  the  ordinary  liquid  is  very  little  ;  and  the  amor- 
phous solid  is  more  closely  related  to  liquids  than  to  crystalline 
solids. 

Water  and  ice  are  two  modifications  of  the  same  chemical 
substance,  between  which  there  is  an  abrupt  change  or  transition 
from  one  to  the  other.  Each  of  these  modifications  is  known  as  a 
phase.  Water  can  exist  in  three  phases :  solid,  ice ;  liquid,  water ; 
and  vapor  or  gas,  steam,  depending  upon  the  temperature  and  pres- 
sure. Sulphur  may  exist  in  two  solid  crystalline  phases,  mono- 
clinic  and  orthorhombic  sulphur;  the  transition  temperature 
between  these  two  phases  at  ordinary  pressure  is  95.6°.  Many 
solid  compounds  as  minerals  have  different  solid  phases.  Leucite 


THE  ORIGIN   OF  MINERALS  239 

is  orthorhombic  at  ordinary  temperatures,  but  at  500°  C.  it  passes 
over  to  an  isometric  phase.  In  like  manner  quartz,  when  heated 
to  a  temperature  of  575°,  /3-quartz  is  formed,  and  crystals  in  passing 
this  inversion  temperature  are  shattered ;  if  the  temperature  is  still 
raised,  at  800°  C.  another  inversion  point  or  temperature  is 
reached,  when  a  third  phase,  or  tridymite,  is  the  stable  form. 

Different  phases  of  the  same  substance  can  exist  in  equilibrium 
with  each  other  only  when  the  temperature  and  pressure  are  fixed. 

Ice  and  water  are  in  equilibrium  and  can  exist  in  contact  with 
each  other  at  0°  C.  and  under  one  atmosphere  of  pressure.  When 
two  phases  of  the  same  substance  are  in  contact,  for  each  pressure 
there  is  a  fixed  definite  temperature  of  equilibrium,  and  for  every 
fixed  temperature  there  is  also  a  fixed  pressure  of  equilibrium. 

If  ice  is  heated  and  the  heat  is  applied  at  a  constant  rate,  the  tem- 
perature of  the  ice  will  rise  regularly ;  as  for  every  gram  of  ice  a  con- 
stant amount  of  heat  is  required  to  raise  the  temperature  one  de- 
gree, this  quantity  of  heat  is  designated  the  specific  heat  of  the 
substance.  If  the  temperature  is  taken  every  ten  seconds  and  the 
curve  is  plotted  with  temperature  and  time  as  ordinates  and  ab- 
scissa, the  curve  will  be  found  to  be  continuous  and  regular  until 
0°  C.  is  reached,  when  there  is  an  abrupt  change  in  the  behavior 
of  the  system,  as  water,  or  the  liquid  phase,  begins  to  form.  The 
rise  of  temperature  is  stopped  and  remains  the  same,  just  as  long 
as  there  is  any  ice  or  solid  phase  present.  At  this  point,  Fig.  368, 
the  heating  curve  runs  along  horizontally  and  there  is  no  rise  in 
temperature,  since  the  melting  ice  absorbs  heat  in  passing  in  to 
the  liquid  phase.  This  absorbed  heat  is  the  heat  of  fusion;  and 
when  water  is  transformed  to  the  solid  phase,  ice,  the  same  amount 
of  heat  is  liberated,  and  the  cooling  curve  will  show  the  same  hori- 
zontal portion  at  0°  C. ;  such  an  interruption  in  the  regular  trend  of 
the  cooling  or  heating  curve  is  a  halting  point.  Such  halting  points 
mark  the  transition  temperatures  between  two  phases  of  a  sub- 
stance and  are  caused  by  the  absorption  or  liberation  of  heat.  The 
heat  liberated  by  a  solid  crystallizing  is  often  termed  the  heat  of 
crystallization;  this  is  a  constant  source  of  heat,  available,  in 
magmas  that  are  crystallizing,  to  counteract  radiation  and  serves 
to  prolong  the  liquid  state  and  to  modify  the  rate  of  cooling.  In 
such  fluid  and  crystallizing  magmas  each  substance  which  crys- 
tallizes and  separates  from  the  solution,will  have  a  direct  influence 
upon  the  temperature  at  which  other  substances  in  turn  crystallize; 
and  each  compound  dissolved  in  the  magma  will  also  influence 


240  MINERALOGY 

the  temperature  at  which  crystals  of  every  other  compound  con- 
tained in  the  solution  will  begin  to  form. 

If  two  substances  as  salt,  NaCI,  and  water  are  taken,  and  the 
salt  is  dissolved  in  the  water,  a  two-component  system  will  result. 
From  a  saturated  solution  of  salt  in  water,  salt  crystals  separate 
without  water,  and  water  will  separate  as  ice  without  salt ;  these 
conditions  are  the  simplest  possible.  Salt  crystals  and  ice  are  the 
two  solid  phases  and  dissolve  in  each  other  to  the  liquid  phases.  If 
one  substance  is  dissolved  in  another,  as  salt  in  water,  the  temper- 
ature of  fusion  is  always  lowered  or  the  temperature  of  solidifica- 
tion of  both  components  is  lowered.  The  change  in  temperature 
at  which  each  will  solidify  caused  by  the  addition  of  the  other  is 
known  as  the  lowering  of  the  freezing  point.  If  water  at  60°  C. 
is  surrounded  by  a  temperature  of  —  30°  and  its  cooling  curve  is 
plotted,  Fig.  368,  the  curve  will  be  regular  until  0°  C.  is  reached, 
when  ice  crystals  will  form  and  the  temperature  will  halt  at  this 
point  as  long  as  there  is  any  water  to  be  transformed  to  ice  and 
absorb  heat,  when  the  lowering  of  the  temperature  will  proceed 
in  a  regular  manner,  as  before.  If  to  the  water,  or  liquid  phase, 
salt  be  added  to,  say,  an  amount  of  15  per  cent.,  then  if  the  system 
be  cooled  as  before,  when  0°  C.  is  reached,  the  curve  will  show  no 
indication  of  a  halting  point  and  no  ice  crystals'  will  form  until  the 
temperature  has  fallen  far  below  the  freezing  point  of  water,  as 
the  freezing  point  of  water  has  been  depressed  by  the  addition  of 
salt,  and  ice  will  not  form  until  a  temperature  is  reached  corre- 
sponding to  the  amount  of  salt  added.  In  the  case  of  a  15  per  cent, 
solution  this  temperature  is  —  12.2°.  As  ice  crystals  separate, 
the  composition  of  the  remaining  solution  is  constantly  enriched 
in  salt  and  impoverished  in  water  by  the  ice  formed,  which  is 
unmixed  with  salt.  As  the  percentage  of  salt  increases,  this 
lowers  still  further  the  freezing  point  of  the  remaining  solution, 
(indicated  by  the  curve  AB),  until  the  temperature  is  lowered  to 
or  reaches  —  22.4°,  where  there  is  a  halting  point,  until  the  liquid 
phase  disappears.  At  this  point  both  salt  and  ice  are  formed  and 
solidify  as  a  whole ;  the  temperature,  owing  to  the  heat  of  crystal- 
lization, remains  constant  until  the  liquid  phases  have  entirely 
disappeared,  after  which  the  cooling  will  proceed  in  a  regular  man- 
ner. If  the  mixture  of  salt  and  water  which  solidifies  as  a  whole 
when  the  temperature  of  —  22.4°  is  reached  be  analyzed,  its  com- 
position will  always  be  found  constant,  and  it  is  composed  of  77 
per  cent,  of  water  and  23  per  cent,  of  salt,  and  no  matter  what 


THE  ORIGIN  OF  MINERALS 

the  percentage  of  the  original  solution  might  be,  upon  cooling 
either  salt  or  water  will  separate  from  the  solution  as  the  case 
may  be  and  continue  *to  do  so,  as  the  temperature  is  lowered, 
until  the  constant  percentage  mixture  of  77  parts  water  and  23 
parts  salt  is  reached  at  —  22.4°,  when  the  solution  will  solidify 


as  a  whole.  Such  mixtures,  which  have  a  constant  composition 
and  solidify  as  a  whole  at  definite  temperatures  under  like  condi- 
tions, are  termed  eutectic  mixtures.  Fig.  368  is  a  so-called  fusion 
diagram  of  a  two-component  system,  water  and  salt.  The  plan, 
one  in  which  temperatures  are  measured  vertically  and  percen- 
tages horizontally,  is  divided  into  four  fields  of  conditions  by  the 
two  curves  AB  and  BC  and  the  straight  line  DE.  In  field  I, 


242  MINERALOGY 

above  both  curves,  is  the  liquid  field,  in  which  all  possible  mix- 
tures of  water  and  salt  may  exist  as  homogeneous  solutions  and 
be  in  equilibrium  at  the  corresponding  temperatures.  This  field 
is  bounded  by  the  two  curves  AB  and  BC.  The  curve  BC  is  the 
boundary  between  homogeneous  solution  and  field  III,  in  which 
ice  crystals  and  salt  solution  are  in  equilibrium.  If  any  point  in 
field  I  is  selected,  as  p  corresponding  to  a  temperature  of  20°  and  10 
per  cent.  NaCl,  at  20°  this  solution  is  homogeneous,  but  if  cooled, 
the  10  per  cent,  solution  will  meet  the  curve  AB  at  a  point  correspond- 
ing to  —  6.7°,  and  at  that  temperature  ice  crystals  will  begin  to  sepa- 
rate and  the  conditions  of  field  III  will  be  reached,  in  which  ice  crys- 
tals are  in  equilibrium  with  salt  solution.  If  a  point  on  the  opposite 
side  of  the  diagram,  as  P,  be  taken,  on  cooling,  the  perpendicular 
from  P  cuts  the  curve  CB  at  —  5°,  at  which  temperature  the  solution 
will  be  saturated  in  regard  to  the  salt,  and  salt  crystals  will  form, 
when  the  solution  or  conditions  will  be  those  of  field  IV,  in  which 
salt  crystals  and  salt  solution  are  in  equilibrium.  As  the  tempera- 
ture falls,  the  solution  becomes  more  concentrated  in  respect  to  the 
water  content  and  the  temperature  of  separation  follows  the  curve 
BC  until  B  is  reached,  when  both  salt  and  ice  separate  in  eutectic 
proportions  of  77  parts  water  and  23  parts  salt,  at  a  temperature 
of  —  22.4°.  Below  the  line  DE  or  below  the  point  B  or  a  tem- 
perature of  —  22.4°,  liquid  solution  cannot  exist,  and  field  II  is 
the  crystalline  or  eutectic  field,  separated  from  the  other  three 
fields  in  which  liquid  solutions  are  possible  by  the  straight  line  DE, 
known  as  the  eutectic  horizontal.  It  often  happens,  and  it  is 
indeed  the  rule,  that  the  temperature  will  fall  below  that  indicated 
by  the  curve  of  separation  without  crystals  being  formed,  unless  care 
is  taken  to  prevent  it ;  such  a  solution  is  said  to  be  supercooled.  If 
the  10  per  cent,  salt  solution  should  fall  below  -  6.4  without  the 
separation  of  ice,  it  would  be  supercooled  and  in  a  metastable 
condition,  the  solution  being  supersaturated  as  regards  water; 
such  a  condition  could  not  exist  if  the  least  particle  of  ice  were  pres- 
ent. On  dropping  a  particle  of  ice  in  the  supercooled  solution 
there  is  an  immediate  separation  of  ice,  a  rise  of  temperature 
from  the  heat  of  crystallization,  as  well  as  an  increase  of  the  con- 
centration of  salt  in  solution,  and  the  whole  system  comes  to  a 
state  of  equilibrium  on  the  curve  of  separation.  A  good  example 
to  illustrate  supersaturation  is  a  solution  of  sodium  sulphate, 
Na2SO4, 10  H20,  dissolved  in  water  and  heated  in  contact  with  the 
salt,  or  saturated  at  a  temperature  a  little  below  32°  C. ;  if  it  is 


THE   ORIGIN  OF  MINERALS  243 

heated  to  a  temperature  above  32°  the  anhydrous  salt  Na2SO4 
will  separate  at  33°.  The  solution  is  carefully  poured  off  the  crys- 
tals in  a  flask  and  the  mouth  of  the  flask  stoppered  with  cotton  to 
prevent  the  access  of  dust  or  fine  particles  of  sodium  sulphate  from 
the  air,  which  would  start  crystallization  and  prevent  supercooling 
or  supersaturation.  Such  a  solution  may  be  cooled  to  room 
temperature  and  kept  for  a  long  time  without  the  formation  of 
crystals.  This  solution  is  supersaturated  in  regard  to  the  salt 
Na2SO4,  10  H20 ;  if  the  smallest  particle  of  this  salt  is  dropped  into 
it,  the  solution  solidifies  almost  at  once,  with  a  rise  in  temperature, 
and  the  system  soon  reaches  a  state  of  equilibrium,  as  between  the 
solid  Na2SO4, 10  H2O  and  solution. 

The  supercooled  solution  is  in  the  metastable  state,  and  the  tend- 
ency to  form  crystals  spontaneously  is  very  small ;  if  cooling  is 
"continued,  however,  a  temperature  will  be  reached  at  which  a  cloud 
of  small  crystals  will  form  spontaneously,  or  a  number  of  crystal- 
line nuclei  will  form  without  inoculation  with  a  solid  particle,  and 
thereafter  the  system  quickly  reaches  equilibrium. 

In  the  case  of  igneous  rocks,  before  consolidation  they  may  be 
said  to  represent  solutions  of  various  components,  each  of  which  will 
have  an  individual  and  separate  influence  on  the  temperature  of  sep- 
aration of  every  other  dissolved  component  in  the  magma;  and  the 
freezing  curve,  or  the  curve  of  separation  of  any  crystalline  form 
or  phase  separating,  will  be  the  resultant  of  all  these  depressions. 
Where  the  number  of  components  is  quite  large,  six  or  eight,  as 
it  is  in  most  magmas,  the  system  becomes  excessively  complicated. 
Upon  the  whole,  when  cooled  to  the  metastable  stage  of  one  or  more  of 
the  components,  a  temperature  will  be  reached  at  which  crystalline 
nuclei  will  form  spontaneously,  and  the  minerals  will  separate  from 
the  magma  in  the  order  of  their  freezing  points  or  saturation  for 
that  particular  system,  and  each  individual  crystal  will  continue  to 
grow  as  cooling  of  the  magma  continues.  In  the  fusion  diagram, 
fields  III  and  IV,  other  things  being  equal,  would  represent  a 
porphyritic  development  of  one  or  two  species;  the  individual 
crystals  of  each  would  be  contained  in  a  ground  mass  of  fine  crys- 
tals, representing  the  final  disappearance  of  solution,  or  the  eutectic 
mixture  composed  of  the  last  to  crystallize,  Fig.  369.  The  eutectic 
mixture  in  its  crystallization  always  presents  a  peculiar,  intimate 
intergrowth  of  the  components.  This  is  seen  in  the  micropegmatitic 
intergrowths  of  quartz  and  orthoclase,  Fig.  370.  Eutectics  are 
often  formed  between  garnet  and  quartz ;  magnetite  and  horn- 


244 


MINERALOGY 


FIG.  369.  —  Porphyritic   Feldspar  Crystals  in  a  Fine 
Grained  Crystalline  Ground  Mass. 


blende;  quartz  and  tourmaline.    Ternary  and  quaternary,  eutectics 
are  also  possible.     Eutectic  consolidation  will  at  times  represent  a 

second  generation 
of  small  crystals, 
surrounding  the 
porphyritically  de- 
veloped crystals  of 
the  first  generation. 
Minerals  which 
are  crystallized 
from  molten  mag- 
mas are  usually  an- 
hydrous or  contain 
small  amounts  of 
water,  as  the  micas, 
and  are  termed  pri- 
mary minerals,  to 
distinguish  them 
from  secondary  min- 
erals, which  have  resulted  from  water  solutions,  the  components  of 
which  have  resulted  from  the  breaking  down  of  other  minerals. 

The  order  of  crystallization  of  the  primary  minerals  from  magmas 
is  not  rigidly  fixed, 
but  may  vary  accord- 
ing to  the  composition 
of  the  magma,  as  the 
.  curve  of  separation  of 
any  individual  species 
is  the  result  of  the 
lowering  of  the  freez- 
ing point  of  that  in- 
dividual by  all  the 
other  components  of 
the  system,  and  these 
may  have  a  greater 
effect  on  one  than  on 
another.  The  order 
of  crystallization  and 
the  relation  of  the 
crystals  of  one  species  to  those  of  an  another  species  will  depend 
upon  and  reflect  magmatic  peculiarities.  There  are,  however, 


FIG.  370.  —  Section  of  a  Micropegmatite  enlarged, 
showing  Eutectic  Structure. 


THE  ORIGIN  OF  MINERALS  245 

certain  minerals  which  ordinarily  separate  during  the  early  stages 
of  crystallization,  or  the  solution  becomes  saturated  in  respect  to 
them  long  before  it  is  saturated  in  respect  to  others.  Minerals 
which  are  early  to  crystallize  are  well  formed  and  exhibit  crystal- 
line outlines  in  the  rock  section.  They  may  be  contained  as 
inclusions  in  the  crystals  of  those  minerals  which  form  later,  as 
the  crystals  of  apatite  in  magnetite,  or  magnetite  in  feldspar.  The 
crystals  of  minerals  among  the  last  to  separate  are  usually  irregu- 
lar and  act  as  a  ground  ma.ss,  filling  the  spaces  between  crystals 
which  have  separated  at  an  earlier  stage.  In  the  common  mag- 
mas it  may  be  stated  that  separation  takes  place  with  a  progressive 
increase  of  silica.  Those  minerals  high  in  silica,  as  quartz,  are 
near  the  end  products  of  crystallization. 

Among  the  first  to  separate  are  metals,  sulphides,  oxides,  and 
included  here  are  apatite  and  zircon ;  second,  the  ferromagnesian 
silicates,  as  pyroxene,  amphibole,  and  olivine ;  third,  feldspars, 
beginning  with  the  basic  plagioclases  and  ending  with  orthoclase ; 
and  lastly  quartz.  This  order  of  crystallization  must  be  considered 
only  in  a  general  way,  as  several  may  overlap  in  their  periods  of 
separation,  and  this  overlapping  may  be  continued  to  such  an  extent 
that  the  order  of  adjacent  minerals  in  the  list  or  order  of  cyrstalliza- 
tion  is  reversed.  Magmas  homogeneous  at  high  temperatures 
may  upon  cooling  separate  into  several  portions  which  are  immis- 
cible at  lower  temperatures,  each  of  which  will  have  an  individual 
character  and  composition,  even  before  crystallization  has  begun. 

In  the  crystallization  of  magmas,  the  viscosity  has  a  great 
influence  upon  the  crystals  formed  and  upon  crystal  growth.  With 
the  decreasing  temperature  there  is  usually  an  increase  of  viscosity, 
which  tends  to  prevent  the  formation  of  centers  of  crystallization, 
and  is  therefore  a  check  generally  to  crystallization ;  and  the  very 
viscous  magma  has  a  predisposition  to  solidify  as  a  glass.  The 
fluidity  of  minerals  at  their  fusing  point  varies  greatly,  even  within 
isomorphous  groups ;  as  the  potash  feldspar,  orthoclase,  is  exceed- 
ingly viscous  at  its  point  of  fusion,  —  crystals  form  in  a  fusion  of  its 
components  with  great  difficulty  or  not  at  all ;  while  the  calcium  feld- 
spar, anorthite,  crystallizes  with  the  greatest  ease  from  the  simple 
fusion.  Viscosity  decreases  with  the  increase  of  basicity,  as  iron, 
calcium,  magnesium,  and  sodium  promote  fluidity,  potassium  and 
silica  promote  viscosity,  and  aluminium  has  but  little  influence 
either  way.  Those  rocks  containing  pyroxene,  hypersthene,  ensta- 
tite,  or  olivine  are  more  apt  to  contain  well-formed  crystals  than 


246  MINERALOGY 

those  containing  felspathoids,  silica  or  alkali  feldspars,  when  under 
the  same  conditions. 

In  the  synthesis  of  minerals  it  has  been  proven  without  doubt 
that  certain  minerals  are  easily  formed  from  a  simple  fusion,  com- 
posed of  their  chemical  constituents  in  correct  proportions;  in 
all  cases  these  fusions  are  not  highly  viscous  in  nature.  The 
plagioclases,  magnetite,  hematite,  rutile,  spinels,  corundum,  some 
garnets,  leucite,  olivine,  and  enstatite  have  all  been  formed  from 
simple  fusions  in  an  open  crucible.  They  are  all  rock-forming 
minerals,  and  in  nature  they  have  in  many  occurrences  been  crys- 
tallized directly  from  a  fused  magma. 

Mineralizers. —  On  the  other  hand  there  is  a  group  of  minerals 
which  it  has  been  impossible  to  synthesize  by  the  open  fusion 
method,  as  there  is  contained  in  their  molecule  small  amounts  of 
volatile  compounds  as  fluorine,  water  as  hydroxyl,  or  chlorine; 
or  again  the  fusion  at  the  point  or  temperature  of  crystallization 
is  so  viscous  as  to  prevent  the  formation  of  crystals,  when  the  fusion 
cools  as  a  glass.  Many  granites,  syenites,  and  gabbros  are  excep- 
tionally well  crystallized  and  contain  orthoclase,  albite,  quartz, 
amphibole,  and  micas,  minerals  which  from  the  experience  with 
open  crucible  fusions  of  their  chemical  components  cannot  be 
crystallized,  and  the  formation  of  crystals  requires  other  substances 
to  be  present  in  the  fusion,  as  water,  fluorine,  boron,  chlorine, 
tungsten,  etc.,  even  though  these  elements  are  not  a  part  of  the 
mineral  molecule  formed.  Such  elements  are  termed  mineralizers 
from  the  role  they  play  in  the  formation  of  certain  minerals. 
They  do,  however,  enter  the  molecule  of  some  of  these  rock-form- 
ing minerals  in  very  small  quantities,  as  the  micas  always  con- 
tain small  amounts  of  hydroxyl  and  fluorine,  apatite  contains 
fluorine  and  chlorine,  and  tourmaline  contains  boron  as  a  molecu- 
lar essential. 

Mineralizers  are  fluxes  in  that  they  decrease  the  viscosi.ty  of 
magmas  in  the  same  sense  that  fluorite  is  used  in  many  smelting 
operations  to  attain  the  same  end,  that  of  forming  a  liquid  slag. 
They  are  solvents  in  the  same  sense  that  water  is  a  solvent  for  salt 
in  the  two-component  system  illustrated,  and  in  each  case  they  lower 
the  fusing  point  or  the  temperature  of  separation  to  such  an  extent 
that  it  is  possible  for  molecules  to  form  at  a  much  lower  tempera- 
ture and  at  temperatures  far  below  their  actual  melting  points, 
as  many  minerals  are  unstable  at  their  temperature  of  fusion  and 
break  down  into  components  formed  of  molecules  of  a  less  com- 


THE  ORIGIN   OF  MINERALS  247 

plex  nature,  and  it  is  not  possible  for  them  to  form  at  the  temper- 
ature of  their  freezing  point.  Such  minerals  are  termed  the  low 
temperature  minerals.  Their  molecules  are  considered  to  be  more 
complex  than  those  which  form  from  direct  fusion  and  without  the 
aid  of  mineralizers,  or  the  high  temperature  minerals.  In  the  low 
temperature  minerals  are  included  the  amphiboles,  micas,  sodalites, 
nepheline,  tourmaline,  topaz,  beryl,  titanite,  quartz,  albite,  and 
orthoclase,  and  also  many  rare  minerals  of  the  pegmatites.  In  the 
synthesis  of  this  class  a  mineralizer  must  usually  be  present,  either 
to  lower  the  fusing  point  or  to  reduce  the  viscosity  of  the  melt, 
while  some,  as  topaz,  tourmaline,  and  muscovite,  have  never  been 
produced  artificially. 

That  mineralizers  have  been  present  during  the  stage  of  crystal- 
lization of  such  rocks  as  granite,  syenite,  and  pegmatites  is  shown 
by  the  simple  fact  that  the  quartz  of  such  rocks  always  contains 
numerous  cavities  holding  liquid  inclusions,  and  that  the  micas  con- 
tain hydroxyl  and  fluorine.  The  mineralizer  is  usually  a  volatile 
substance  or  forms  volatile  compounds  with  the  bases  as  the 
fluorides,  and  therefore  when  the  magma  is  extruded  they  escape, 
and  with  their  escape  the  tendency  to  quickly  solidify  and  the  for- 
mation of  glass  is  increased.  Basalts  are  more  often  crystallized 
than  are  the  rhyolites  and  andesites,  as  in  the  latter  the  mineral- 
izers have  escaped,  leaving  the  magma  by  nature  too  viscous  to 
crystallize.  The  coarse  crystalline  forms,  granite  and  syenite,  are 
plutonic  rocks  and  have  crystallized  under  conditions  which  pre- 
clude any  escape  of  the  volatile  mineralizers,  or  they  have  escaped 
very  slowly. 

In  the  process  of  cooling,  those  compounds  which  are  the  more 
insoluble  have  separated  first,  with  the  result  that  the  remaining 
liquid  portion  is  a  concentrated  solution  of  the  more  soluble,  and 
the  dykes,  veins,  and  marginal  masses  connected  with  some  granites 
and  known  as  pegmatites  are  the  result  of  and  represent  the  ulti- 
mate concentration  of  some  of  the  constituents  of  the  original  fused 
magma.  Crystals  of  pegmatites  are  large  and  well  formed,  and  such 
dykes  contain  minerals  in  quantity  which  are  rare  accessories  in  the 
rock  mass  as  a  whole.  There  seems  to  be  no  good  reason  why  the 
condition  should  not  change  during  crystallization  from  that  at 
the  beginning,  a  fused  magma,  at  a  high  temperature,  far  above  the 
critical  temperature  of  water,  365°  C.,  to  that  at  the  end,  when  the 
final  product  of  crystallization  may  be  separated  from  a  solution  in 
water,  though  hot  and  under  pressure. 


248  MINERALOGY 

Pneumatolysis.  —  Most  mineralizers  form  gases  and  volatile 
compounds  which  penetrate  the  cracks,  cavities,  and  pores  of  adja- 
cent rock  formations,  where  they  may  be  condensed,  or  by  decom- 
position deposit  compounds ;  or  by  the  reduction  of  temperature 
and  pressure  deposit  compounds  carried  in  solution;  or  by  the 
direct  interaction  with  the  rock  mass  form  minerals,  all  of  which 
are  concentrated  near  the  margin  of  large  intruded  igneous  masses 
and  extending  out  as  impregnations  in  the  sedimentary  formations 
of  the  immediate  vicinity.  Numerous  ore  deposits  are  of  this  na- 
ture, especially  those  of  tin  or  cassiterite,  where  the  tin  has  been 
concentrated  by  a  squeezing  out  of  the  volatile  tin  fluoride  to  be 
decomposed  by  contact  with  steam ;  it  deposits  SnO2  in  the  open 
veins  and  cracks  of  the  formation;  this  decomposition  produces 
hydrofluoric  acid  to  further  react  with  the  walls  of  the  formation 
to  produce  such  minerals  as  fluorite  and  topaz.  Minerals  formed 
by  the  action  of  gases  or  volatile  compounds  are  said  to  be  pneu- 
matolytic  or  formed  by  pneumatolysis.  The  gases  most  active  in 
pneumatolysis  are  fluorine,  water  or  steam,  hydrogen  sulphide, 
boron,  chlorine,  and  their  volatile  compounds. 

Typical  minerals  formed  by  this  process  are  tourmaline,  topaz, 
cassiterite,  rutile,  oxides  of  iron,  micas,  fluorite,  quartz,  sulphides 
of  copper,  lead,  arsenic,  also  titanite  and  axinite,  and  numerous 
rare  minerals  as  wolframite,  scheelite,  uraninite,  and  allanite. 

Intimately  connected,  possibly  with  the  last  stages  of  cooling  of 
large  intruded  igneous  rock  masses,  is  the  formation  of  the  so- 
called  contact  minerals ;  produced  by  the  interaction  of  and  the 
impregnation  of  the  formation  by  steam  and  associated  with  the 
heat  of  contact  and  pressure.  This  stage  directly  follows  that  of 
the  pneumatolytic  action  of  gases  and  is  usually  termed  thermal 
metamorphism,  though  heat  is  connected  with  all  metamorphic 
changes;  m  this  case  the  temperature  is  comparatively  high; 
especially  near  the  contact  of  the  igneous  rock  intruded.  The  min- 
erals formed  under  such  conditions,  when  the  volatile  gases  are  pre- 
vented from  escape  and  are  held  under  pressure,  are  of  a  complex 
nature  when  contrasted  with  those  formed  where  there  is  a  ready 
escape  of  gases. 

The  materials  at  hand  for  the  formation  of  new  minerals  are  not 
only  those  contained  in  the  sedimentary  formation  under  the  pro- 
cess of  alteration,  but  also  those  elements  carried  in  solution,  by 
the  interaction  of  which  at  high  temperatures  and  pressure  and  con- 
tinued through  long  periods  of  time  large  and  most  beautifully  de- 


THE  ORIGIN  OF  MINERALS  249 

veloped  crystalline  specimens  are  formed.  In  such  regions  hydrated 
minerals  as  the  zeolites,  kaolinite,  etc.,  are  dehydrated,  forming 
feldpars ;  silicic  acid  will  replace  carbonic  acid  in  limestones ;  and 
large  areas  of  limestone  have  been  almost  entirely  replaced  by  such 
silicates  as  scapolite,  pyroxenes,  micas,  tourmaline,  and  feldspars. 
Other  characteristic  minerals  of  contact  metamorphism  are  epidote, 
garnets,  vesuvianite,  spinels,  wernerite,  andalusite,  corundum, 
apatite,  biotite,  phlogopite,  and  all  those  minerals  so  common  in 
the  granular  limestones.  Practically  there  is  no  sharp  line  to  be 
drawn  separating  the  two  processes  of  thermal  or  contact  metamor- 
phism and  pneumatolysis ;  one  begins  where  the  other  leaves  off, 
and  the  two  are  so  intimately  related  in  the  formation  of  some 
contact  minerals  as  to  be  quite  impossible  of  separation. 

The  volatile  matter  given  off  by  hot  intruded  magmas  may  pos- 
sibly be  exemplified  by  a  study  of  the  gases  escaping  from  active 
volcanoes,  though  the  difference  here  would  be  that  of  a  great  de- 
crease of  pressure.  With  the  relief  of  all  pressure,  only  those  com- 
pounds would  exist  which  are  stable  at  atmospheric  pressures, 
compounds  possibly  quite  different  from  those  which  are  in  solu- 
tion and  which  are  stable  under  high  pressures  and  where  the 
volatile  gases  do  not  escape  freely.  The  common  gases  emitted 
from  volcanoes,  solfataras,  and  fumaroles  are  hydrogen  sulphide, 
sulphur  dioxide,  carbon  dioxide,  hydrochloric  acid  and  volatile 
chlorides  and  fluorides,  steam,  nitrogen,  and  other  gases  in  much 
smaller  quantities.  Many  minerals  are  formed  as  sublimates  by 
the  direct  condensation  and  interaction  of  these  gases.  They  are 
usually  simple  in  their  molecular  structure,  in  contrast  to  those 
formed  at  depths  and  under  pressure.  Sulphur  is  usually  present, 
a  product  formed  by  the  interaction  of  S02  and  H2S,  also  sulphates 
and  chlorides,  as  NaCl,  PbCl2,  and  where  the  temperature  is 
high,  the  chlorides  are  decomposed,  forming  oxides  as  melaconite 
(CuO),  cuprite,  magnetite,  and  hematite;  all  these  minerals  are 
known  in  the  lavas  around  Vesuvius. 

Hot  Solutions.  —  After  the  cooling  of  an  injected  magma  has 
progressed  to  such  a  degree  that  it  is  possible  for  the  water  to  exist 
as  such  or  the  temperature  has  fallen  below  365°,  the  critical  tem- 
perature of  water,  then  many  minerals  are  formed  or  deposited 
from  the  water  solutions.  Water  under  pressure  dissolves  many 
compounds  with  ease  which  at  the  surface  or  under  normal  temper- 
ature and  pressure  are  but  slightly  soluble  or  are  considered 
to  be  insoluble.  The  solubility  of  many  substances  is  greatly 


250  .MINERALOGY 

increased  by  other  components  in  the  solution,  especially 
CO2,  H2S,  HC1,  HF,  or  alkalies. 

All  substances,  even  the  most  insoluble,  are  dissolved  in  slight 
amounts;  insolubility  is  only  a  relative  term.  Gold  itself  is 
soluble  in  water  and  especially  so  in  the  presence  of  ferric  chloride, 
and  solutions  of  choloidal  gold  will  retain  the  metal  without  sepa- 
ration for  a  long  time.  Solubility  is  also  increased  with  an  increase 
of  pressure,  provided  that  the  total  volume  of  solute  and  solvent 
is  decreased  by  solution.  Hot  flowing  waters  under  pressure  are 
therefore  very  complex  solutions,  and  are  usually  nearly  saturated 
with  compounds  which  at  the  surface  would  be  considered  insoluble. 
When  such  solutions,  flowing  through  the  veins  and  fissures  of  the 
adjacent  formation,  gain  access  to  regions  of  lower  temperatures 
and  pressures,  they  are  in  a  state  of  unstable  equilibrium,  and  some 
of  their  components  are  deposited.  As  the  directional  flow  is 
constant  for  long  times,  these  streams  of  solutions  serve  to  concen- 
trate the  soluble  components  in  the  fissures  and  veins,  where  they 
are  deposited,  often  in  definite  order  and  at  times  filling  completely 
the  original  veins  and  fissures  along  which  the  solutions  flowed. 
Many  ore  deposits  filling  veins,  fissures,  and  pipes  have  been  con- 
centrated by  this  method ;  and  often  where  such  heated  solutions 
reach  the  surface,  as  at  the  Steamboat  Springs  of  Nevada,  the  depo- 
sition of  sulphides  has  been  noted.  Pyrite,  chalcopyrite,  galena, 
arsenides,  antimonides,  and  many  minerals  of  like  character  have 
been  deposited  on  the  walls  of  channels  by  the  hot  solutions  flow- 
ing through  them. 

When  minerals  are  crystallized  under  high  pressures,  the  molecules 
are  compact,  and  the  minerals  formed  are  of  a  phase  in  which  the 
specific  gravity  is  high ;  as  quartz,  with  a  specific  gravity  of  2.65,  will 
form'  rather  than  tridymite,  with  a  specific  gravity  of  2.3.  Tridy- 
mite  occurs  in  surface  lavas  where  it  has  crystallized  at  reduced  or 
atmospheric  pressures.  High  pressures,  other  things  being  equal, 
will  induce  the  formation  of  pyrite,  with  a  specific  gravity  of  5 
rather  than  marcasite  with  a  specific  gravity  of  4.9.  As  solids  in 
the  crystalline  phase  occupy  less  space  or  have  a  higher  specific 
gravity  than  the  amorphous  phase,  high  pressures  will  tend  to  dis- 
solve the  amorphous  phase  and  to  redeposit  the  chemical  com- 
pound as  crystalline.  Large  areas  of  amorphous  limestones  have 
been  thus  crystallized  under  pressure.  Ordinary  obsidian  when 
crystallized  will  occupy  less  space  and  during  crystallization  will 
shrink  from  3  to  11  per  cent,  of  its  original  volume.  Garnets,  feld- 


THE   ORIGIN   OF  MINERALS  251 

spars,  micas,  pyroxene,  amphibole,  epidote,  spinels,  and  andalusite 
are  minerals  which  form  under  high  pressures ;  the  latter  are  dense 
minerals  and  differ  entirely  from  minerals  formed  at  or  near  the 
surface,  where  hydrated  minerals  are  the  rule,  a  class  quite  unstable 
under  high  pressures  and  heat.  Under  such  conditions  the  latter  are 
quickly  transformed  to  the  more  stable  compact  molecule.  If  the 
minerals  which  have  been  formed  at  depths  are  brought  to  the  sur- 
face, they  become  the  less  stable  phase,  when  they  are  subject  to 
change  and  decomposition,  either  through  the  solution  of  some  of 
their  constituents,  or  oxidation,  or  by  replacement  of  some  ele- 
ments by  others. 

The  action  of  ground  waters.  —  Water  falling  as  rain  passes 
through  and  over  the  soil,  following  the  course  of  least  resistance, 
constantly  taking  up  and  carrying  along  soluble  compounds.  As 
CO2  is  absorbed  from  the  soil  and  the  air,  its  solvent  power  on  other 
carbonates,  as  calcium  carbonate,  is  thereby  increased ;  such  car- 
bonates in  solution  are  considered  as  the  bicarbonates  and  are 
more  soluble  than  the  normal  salts. 

There  is  an  area  near  the  surface,  usually  termed  the  area  of  oxi- 
dation or  weathering,  in  which  the  pores  and  cavities  of  the  soil 
and  rocks  are  not  completely  filled  by  the  percolating  waters.  In 
this  area  oxygen  is  an  active  agent,  and  many  minerals  are  oxidized 
as  they  are  carried  in  solution,  or  oxidation  is  the  cause  of  precipita- 
tion. In  this  area  sulphides,  as  pyrite,  are  oxidized  to  sulphates, 
forming  ferric  and  ferrous  sulphates,  both  of  which  are  soluble  and 
carried  in  solution  by  the  descending  ground  waters,  to  act  as  power- 
ful reagents  in  the  transformation  and  solution  of  other  minerals. 
In  the  area  of  oxidation,  such  processes  as  carbonation  and  hydra- 
tion,  in  addition  to  oxidation  and  solution,  are  active,  resulting 
in  the  formation  of  carbonates  and  many  hydrated  minerals  and 
hydroxides,  as  limonite  and  the  oxides  of  manganese,  and  alumin- 
ium, as  well  as  sulphates,  arsenates,  and  phosphates.  In  this  region 
carbon  dioxide  can  replace  silica,  the  silica  being  carried  in  solu- 
tion to  be  again  deposited  either  as  the  amorphous  form,  opal,  or  as 
quartz,  according  to  conditions.  This  reaction  is  the  reverse  of 
that  which  takes  place  at  greater  depths  and  under  the  influence  of 
heat  and  pressure,  where  silica  replaces  CO2.  Upon  the  whole 
minerals  in  the  zone  of  oxidation  or  weathering  are  decomposed  and 
disintegrated,  forming  products  which  with  the  addition  of  water 
and  oxygen  have  increased  in  volume,  and  in  the  production  of  which 
hydration  and  oxidation  are  most  important  processes. 


252  MINERALOGY 

In  the  percolating  ground  waters  many  sulphides  are  soluble, 
and  especially  so  as  these  waters  are  acid  in  some  cases  and  alkaline 
in  others.  These  solutions  finally,  in  their  descent,  reach  the  area 
where  the  pores,  fissures  and  cavities  of  the  rocks  are  completely 
filled;  and  the  solution  joins  that  reservoir  of  ever  flowing  water 
termed  the  ground  water;  where  through  the  action  of  diffusion 
and  flowage,  solutions  of  various  substances  are  mixed  as  reagents 
in  a  beaker,  with  the  resulting  precipitations.  Here,  however,  the 
walls  of  the  containing  cavities  are  active  agents  and  by  the  inter- 
change of  their  elements  enter  the  reactions  as  important  factors  in 
the  process  of  chemical  replacement,  or  metasomasis. 

The  products  of  the  oxidation  of  pyrite  are  varied,  according  to 
the  conditions  and  the  amount  of  oxygen  available. 

FeS2  +  6  O  =  FeS04  +  SO2,  which  yields  a  solution  of  ferrous 
sulphate  and  an  acid  solution.  2  FeS2  +  14  O  =  Fe2(SO4)3  +  SO2, 
yielding  a  solution  of  ferric  sulphate  and  an  acid  solution.  This 
solution  may  interact  with  more  pyrite,  FeS2  +  Fe2(SO4)3  = 
3  Fe(S04)  +  2  S,  yielding  ferrous  sulphate  and  free  sulphur,  also 
hydrogen  sulphide  and  sulphites  may  be  produced.  Ferrous  sul- 
phate in  solution  is  a  powerful  reducing  agent  and  results  in  many 
cases  in  the  precipitation  of  metals,  as  silver  and  copper,  and  as 
oxides,  as  cuprite.  Many  deposits  of  iron  ore  have  resulted  from 
the  interaction  of  solutions  of  iron  with  carbonates,  in  which  the 
iron  has  replaced  the  calcium  of  limestones  and  shells.  CaCO3 
+  FeS04  =  FeCOs  +  CaS04,  when  siderite  and  gypsum  are 
formed. 

It  is  by  means  of  a  similar  replacement  or  interaction  of  solutions 
containing  sulphates  resulting  from  the  oxidation  of  sulphides  near 
the  surface  that  such  carbonates  as  smithsonite,  rhodochrosite, 
witherite,  cerussite,  azurite,  and  malachite  are  formed ;  these  min- 
erals may  also  be  formed  from  solutions  of  carbonates.  The  more 
soluble  sulphate  gypsum  may  be  replaced  by  barium,  strontium, 
or  lead  sulphates.  The  minerals  barite,  celestite,  and  anglesite 
are  often  precipitated  on  the  walls  of  fissures  and  veins  as  the  result 
of  the  intermingling  of  solutions.  Minerals  formed  by  chemical 
replacement  occur  usually  as  lens-shaped  masses  embedded  in  the 
rock  which  has  been  the  cause  of  their  formation.  Large  areas  of 
limestone  have  been  changed  to  dolomite  by  the  replacement  of 
calcium  by  magnesium.  Most  of  the  galena  and  sphalerite 
deposits  of  the  Mississippi  Valley  have  resulted  from  replace- 
ments in  limestones. 


THE  ORIGIN   OF  MINERALS  253 

The  surface  sulphides  of  many  deposits  have  been  completely 
oxidized.  Most  of  the  products  of  oxidation  are  carried  off  in  solu- 
tion ;  only  a  small  proportion  remaining  to  indicate  the  nature  of  the 
original  minerals.  The  red  oxide  of  iron,  hematite,  is  especially 
characteristic  of  such  conditions  and  often  covers  the  lower  un- 
oxidized  sulphides  like  a  cap ;  from  its  position  it  has  been  termed 
the  "  iron  cap."  The  upper  oxidized  surface  areas  of  ore  deposits 
are  also  termed  the  gossan.  Minerals  characteristic  of  the  gossan 
are  the  carbonates,  hydrates,  sulphates,  arsenates,  and  oxides. 
The  depth  of  these  oxidized  minerals,  or  the  gossan,  will  depend 
upon  climatic  conditions,  the  nature  of  the  formation,  and  the  depth 
of  the  ground  water.  The  oxidized  areas  gradually  give  way  to 
the  region,  in  ore  deposits,  termed  the  areas  of  secondary  enrich- 
ment, where  the  sulphides  have  not  only  not  been  oxidized,  but 
have  been  increased  in  some  of  their  constituents.  The  metallic 
sulphides  carried  down  in  solution  from  the  gossan  above  are  pre- 
cipitated and  added  to  the  original  metallic  content  of  sulphides  be- 
neath. That  the  waters  descending  from  the  oxidized  areas  above 
do  carry  sulphates  in  solution  is  abundantly  proven  by  large  num- 
bers of  analyses  of  mine  waters,  and  also  by  the  fact  that  in  many 
mining  regions  the  mine  waters  carry  copper  sulphate  in  such  quan- 
tities that  it  pays  to  precipitate  it  from  the  solutions  with  scrap 
iron.  Of  the  metallic  sulphides,  pyrite,  marcasite,  and  pyrrhotite 
are  the  most  readily  oxidized,  and  when  in  contact  with  solutions 
containing  copper  sulphate  they  precipitate  the  copper  as  sulphide, 
forming  chalcopyrite.  At  the  Copper  Queen  Mine,  at  Bisbee, 
Arizona,  pyrite  too  poor  in  copper  to  pay  for  smelting  was  used  to 
fill  old  stopes,  is  now,  after  some  ten  or  twelve  years,  remined  and 
smelted,  having  collected  nearly  10  per  cent,  of  copper  by  precipita- 
tion from  the  mine  waters  through  replacement  of  the  iron.  Pyrite 
not  only  serves  as  a  precipitant  for  copper^  but  also  for  lead,  silver, 
zinc,  and  other  elements  less  basic. 

By  this  process  of  oxidation  and  precipitation  the  area  of  second- 
ary enrichment,  or  the  unoxidized  sulphides  directly  underneath 
the  gossan,  contains  not  only  the  valuable  metals  which  were  their 
original  content,  but  added  to  this  by  replacement  is  a  large  propor- 
tion of  that  formerly  contained  in  the  gossan  above.  The  ore  in 
many  mines  has  been  found  too  poor  to  work  when  the  regions  be- 
low the  areas  of  secondary  enrichment  have  been  reached.  This 
process  is  therefore  one  of  prime  importance,  and  must  be  consid- 
ered in  the  valuation  of  ore  deposits.  Surface  waters  and  spring 


254  MINERALOGY 

waters  are  complex  solutions  of  carbonates,  sulphates,  and  chlorides 
of  the  more  soluble  bases,  as  the  alkalies,  alkali  earths,  iron,  man- 
ganese, aluminium,  as  well  as  silica  and  a  large  number  of  other  ele- 
ments in  very  small  quantities.  The  amount  of  solid  residue  in 
the  ordinary  natural  waters  will  vary  greatly,  depending  upon  the 
nature  of  the  soil  and  the  geological  formations  over  and  through 
which  the  waters  flow.  Rivers  of  limestone  regions  are  usually 
high  in  their  content  of  calcium  and  magnesium  carbonates,  as 
these  carbonates  are  carried  into  solution  as  bicarbonates  and  they 
are  termed  hard  waters.  Such  waters  are  not  saturated  solutions, 
containing  in  solids  from  10  to  40  parts  to  the  100,000,  unless 
greatly  concentrated  by  evaporation  or  by  the  loss  of  carbon  diox- 
ide, when  the  normal  carbonates  may  crystallize  or  be  deposited,  as 
is  the  case  in  the  formation  of  stalactites  and  stalagmites  in  caves 
and  the  calcite  cement  of  some  conglomerates.  Though  the  amount 
of  dissolved  solids  in  a  river  water  seems  very  small,  yet  when  the 
contant  flow  is  considered,  enormous  quantities  of  soluble  com- 
pounds are  carried  in  solution.  It  has  been  estimated  that  the  St. 
Lawrence  at  Ogdensburg,  New  York,  having  a  flowage  of  248,518 
cubic  feet  a  second,  and  a  salinity  of  13.2  parts  per  100,000,  will 
transport  by  this  point,  annually,  29,278,000  metric  tons  of  salts 
in  solution.  Taking  into  consideration  the  area  drained,  exclusive 
of  water  areas,  this  is  equal  to,  if  evenly  distributed,  102  tons  of 
matter  carried  off  in  solution  per  square  mile  each  year.  The  vol- 
ume of  the  ocean  is  so  great  that  its  saline  content  is  not  apparently 
increased  by  these  enormous  quantities  of  dissolved  salts  which  are 
being  poured  into  it  by  every  river;  but  if  at  any  time  such  waters  are 
confined  and  evaporation  equals  or  surpasses  the  annual  addition  by 
rivers  and  rain,  the  inclosed  lake,  basin,  or  arm  of  the  sea  will  become 
concentrated,  as  in  the  case  of  salt  and  alkali  lakes  of  arid  regions. 
When  cencentration  advances  to  saturation,  their  salts  are  usually 
deposited  in  a  definite  order,  forming  stratified  deposits,  following 
the  order  of  saturation  in  regard  to  the  various  minerals  separated. 
There  is  therefore  in  such  saline  deposits  an  order  or  sequence  from 
the  bottom  to  the  top,  except  where  this  order  may  have  been 
modified  by  some  peculiar  local  characteristic  or  condition,  caused 
by  an  unusual  component  in  the  solutions. 

When  ordinary  sea  water  is  concentrated  to  about  one  tenth  of 
its  original  volume,  crystals  begin  to  form.  In  the  normal  concen- 
tration of  sea  water  these  crystals  are  gypsum,  and  gypsum  usually 
forms  the  lower  stratum  of  such  deposits.  Anhydrite  may  re- 


THE  ORIGIN  OF  MINERALS  255 

place  the  gypsum  deposit,  as  gypsum  is  transformed  to  the  anhy- 
drous sulphate  as  concentration  advances,  being  induced  by  a 
raise  of  temperature  and  a  concentration  of  sodium  chloride.  On 
top  of  the  calcium  sulphate,  halite  or  sodium  chloride  is  deposited, 
and  when  finally  the  mother  brines  reach  the  point  of  saturation  for 
the  more  soluble  sulphate  and  chloride  of  magnesium,  then  double 
salts  are  precipitated,  as  at  Stassfurt  in  Prussia,  where  some  thirty 
species  of  minerals  have  separated  from  a  concentrating  brine. 
Rock  salt  and  gypsum  or  anhydrite  are  constant  companions,  their 
positions  indicating  their  separation  from  concentrating  brines, 
though  deposits  of  gypsum  will  occur  without  the  salt,  or  it  is  often 
interbedded  with  clay  and  salt,  indicating  periods  of  changed  con- 
ditions in  the  concentration  of  the  mother  brine  caused  by  an 
influx  of  the  sea  water  or  by  periodic  additions  of  a  dilute  solution. 
Often  the  concentration  has  never  reached  the  stage  when  rock  salt 
is  deposited,  and  in  such  cases  the  deposit  of  calcium  sulphate  will 
exist  unassociated  with  the  usual  stratum  of  halite ;  or  again  the 
salt  may  have  been  entirely  carried  away  in  solution  by  the  ground 
water,  as  salt  springs  are  not  uncommon.  The  minerals  of  saline 
deposits  include  borates,  carbonates,  chlorides,  nitrates,  and  double 
salts  with  various  amounts  of  water  of  crystallization.  They  are 
all  quite  soluble  in  water,  and  as  concentration  of  natural  solutions 
is  favored  only  in  very  dry  climates,  they  are  all  characteristic  of 
arid  regions. 


CHAPTER  III 
PHYSICAL    PROPERTIES 

Cleavage  and  fracture.  —  When  a  mineral  is  broken  by  the  blow 
of  a  hammer  on  a  sharp-edged  instrument,  as  a  cold  chisel,  held 
on  the  specimen,  it  either  breaks  in  a  smooth  plane  face  or  irregu- 
larly. The  former  is  cleavage ;  the  latter  is  fracture.  Cleavage 
is  caused  by  the  separation  along  and  between  layers  of  molecules. 
Taken  in  this  sense,  only  crystalline  minerals  may  possess  cleavage. 


FIG.  371.  —  A  Crystal  of  Galena,  showing  the  Perfect  Cubical  Cleavage. 

Missouri. 


Aurora, 


Cleavage  is  named  from  the  crystal  form  to  which  the  separation 
is  parallel,  and  is  represented  by  the  letter  representing  the  form, 
as  cleavage  m  =  prismatic  or  parallel  to  the  unit  prism,  c  =  basal, 
etc.  Galena  and  halite  have  cubical  cleavage,  Fig.  371 ;  cal- 
cite,  rhombohedral ;  spodumene,  prismatic,  etc.  A  perfect  cleav- 
age is  one  in  which  the  plane  face  obtained  is  smooth,  even,  and 
polished  and  is  generally  easy  to  obtain  but  not  necessarily  so; 

256 


PHYSICAL  PROPERTIES  257 

such  a  cleavage  is  the  cubic  cleavage  of  galena,  in  which  it  is 
almost  impossible,  even  on  grinding,  to  obtain  particles  not  cubical 
in  form.  Cleavage  faces  may  be  highly  polished  and  smooth, 
even  though  difficult  to  obtain,  as  the  rhombohedral  cleavage  of 
quartz. 

Cleavage  surfaces  parallel  to  the  faces  of  the  same  crystal  form 
are  all  of  the  same  character ;  they  possess  the  same  luster  and  are 
obtained  with  the  same  ease,  as  the  three  directions  of  the  rhombo- 
hedral cleavage  in  calcite,  or  the  four  directions  of  the  octahedral 
cleavage  of  fluorite.  In  anhydrite,  where  there  are  also  three  cleav- 
age directions  at  right  angles,  they  are  obtained  with  unequal  ease 
and  each  differs  from  the  other  in  luster.  In  gypsum  there  is 
one  very  perfect  cleavage,  parallel  to  the  clinopinacoid,  cleavage  b, 
and  another  parallel  to  the  orthopinacoid,  cleavage  a,  which  is 
obtained  with  more  difficulty;  the  resulting  flat  cleavage  plates 
are  parallel  to  the  easy  clinopinacoidal  cleavage,  and  are  bounded  by 
two  straight  edges  parallel  to  the  less  easy  orthopinacoidal  cleav- 
age. This  is  also  the  usual  shape  of  cleavage  fragments  in  ortho- 
clase.  The  comparative  ease  of  coexisting  cleavages  materially 
influence  the  general  shape  of  the  fragments  into  which  a  mineral 
breaks.  The  descriptive  terms  as  applied  to  cleavage  are:  per- 
fect, as  the  rhombohedral  cleavage  of  calcite ;  distinct,  as  the  pris- 
matic cleavage  of  rutile ;  imperfect,  as  the  basal  cleavage  of  beryl 
or  apatite ;  traces  or  indistinct,  as  the  cubic  and  octahedral  cleavage 
of  pyrite ;  difficult,  as  the  cleavage  of  quartz  or  tourmaline. 

When  the  cleavage  pieces  are  flat  and  it  is  possible  to  cleave  very 
thin  laminae,  the  cleavage  is  said  to  be  micaceous,  as  in  muscovite 
or  biotite.  The  laminae  are  described  as  flexible  when  they  can 
be  bent,  even  though  they  may  crack,  but  without  parting,  as  in 
chlorite;  elastic  when  they  bend  with  an  even  curve  without 
cracking  and  on  being  released  spring  back  to  their  original  flat 
condition,  as  in  muscovite.  The  laminae  are  brittle  when  they 
break  easily  on  bending,  as  in  margarite;  tough  in  one  direction 
and  brittle  in  another,  as  in  gypsum. 

Parting  is  not  a  true  cleavage,  but  is  the  result  of  pressure  or 
strain  to  which  minerals  have  been  subjected  and  is  therefore  not 
characteristic  of  all  specimens  of  the  same  species,  but  is  peculiar 
to  localities  and  formations  and  not  necessarily  confined  to  crys- 
tals. Some  garnets  have  a  parting  parallel  to  the  rhombic  dodec- 
ahedral  faces,  others  have  no  indication  of  it.  Magnetite  and 
franklinite  have  an  octahedral  parting. 


258 


MINERALOGY 


Fracture  is  where  the  specimen  breaks,  not  along  a  smooth  plane 
face,  but  irregularly  in  no  definite  direction.  The  appearance  of 
the  uneven  surface  of  fracture  is  also  characteristic,  and  the  follow- 
ing terms  are  used  in  describing  the  fracture  of  minerals:  Conchoi- 
dal,  when  the  break  results  in  curved  or  warped  surfaces,  as  in  glass, 
chrysocolla,  or  flint,  Fig.  372.  Subconchoidal,  when  the  curves  are 
not  well  marked,  are  uneven,  and  the  surfaces  are  slightly  rough,  as 
in  most  minerals.  Hackly,  when  the  roughness  consists  of  sharp 
points,  as  in  copper  and  most  metals.  Splintery,  when  the  fracture 


FIG.  372.  —  Obsidian,  showing  a  Conchoidal  Fracture. 

shows  a  fibrous  structure,  as  in  some  steatites.  Scaly,  where  the 
mineral  is  formed  of  fine  crystal  scales,  as  in  lepidolite. 

Tenacity  and  hardness  both  depend  upon  cohesion.  The  force 
necessary  to  overcome  this  attraction  of  one  molecule  for  its 
neighbor  will  vary  with  the  molecule  and  the  direction  in  the  crys- 
tal. 

A  sectile  mineral  may  be  cut  with  a  knife,  and  the  shavings 
remain  whole  and  possibly  curl  like  the  shaving  of  a  quill  or  horn, 
as  graphite,  molybdenite,  and  most  micas  and  metals. 

A  mineral  is  malleable  when  it  flattens  on  hammering  and 
spreads  out,  increasing  in  area  without  cracking,  as  lead,  silver,  or 
tin;  ductile  when  it  may  be  drawn  out  in  wire,  as  copper,  silver, 
iron ;  brittle  when  on  hammering  it  breaks  down  in  a  powder  as  do 
most  minerals,  —  though  the  ease  with  which  this  occurs  is  modified 
by  such  terms  as  tough,  as  rhodonite;  soft,  as  wad;  friable,  as 
kaolinite. 


PHYSICAL  PROPERTIES 


259 


Hardness  is  an  uncertain  term,  but  in  mineralogy  it  may  be 
taken  to  indicate  the  ease  or  difficulty  with  which  a  mineral  may 
be  scratched.  It  is  a  directional  quality  and  not  only  varies  with 
the  crystal  form,  but  with  the  direction  on  the  same  crystal  face. 
The  hardness  of  cyanite  when  tested  on  the  macropinacoid  parallel 
to  the  vertical  axis  is  4-5,  but  tested  on  the  same  face  at  right 
angles  to  this  direction  it  is  nearly  7.  When  the  hardness  is  tested 
in  all  directions  on  a  face  like  that  of  cyanite,  and  the  values 
plotted  in  a  curve,  this 
curve  will  be  found  to 
conform  to  and  reflect 
the  symmetry  of  the  face, 
Fig.  373. 

Where  it  is  wished  to 
determine  the  hardness 
with  some  degree  of  ac- 
curacy, the  sclerometer 
is  used,  the  principle 
of  which  depends  upon 
weighting  a  sharp  point 
of  a  very  hard  substance, 
as  a  pointed  diamond, 
until  it  just  scratches  the 
surface  of  the  specimen  when  drawn  across  in  any  particular  direc- 
tion in  which  the  hardness  is  wished  to  be  determined.  The  weight 
of  the  load  which  just  produces  a  scratch  is  taken  as  a  measure  of 
the  hardness.  Again,  the  material  removed  by  the  point  under  a 
constant  load  after  being  drawn  across  the  face  a  determined  num- 
ber of  times  may  also  be  taken  as  a  measure  of  the  hardness,  but 
here  the  hardness  will  be  inversely  as  the  weight  of  material  re- 
moved. The  loss  of  weight  or  the  material  removed  is  determined 
by  weighing  the  crystal. 

Hardness  as  a  test  in  the  identification  of  mineral  species  is  deter- 
mined by  reference  to  a  series  of  minerals  arranged  in  a  table,  in 
ascending  scale,  from  1,  the  softest,  to  10,  the  hardest.  The  series 
of  ten  test  minerals  is  known  as  Mori's  scale  of  hardness,  a  purely 
arbitrary  scale  and  in  no  way  representing  the  true  relation,  as  the 
difference  between  9  and  10  in  the  scale  is  ever  so  much  greater 
than  the  difference  between  1  and  2.  Moh's  scale  of  hardness 
below  is  also  expressed  in  terms  of  hardness  as  determined  by  the 
sclerometer,  in  which  the  hardness  of  the  sapphire  is  taken  as  1000. 


FIG.  373.  —  Curve  of  Hardness  on  the  Cube 
Face  of  Fluorite. 


260  MINERALOGY 

I  Talc          1.13  6.  Orthoclase  ...  191 

2.  Gypsum 12.03  7.  Quartz    ....  254 

3.  Calcite 15.3  8.  Topaz     ....  459 

4.  Fluorite 37.3  9.  Sapphire      .     .     .  1,000 

5.  Apatite 53.5  10.  Diamond     .     .     .  140,000 

The  specimens  used  for  the  scale  should  be  crystalline,  cleavable, 
and  as  nearly  transparent  as  possible. 

In  testing  a  mineral  for  hardness,  a  sharp  point  of  the  mineral 
in  the  scale  or  test  mineral  is  pressed  firmly  on  a  smooth  surface 
of  the  mineral  to  be  tested  and  drawn  across  with  a  quick  move- 
ment. The  harder  the  mineral  the  more  pressure  will  be  re- 
quired to  make  the  scratch.  Care  must  also  be  taken  not  to  mis- 
take the  mark  left  by  a  soft  mineral  on  a  harder  surface  as  a 
scratch.  This  mark,  like  a  chalk  mark,  may  be  easily  rubbed 
off,  while  a  scratch  may  be  tested  by  drawing  the  finger  nail  across 
it.  Minerals  of  the  same  hardness  will  scratch  each  other  when 
tested  in  this  way. 

In  ordinary  cases,  as  the  determination  of  hardness  for  use  with 
the  blowpipe  tables,  hardness  above  or  below  3,  5,  and  7  is  an  im- 
portant point.  If  a  mineral  will  scratch  a  copper  coin,  it  may  be 
considered  as  above  3  in  hardness,  and  failing  to  scratch  glass 
it  would  be  below  5 ;  failing  to  scratch  quartz  and  scratching  glass, 
it  would  be  between  5  and  7  in  hardness.  With  experience  the 
approximate  hardness  of  a  specimen  may  be  determined  by  the 
ease  or  difficulty  with  which  it  is  cut  or  scratched  by  an  old  knife- 
blade  or  file.  When  the  knife  fails  to  have  any  effect  on  the  speci- 
men, it  is  above  6  in  hardness.  In  testing  the  hardness  of  a  min- 
eral, care  must  always  be  taken  that  it  is  as  pure  as  possible  and 
free  from  decomposition  products,  remembering  that  impurities, 
as  sand,  etc.,  will  cause  a  soft  mineral  to  appear  much  harder  than 
in  reality  it  is,  and  by  decomposition  minerals  which  when  unaltered 
are  very  hard,  as  corundum  or  andalusite,  will  appear  softer. 

Specific  gravity  =  G,  is  the  expression  in  figures  of  the  ratio  of 
the  weight  of  unit  volume  of  the  substance  to  the  weight  of  unit 
volume  of  water  at  4°  C.  A  'mineral  in  which  G  =  2.65  (quartz) 
will  weigh  2.65  gm.  per  cubic  centimeter,  since  one  cubic  centimeter 
of  water  weighs  one  gram.  Knowing  the  specific  gravity  of  any 
mineral  or  rock,  it  is  an  easy  matter  to  determine  the  weight  of  any 
cubic  amount,  as  a  yard  or  a  foot.  For  the  identification  of  min- 
erals, the  specific  gravity  is  determined  approximately  to  the  second 


PHYSICAL  PROPERTIES 


261 


decimal  place.  Various  specimens  of  the  same  species  will  differ 
according  to  their  purities,  but  chemically  pure  specimens  of  a  sub- 
stance determined  under  the  same  conditions  will  not  vary  in  their 
specific  gravity. 

The  specific  gravity  of  most  minerals,  including  most  of  the  sili- 
cates, will  lie  between  2.25  and  3.5.  Minerals  with  metallic  luster 
are  usually  high  and  will  lie  between  4.5  to  10, 
while  the  specific  gravity  of  the  naturally  occur- 
ring metals  reaches  as  high  as  23  in  iridium. 
The  specific  gravity  of  ice  is  0.92. 

Two  general  methods  are  followed  in  the  de- 
termination of  the  specific  gravity :  (a)  weighing 
the  substance  in  air  and  weighing  it  in  water ; 
(b)  suspending  the  substance  in  a  liquid  of  the 
same  specific  gravity,  then  determining  the  spe- 
cific gravity  of  the  liquid  by  weighing. 

(a)  There  are  several  modifications  of  this  first 
method,  depending  upon  the  accuracy  required, 
or  the  solubility  and  physical  condition  of  the 
material. 

I.  Joly  balance.  —  A  quick  but  only  an  ap- 
proximate method.  The  balance,  Fig.  374,  con- 
sists of  a  spiral  spring  S,  two  scale  pans  P  and 
P',  a  long  mirror  graduated  in  units  and  tenths, 
a  white  bead  b,  just  above  the  top  scale  pan,  to 
assist  in  reading  the  scale,  and  a  movable  bracket, 
which  supports  a  beaker  of  water.  If  it  is  wished 
to  determine  the  specific  gravity  of  a  crystal  of 
quartz,  as  an  example,  three  readings  are  neces- 
sary, and  in  each  reading  the  lower  glass  scale 
pan  P'  should  be  submerged  to  the  same  depth 
in  the  water,  should  not  touch  the  walls  of  the  beaker,  and 
should  be  free  of  air  bubbles.  The  first  reading  is  taken  with  both 
pans  empty  and  the  lower  one  submerged  in  the  beaker  of  water ; 
the  eye  is  held  in  such  a  position  that  the  bead  b  will  exactly  cover 
its  image  in  the  mirror  =  C  =  2.8  in  this  case.  The  specimen  is 
now  placed  in  the  top  pan  P,  care  being  taken  that  with  the  extra 
weight  it  does  not  sink  in  the  water  and  get  wet,  increasing  its 
weight ;  the  beaker  is  now  lowered  until  the  pan  P'  rests  at  the 
same  depth  in  the  water  as  before,  when  the  second  reading  is 
taken  =  16.1.  The  specimen  thoroughly  wet  is  now  placed  in  the 


FIG.  374.  — Joly  Bal- 
ance. 


262 


MINERALOGY 


lower  pan  P'.  There  should  be  no  air  bubbles  sticking  to  it ;  the 
beaker  is  now  raised  until  equilibrium  is  established,  with  the  lower 
pan  at  the  same  depth  in  water  as  before,  when  the  third  reading 
is  taken  =  11.1.  •  To  determine  the  specific  gravity: 


G  = 


the  weight  in  air       =      W      =  16.1  -  2.8       13.3 
loss  of  weight  in  water      W  —  w      16.1  —  11.1        5 


=  2.66. 


The  specific  gravity  of  pure  quartz  is  2.653.  In  this  method 
the  specimen  used  should  weigh  between  two  and  three  grams. 
The  chemical  balance  may  be  substituted  for  the  Joly  balance  and 
accurate  weighings  made,  with  a  resulting  increase  in  accuracy. 

II.  Pycnometer  method.  —  The  pycnometer,  Fig.  375,  is  a  small 
flask  of  usually  10  cc.  capacity,  having  a  nicely  fitted  ground 

glass  stopper  with  a  capillary  bore 
running  through  it.  The  weight  of 
distilled  water  which  exactly  fills  the 
pycnometer  at  20°  C.  is  determined 
once  for  all  =  C.  In  each  case  the 
pycnometer  is  weighed  empty  and 
dry  =  P.  The  specimen  is  powdered 
to  avoid  any  internal  cavities  and 
about  two  and  one  half  grams  are 
placed  in  the  pycnometer,  it  having 
been  dried  at  100°  C.  when  there  is 
no  danger  of  loss  at  that  temperature, 
and  carefully  weighed  =  S.  The  pyc- 
nometer is  now  filled  about  half  full 
with  distilled  water  and  very  carefully 
boiled  for  a  few  minutes  to  expel  any 

air  that  may  be  included  in  the  powder.  After  cooling,  the  flask 
is  filled  with  recently  boiled  distilled  water,  care  being  taken  that 
none  of  the  sample  is  lost  and  that  the  pycnometer  is  filled  to  the 
top  of  the  small  capillary  bore  when  the  stopper  is  in  place,  and 
cooled  to  20°  C.,  when  the  whole  is.  weighed  =  D.  S  =  sample 
and  pycnometer  weighed  together. 

S  —  P  =  W  =  weight  of  sample  in  air. 
(S  +  C)  —  D  =  loss  of  weight  in  water. 


FIG.  375.  —  Pycnometer. 


S-P 


(S  +  C)  -  D' 


PHYSICAL  PROPERTIES  263 

When  all  precautions  are  taken,  this  method  will  yield  results 
accurate  to  ±  1,  in  the  fourth  decimal  place. 

When  the  substance  is  soluble  in  water  or  affected  by  water, 
some  other  liquid  than  water  is  used  the  specific  gravity  of  which 
has  been  determined,  as  benzene  or  carbon  tetrachloride.  The 
experiment  is  carried  out  as  with  water,  but  the  result  must  be 
multiplied  by  the  specific  gravity  of  the  fluid  used  in  order  to  re- 
duce it  to  terms  of  water  as  the  unit. 

III.  The  suspension  method.  —  The  Westphal  balance  and  the 
use  of  heavy  liquids.  The  advantage  of  this  method  is  that  the 
gravity  of  small  crystals  or  fragments  may  be  determined  with 
accuracy  to  the  third  decimal  place  as  in  the  pycnometer  method. 
The  method  consists  in  placing  the  fragment  in  a  liquid,  the  spe- 
cific gravity  of  which  is  higher  than  that  of  the  fragment,  which 
will  then  float  on  the  surface  of  the  heavy  liquid. 

The  fluid  is  now  diluted  carefully  with  a  lighter  one,  stirring  after 
each  addition;  a  point  will  be  reached  when  the  fragment  of 
mineral  will  neither  float  nor  sink ;  the  specific  gravity  of  the  liquid 
and  fragment  are  under  these  conditions  identical  and  the  fragment 
remains  suspended.  The  specific  gravity  of  the  fluid  is  now  deter- 
mined with  the  Westphal  balance  or  the  pycnometer  when  accu- 
racy is  required. 

The  heavy  solutions  in  general  use  are : 

Thoulet's  solution.  —  A  solution  of  potassium  iodide,  KI,  1 
part,  and  mercuric  iodide,  HgI2,  1.24  parts,  dissolved  in  an  excess 
of  distilled  water.  The  solution  is  then  evaporated  on  the  water 
bath  until  a  fragment  of  fluorite  floats  on  it ;  on  cooling,  the  gravity 
rises  to  the  maximum  3.196.  It  is  then  filtered,  and  well  corked,  as 
on  exposure  to  the  air  it  absorbs  water  and  its  specific  gravity 
will  fall  to  3.1,  where  it  remains  constant.  It  is  a  convenient  solu- 
tion, as  it  may  be  diluted  with  water  to  any  extent,  and  if  left  stand- 
ing exposed,  the  water  will  evaporate  until  the  specific  gravity 
reaches  3.1 ;  by  evaporation  on  the  bath  it  may  again  be  brought 
back  to  3.196.  If  darkened  by  the  separation  of  iodine,  this  may 
be  corrected  by  adding  a  little  mercury  during  the  evaporation. 
Methylene  iodide,  CH2I2,  G  =  3.315  at  20°  C.,  is  a  little  heavier 
than  Thoulet's  solution,  but  has  the  disadvantage  of  not  being 
miscible  with  water,  and  benzole  must  be  used  to  dilute  it.  It  does 
not  attack  metals  as  does  the  mercury  solution. 

The  heaviest  solution  of  all  is  that  suggested  by  Penfield,  silver 
thallium  nitrate,  which  is  liquid  at  75°  and  has  a  specific  gravity 
of  4.5  and  may  be  diluted  with  water. 


264  MINERALOGY 

Where  only  an  approximate  determination  is  all  that  is  required, 
as  in  the  identification  of  minerals,  the  fragment  is  placed  on  the 
liquid,  and  the  liquid  is  diluted  until  suspension  is  reached,  then 
small  fragments  of  minerals,  the  specific  gravity  of  which  are  known, 
are  used  as  tests.  Thus  if  calcite  sinks  and  quartz  floats,  the  spe- 
cific gravity  of  the  mineral  to  be  determined  must  lie  between  2.65 
and  2.72  or  not  far  from  2.70. 

A  list  of  convenient  test  minerals  is  as  follows  : 

Heulandite 2.20         Calcite 2.72 

Analcite 2.26  Prehnite    ......  2.87 

Gypsum 2.32         Datolite 2.95 

Leucite 2.47         Tremolite 3.00 

Orthoclase 2.56         Actinolite 3.10 

Quartz.     .     .     .     .     .     .2.65         Fluorite 3.18 

When  accuracy  is  required  the  Westphal  balance,  Fig.  376,  is 
used.  This  consists  of  a  bob  n  which  just  balances  the  arm  H 
in  air.  The  arm  is  divided  into  a  decimal  scale.  Three  sizes  of 
weights  are  furnished  with  the  instrument,  A  =  units,  B  =  .1 
and  C  =  .01,  when  hung  on  the  hookas  A2,  and  corresponding  tenths 
of  these  values  when  on  the  arm. 

In  the  determination,  the  bob  is  immersed  in  the  liquid  and  the 
balance  arm  weighted  to  balance,  when  the  specific  gravity  is  read 
directly  from  the  weights  and  the  arm. 

The  separation  of  minerals  with  heavy  solutions.  —  The  different 
mineral  components  of  a  rock  such  as  granite  may  be  separated  by 
the  use  of  heavy  liquids.  The  specimen  is  ground  only  sufficiently 
fine  to  insure  each  particle  as  consisting  of  one  mineral  species 
only;  the  fineness  necessary  is  determined  by  an  examination  of 
the  powder  with  a  microscope.  The  sample  is  then  charged  in 
chamber  A  of  the  separatory  apparatus,  Fig.  377,  and  Thoulet's 
solution,  density  3.00,  poured  in,  stoppered  and  shaken;  all  acces- 
sory minerals,  as  zircon,  apatite,  magnetite,  amphibole,  and  tour- 
maline, will  settle  in  the  chamber  A,  while  the  others  will  float  on  the 
surface  of  the  liquid.  The  heavy  minerals  may,  after  settling,  be 
drawn  into  B  by  means  of  the  cock  C,  and  finally  after  shaking  a 
second  time,  out  at  the  bottom  by  turning  the  cock  C'.  The  solu- 
tion in  chamber  A  is  now  diluted  until  calcite  just  sinks,  when  it  is 
again  shaken,  allowed  to  settle,  and  all  minerals  above  2.72,  as  the 
micas,  are  drawn  off.  The  solution  in  A  is  again  diluted  until 
quartz  just  sinks,  which  is  separated  from  the  orthoclase. 


PHYSICAL  PROPERTIES 


265 


Where  samples  for  analysis  are  required  it  is  advisable  to  pass 
the  material  through  the  separator  several  times,  care  being  taken 


FIG.  376.  — The  Wesphal  Balance. 

to  have  the  specific  gravity  of  the  heavy  liquid  nicely  adjusted  to 
the  specific  gravities  of  the  minerals  to  be  separated. 

Terms  used  in  the  description  of  mineral  aggregates.  —  Indi- 
vidual crystals,  even  though  of  the  same  species  and  combinations 
of  the  same  forms,  may  differ  greatly  in  appearance.  The  general 
shape  of  a  crystal  will  depend  upon  the  crystal  form 
which  predominates  in  the  combination;  thus  various 
habits  arise  which  are  described  by  the  following 
terms  : 

Tabular  is  when  a  pair  of  parallel  faces,  as  the  base 
or  pinacoid,  predominate.  The  crystal  is  short  in  one 
direction  and  extended  in  the  other  two,  as  in  some 
barites,  tabular  parallel  to  the  base,  Fig.  378. 

Prismatic  is  when  the  individuals  are  elongated  in  the 
direction  of  any  one  axis,  usually  the  vertical  axis,  as  in 
quartz. 

Short  and  stout  prismatic  crystals  are  said  to  be  of 
columnar  habit,  as  beryl.  FIG.  377. 


266 


MINERALOGY 


Acicular  is  where  the  prismatic  habit  is  so  accentuated  that  the 
crystals  are  long  and  needle-like,  as  stibnite  or  rutile. 


FIG.  378.  —  Tabular  Barite  and  Acicular  Stibnite.     Felsobanya,  Hungary. 

When  still  more  attenuated  or  hairlike,  they  are   said  to   be 
capillary,  as  millerite,  Fig.  379. 

Fibrous  is  when  the  fine  hairlike  crystals  are  parallel  in  arrange- 

ment,  and  usually 
easily  separable,  as 
asbestos,  Fig.  380. 

All  the  above  habits 
may  be  illustrated  by 
specimens  of  the  same 
mineral  species. 

The  surfaces  of 
crystal  faces  and 
mineral  aggregates 
differ  in  appearance 
and  the  following  de- 
scriptive terms  are  in 
use : 

Striated.  —  Crystal  faces  are  often  crossed  by  striations,  which 
are  always  parallel  to  some  definite  crystallographic  direction  and 


FIG.  379.  —  Millerite,    Antwerp, 
New  York. 


Jefferson   County, 


PHYSICAL  PROPERTIES  267 

often  serve  for  the  identification  of  zones  or  individual  forms. 
They  are  of  two  varieties :  those  caused  by  alterations  of  growth, 
between  two  crystal  forms  and  therefore  parallel  to  their  inter- 
section or  edge,  as  the  horizontal  striations  on  the  prism  faces  of 
quartz. 

When  the  striations  are  very  marked,  the  crystal  is  said  to  be 
furrowed.  Twinning  striations  differ  from  oscillatory  striations, 
or  those  caused  by  the  alterations  in  growth  between  two  forms,  in 
that  they  represent  the  composition  plane  between  twins  which 


FIG.  380.  —  Asbestos.     Thatf ord,  Quebec,  Canada. 

penetrate  and  pass  through  the  body  of  the  crystal.  They  will 
therefore  appear  on  cleavage  pieces  as  well  as  on  crystal  faces, 
either  singly,  as  in  the  Alaska  epidotes,  or  often  repeated  as  parallel 
lines,  as  in  the  plagioclases. 

Vicinal  faces.  —  It  has  often  been  observed  that  each  face  of  a 
simple  form,  which  may  be  represented  by  parameters  or  in- 
dices of  normal  value,  as  the  cube  or  octahedron  in  the  isometric 
system,  is  replaced  by  a  low  and  very  flat  pyramid.  The  number 
of  faces  of  the  pyramid  will  depend  upon  the  symmetry  of  the  face 
replaced  —  in  the  cube  four  and  in  the  octahedron  three.  These 
flat  vicinal  faces  intersect  the  axes  at  long  distances  and  their 
indices,  contrary  to  the  general  rule  of  simple  indices,  are  large  and 
indefinite,  though  the  faces  conform  to  the  symmetry  of  the  type 


268 


MINERALOGY 


and  represent  always  the  more  general  form  of  the  type.     Vicinal 
faces  are  common  on  fluorite,  where  the  face  of  the  cube  seems  to 

be  replaced  by  a  very  flat 
tetrahexahedron,  Fig.  381. 
When  a  zone  of  vicinal 
faces  occurs,   the    crystal 
appears    rounded    or 
bounded  by  curved  faces. 
Drusy.  —  A    surface 
formed  of  numerous  small 
individual  crystals  and 
therefore    dull   is   said  to 
be  drusy.     Drusy  surfaces 
may  be  caused  by  a  sec- 
ondary mineral  produced 
by  surface  decomposition, 
or  it  may  be  the  result  of 
very  fine  crystals  usually 
in  parallel  position,  repre- 
senting a  second  generation  of  the  same  species;   or  it  may  be 
caused  by  a  parallel  or  radiated  aggregate  of  fine  crystals,  each 
terminated  at  the  surface,  as  in  crusts  of  millerite  or  smithsonite. 
Internal    structures.  —  Granular  is  where  the  individual  crys- 


FIG.  381.  —  A  Cube  of  Fluorite,  showing  Four 
Vicinal  Faces  replacing  the  Cube  Face. 


FIG.  382.  —  Calamine  with  Radiated  Structure.     Franklin,  New  Jersey. 

tals  are  equidimensional  and  irregularly  packed  together,  the  indi- 
viduals being  distinguishable  by  the  unaided  eye. 

Massive  or  compact.  —  A  granular  structure  in  which  the  indi- 
viduals are  not  to  be  distinguished  by  the  unaided  eye. 


PHYSICAL  PROPERTIES 


269 


Powdery.  —  Is  when  the  fine  individuals  are  easily  parted  and 
break  down  under  slight  pressure. 

Lamellar.  —  Is  when  the  individuals  are  flattened  and  in  paral- 
lel position  and  easily  parting,  as  some  talc.  Foliated  is  an  equiva- 
lent term,  only  the  laminae  are  thicker. 

Radiated.  —  Formed  of  individual  crystals,  usually  acicular, 
which  radiate  from  a  point,  the  nucleus  of  crystallization,  like  the 
sticks  of  a  fan.  Sometimes  termed  fan-shaped,  as  calamine  or 
wavellite,  Fig.  382. 

Reticulate.  —  Individual  prismatic  crystals  arranged  like  lat- 
ticework, with  definite  twinning  angles,  as  the  rutile  from  Tavetsch, 


FIG.  383. — Reticulate  Rutile,  enlarged.    Tavetsch,  Switzerland. 

Switzerland,  Fig.  383,  or  irregularly  matted,  as  the  cerussite  from 
Cornwall,  England. 

Dendritic.  —  Branching  forms  deposited  from  solutions,  in 
cleavage  cracks  and  rock  fissures,  as  pyrolusite  or  metallic  copper, 
Fig.  384. 

Wiry.  —  Like  wire,  usually  native  metals,  as  silver  or  gold,  also 
in  sheets  or  leafy. 

Nuggets.  —  Are  irregular  rounded  or  rolled  lumps,  usually  applied 
to  the  precious  metals,  as  gold,  silver,  or  platinum,  Fig.  385. 

Nodular.  —  Applies  to  rounded  individual  masses  of  minerals, 
as  the  rounded  balls  of  pyrite  occurring  in  some  clays,  Fig.  386. 


270 


MINERALOGY 


FIG.  384.  —  Dendritic  Psilomelane.     Leadville,  Colorado. 

Geodes.  —  Are  cavities  in  clays  or  other  formations,  which  have 
been  encrusted  with  a  wall  of  quartz  or  other  mineral,  and  which 


FIG.  385.  —  Platinum  Nugget.  Nischi- 
Tagilsk,  Siberia. 


FIG.  386.  —  Nodular  Pyrite. 


separate  as  a  hollow  mass,  the  interior  walls  of  which  are  usually 
studded  with  crystals,  Fig.  387.  Small  almond-shaped  steam  cavi- 
ties occurring  in  lavas  are  often  filled  by  minerals,  deposited  from 


PHYSICAL   PROPERTIES 


271 


solution ;   such  bodies  of  minerals  are  termed  amygdules  and  the 
structure  amygdaloidal. 


FIG.  387.  —  Geode  studded  with  Quartz  Crystals. 

Pisolitic. — Spherical  forms  with  concentric  shell-like  structure,  as 
in  some  calcites  from  Freiburg,  Saxony,  Fig.  388.  When  the  indi- 
vidual spherules  are  small,  the  mass  resembles  fish  roe,  or  oolitic. 


FIG.  388.  —  Oolitic  Calcite.    Carlsbad,  Bohemia. 


272 


MINERALOGY 


FIG.  389.  —  Botryoidal  Prehnite.    Bergen  Hill,  New  Jersey 


FIG.  390.  -  Stalactite  of  Calcite.    Copper  Queen  Mine,  Bisbee,  Arizona. 


PHYSICAL  PROPERTIES  273 

Botryoidal.  —  When  in  rounded  irregular  masses,  usually  with  a 
smooth  surface  and  an  internal  radiated  structure,  as  prehnite, 
Fig.  389.  When  in  forms  resembling  kidneys,  Often  termed  reni- 
form,  as  some  hematites. 

Stalactitic.  —  Minerals  are  often  formed  by  the  evaporation  of 
solutions  dripping  from  the  walls  and  ceilings  of  caves ;  the  solids 
are  deposited  in  the  form  of  icicles  or  stalactites,  as  calcite,  mala- 
chite, or  limonites,  Fig.  390.  Stalactites  are  often  long,  slender, 
and  hollow,  the  solution  flowing  down  the  hollow  tube  and  deposit- 


FIG.  391.  —  Stalagmite  of  Calcite. 

ing  the  mineral  substance  only  on  evaporating  at  the  lower  end. 
Such  stalactites  are  strawlike. 

Stalagmites.  —  Are  the  reverse  of  stalactites,  or  the  structure 
which  forms  on  the  floor  of  caves  under  the  drip  from  a  stalactite, 
Fig.  391. 

Mammillary.  —  When  the  crusts  are  composed  of  rounded  masses 
or  nipple-like  structures. 

Other  terms  in  common  usage  in  the  description  of  minerals  will 
need  no  particular  explanation. 

Color  of  minerals.  —  The  outward  appearance  of  minerals,  due 
to  other  causes  than  form  or  structure,  is  caused  by  light,  or  color 
and  luster.  The  color  of  a  specimen  when  opaque  is  caused  by  re- 
flected light  wholly;  when  transparent,  to  transmitted  light  in 
combination  with  reflected  light.  When  white  light  falls  upon  a 
mineral,  some  rays  are  absorbed  and  the  complementary  color  is 


274  MINERALOGY 

reflected  as  the  color  of  the  surface,  while  the  depth  or  tone  is 
modified  by  the  physical  condition  of  the  surface,  for  a  highly  pol- 
ished surface  will  glisten  with  the  quantity  of  light  reflected,  and  a 
rough,  earthy,  or  powdery  surface  will  appear  dull  from  the  amount 
of  light  diffused.  Combined  with  these  surface  effects,  in  trans- 
parent minerals,  is  the  color  of  transmitted  light,  characteristic 
in  some  cases,  as  in  crocoite,  of  the  compound,  in  others,  as  in 
quartz,  due  entirely  to  impurities. 

Since  the  nature  of  the  surface  is  variable,  it  has  been  found  more 
accurate  in  comparing  the  color  of  mineral  specimens  to  use  the 
very  finely  ground  powder,  which  insures  a  surface  always  of  the 
same  nature,  from  which  the  same  amount  of  light  will  be  reflected 
or  diffused,  and  the  color  will  not  depend  upon  fortuitous  causes. 
The  most  convenient  method  of  obtaining  the  fine  powder  quickly 
is  to  draw,  with  a  firm  pressure,  the  rounded  corner  of  the 
specimen  quickly  across  an  unglazed  porcelain  surface,  when 
most  minerals  will  leave  a  mark,  termed  the  streak,  the  color 
of  which  is  that  of  the  fine  powder.  The  color  of  the  streak  is 
of  great  help  in  the  rapid  determination  of  minerals.  The  streak 
is  little  affected  by  the  structure  of  the  specimen,  as  hematite 
occurs  in  well-formed  crystals,  micaceous,  compact,  massive,  and 
earthy,  with  a  blood-red  color,  gray,  steel-gray  to  nearly  black, 
iridescent,  or  brown;  all  of  these  varieties  will  yield  a  cherry-red 
streak. 

A  mineral  is  described  as  of  metallic  luster  when  the  streak  is 
dark  in  color  and  the  specimen  is  opaque  on  the  thin  edges  or  in  thin 
sections  and  has  a  shiny  surface,  common  to  most  metals,  as  tin 
or  iron.  When  the  streak  is  dark  and  the  surface  is  not  shiny,  it  is 
of  submetallic  luster. 

If  the  streak  is  light  in  color  or  the  thin  edges  and  sections  trans- 
mit light,  it  is  non-metallic  in  luster. 

The  color  of  metallic  and  submetallic  minerals  is  more  character- 
istic and  less  variable  than  in  non-metallic.  It  is  therefore  more 
reliable  as  a  means  of  identification. 

The  true  color  of  a  mineral  is  shown  only  on  a  freshly  broken 
surface,  as  by  oxidation  or  weathering  the  natural  surface  may  be 
entirely  changed.  The  beautiful  iridescence  of  some  pyrites, 
chalcopyrite,  limonite,  and  hematite,  is  caused  by  a  thin  film  of 
oxides  or  hydroxides  which  yield  spectral  colors  through  the  inter- 
ference of  light. 

The  surface  change  in  color  due  to  chemical  change  is  termed 


PHYSICAL  PROPERTIES  275 

tarnish.  Non-metallic  minerals  are  transparent  on  thin  edges  or 
in  thin  sections,  such  as  rock  sections,  which  are  under  .04  mm. 
Many  minerals  which  are  opaque  in  coarse  fragments,  as  the  black 
tourmalines  or  rutile,  will  be  transparent  in  thin  sections;  even 
metals,  as  gold,  will  transmit  light  if  the  sheets  are  thin  enough. 
The  opacity  to  light  is  relative,  depending  upon  the  thickness  of 
the  section.  A  mineral  is  said  to  be  transparent  if  the  outline  of 
objects  can  be  distinguished  through  it;  translucent  when  the 
light  is  diffused  and  the  outlines  of  objects  are  no  longer  distin- 
guishable through  the  specimen.  Some  effects  are  due  to  both 
diffusion  and  interference  of  light,  as  opalescence,  and  the  milki- 
ness  of  some  quartz,  the  play  of  colors  in  the  fire  opal,  or  the  color 
yielded  by  some  labradorite  when  viewed  at  certain  angles  with 
the  twinning  planes.  The  star  sapphires  and  cat's-eye  are  due  to 
reflections  and  a  fibrous  structure.  The  prism  colors  or  banded 
effects  produced  by  films  of  air  in  thin  cleavage  cracks  are  due  to 
interference. 

The  variability  of  color  in  non-metallic  minerals  is  well  illus- 
trated in  quartz,  a  mineral  which,  when  pure,  is  colorless.  Very 
small  amounts  of  some  oxides  which  are  present  as  impurities  act 
as  a  pigment,  yielding  decided  colors  even  when  the  amount  of 
coloring  material  is  so  small  as  not  to  be  detected  by  the  ordinary 
chemical  tests. 

Certain  colors  are  characteristic  of  chemical  elements,  as  most 
copper  minerals  are  blue,  green,  or  red;  and  copper  compounds  will 
yield  these  colors  when  present  in  other  minerals  as  impurities  or 
inclusions.  Calcite,  smithsonite,  and  quartz  are_  often  colored 
green  by  copper  compounds.  Chromium  yields  a  green  color,  as 
in  the  green  of  some  garnets  and  in  the  emerald.  The  red  of  the 
ruby  as  well  as  the  red  of  crocoite  is  chromium  in  a  different  form 
or  ion.  Nickel  also  yields  a  green  color,  as  in  chrysoprase,  a  variety 
of  quartz.  Iron  as  an  impurity  will  yield  shades  of  red,  brown, 
yellow,  green,  or  blue  according  to  its  state  of  oxidation  or  combi- 
nation. The  yellow  calcites  of  Joplin,  Missouri,  contain  ferrous 
iron  carbonate  and  the  red  jaspers  and  bloodstones  are  colored  by 
ferric  iron.  Very  small  quantities  of  manganese  yield  intense  colors; 
pinks,  amethystine,  and  some  greens  are  caused  by  manganese. 
The  rqse  color  of  some  quartz  and  tourmaline  is  caused  by  titanium, 
which  also  when  in  a  lower  state  of  oxidation  will  yield  a  blue. 
Cobalt  is  the  most  intense  of  all  mineral  pigments  and  many  blue 
minerals  owe  their  color  to  cobalt ;  when  in  cobaltic  form,  the  ion  is 


276 


MINERALOGY 


pink,  as  in  cobalt  bloom.  Uranium  minerals  are  yellow  or  green. 
Molybdenum  yields  greens  and  yellows,  and  tungsten  yellow  or 
blue;  while  the  browns  and  yellows  of  most  quartz,  as  smoky 
quartz,  are  caused  possibly  by  organic  matter,  and  the  delicate 
blues,  violets,  greens,  and  yellows  of  fluorite,  apatite,  barite,  and 
topazes  are  also  attributed  to  organic  matter. 

Inclusions  of  colored  minerals  within  the  body  of  a  colorless  min- 
eral are  often  the  cause  of  color  in  minerals,  as  the  red  or  flesh- 


FIG.  392.  —  Section  of  Tourmaline,  with  an  Uneven  Distribution  of  Color.    Brazil. 

colored  orthoclases  of  some  granites  is  caused  by  included  scales  of 
hematite,  as  is  also  the  peculiar  appearance  of  aventurine  quartz. 
The  color  of  minerals  caused  by  pigments  is  not  always  evenly 
distributed,  as  during  the  period  of  growth  of  some  crystals  the 
chemical  composition  of  the  mother  solution  may  have  changed  as 
regards  these  minor  constituents,  resulting  in  the  change  of  color 
of  the  forming  crystals,  producing  phantoms  or  outlined  crystals 
within  the  body  of  larger  individuals,  as  the  phantom  amethysts  of 
Schemnitz,  Hungary,  and  the  phantom  fluorides  of  Cumberland, 
England.  The  best  illustration  of  the  uneven  or  irregular  distribu- 
tion of  color  is  the  variegated  tourmaline  of  many  localities,  as  of 
Haddam,  Connecticut ;  Pala,  California,  or  Brazil.  In  the  latter 
the  colors  appear  in  concentric  bands  as  illustrated  in  the  section, 


PHYSICAL  PROPERTIES  277 

Fig.  392,  at  right  angles  to  the  vertical  axis  of  a  tourmaline  from 
Brazil.  In  the  center  there  is  an  area  of  concentric  bands  of  differ- 
ent shades  of  pink,  then  a  colorless  band,  outside  of  which  is  a  band 
of  green.  Again  the  colors  may  be  distributed  lengthwise  the 
crystal,  as  many  specimens  from  Mesa  Grande,  California,  in  which 
one  end  may  be  green,  the  other  pink,  and  often  separated  by  a 
colorless  middle  band  across  the  body  of  the  crystal. 

The  delicate  coloring  of  some  transparent  crystals  is  often 
changed  by  heating ;  in  this  way  the  pink  topazes  are  produced 
by  heating  the  originally  yellow  stones  from  Brazil.  This  process 
is  known  as  "  pinking,"  and  if  the  heat  is  too  great,  the  crystal 
becomes  colorless. 

In  the  description  of  colors  in  minerals  Werner,  nearly  two  hun- 
dred years  ago,  fixed  the  following  eight  colors  as  primary,  and  since 
that  time  mineralogists  have  been  accustomed  to  describe  minerals 
in  terms  of  these  eight  colors  with  their  variations  as  follows : 

WHITE,  either  clear,  transparent,  or  translucent 

Snow-white,  as  Carrara  marble.  Greenish  white,  as  talc.  • 

Reddish  white,  as  some  calcite.  Bluish  white,  as  some  calcite. 

Yellowish  white,  as  some  calcite.  Milk-white,  as  some  quartz. 
Grayish  white,  as  some  calcite. 

GRAY 

Blue-gray,  as  some  wernerite.  Yellowish  gray,  as  some  dolo- 
Pearl-gray,  as  some  dolomites.  mites. 

Smoke-gray,  as  flint.  Ash-gray,  as  leticite. 
Greenish  gray,  as  tremolite. 

BLACK 

Grayish  black,  as  ilvaite.  Brownish  black,  as  allanite. 

Velvet-black,  as  black  tourma-  Reddish  black,  as  acmite. 

line.  Bluish  black,  as  tourmaline  (in- 
Greenish  black,  as  augite.  dicolite). 

BLUE 

Blackish  blue,  as  dark  azurite.  Prussian  blue,  as  cyanite. 

Azure-blue,  as  light  azurite.  Smalt-blue,  as  dumortierite. 

Violet-blue,  as  some  fluorites.  Indigo-blue,  as  some  vivianites. 

Lavender-blue,  as  some  sodalites.  Sky-blue,  as  turquoise. 


278 


MINERALOGY 
GREEN 


Verdigris-green,  as  amazon  stone. 
Celandine-green,  as  some  beryl. 
Mountain-green,  as  light  beryl. 
Leek-green,  as  prase. 
Emerald-green,  as  emerald. 
Apple-green,  as  chrysoprase. 


Olive-green,  as  olivine. 
Grass-green,  as  some  diallage. 
Pistachio-green,  as  some  epidote. 
Black-green,  as  some  serpentine. 
Oil-green,  as  some  beryl. 
Siskin-green,  as  torberniteo 


YELLOW 


Sulphur-yellow,  as  sulphur. 
Straw-yellow,  as  yellow  topaz. 
Wax-yellow,  as  orpiment. 
Honey-yellow,  as  some  blende 

sphalerite. 
Lemon-yellow  as  sulphur. 


Ocher-yellow,  as  some  limonites. 
Wine-yellow,  as  topaz. 
Cream-yellow,  as  some  kaolinite. 
Orange-yellow,   as    some    orpi- 
ment. 


Aurora-red,  as  realgar. 
Hyacinth-red,  as  crocoite. 
Brick-red,  as  some  jasper. 
Scarlet-red,  as,  cinnabar. 
Blood-red,  as  pyrope. 


RED 


Carmine-red,  as  ruby. 
Rose-red,  as  rose  quartz. 
Crimson-red,  as  ruby. 
Peachbloom-red,  as  erythrite. 
Flesh-red,  as  some  feldspars. 


BROWN 


Reddish-brown,  as  zircon.  Pinchbeck-brown,  bronzite. 

Clove-brown,  as  axinite.  Wood-brown,  as  some  asbestos. 

Hair-brown,  as  some  wood  opals.  Liver-brown,   as   some  jaspers. 

Chestnut-brown,  as  some  hema-  Blackish      brown,      as       some 

tites.  chromites. 

Luster.  —  Luster  is  a  term  used  to  describe  or  denote  the  peculiar 
character  of  light  reflected  from  the  surfaces  of  minerals.  The  differ- 
ence in  quality  of  reflected  light  is  caused  not  only  by  the  character 
of  the  surface,  but  also  by  the  structure  and  index  of  refraction  of  the 
specimen.  All  faces  of  the  same  crystal  form,  as  the  cube  faces  or 
cubical  cleavage  surfaces  of  galena,  will  have  the  same  luster,  while 
the  three  pinacoids  are  at  right  angles  to  each  other,  as  the  three 
directions  of  the  cubical  cleavage,  but  each  of  the  pinacoids  may 
have  a  distinguishing  luster. 


PHYSICAL  PROPERTIES  279 

In  addition  to  metallic,  submetallic,  or  non-metallic  lusters,  which 
are  in  a  large  measure  dependent  upon  opacity  to  light,  the  follow- 
ing terms  are  used  :  Adamantine  luster  is  a  high,  shiny,  and  brilliant 
luster,  usually  connected  with  minerals  with  a  high  specific  gravity 
and  index  of  refraction.  It  also  gives  the  impression  of  being  very 
hard.  Good  examples  are  cuprite,  rutile,  cassiterite,  sphalerite, 
cerussite,  and  diamond. 

Vitreous  or  glassy  luster,  as  of  broken  glass,  bright  and  shining, 
like  quartz,  apatite,  beryl,  and  most  of  the  silicates. 

Greasy  or  resinous  is  a  vitreous  luster  as  if  oiled  or  like  resins,  as 
serpentine. 

Waxy,  very  much  like  resinous,  like  calcedony. 

Pearly  luster  is  well  shown  in  mother-of-pearl,  due  to  a  combina- 
tion of  surface  reflection  and  a  shelly  structure,  as  in  brucite,  talc, 
and  the  basal  cleavage  of  apophyllite  and  the  pinacoidal  cleavage 
of  heulandite. 

Silky  luster  is  the  luster  of  satin,  due  to  a  fibrous  structure,  as  in 
satin  spar,  asbestos,  and  enstatite. 

Dull,  as  in  chalk  or  kaolinite,  is  where  the  reflected  light  is 
diffused. 

Phosphorescence.  —  Some  minerals  and  chemical  compounds 
possess  the  property  of  transforming  energy  of  other  forms  into 
light  and  continue  to  emit  a  characteristic  glow  long  after  the 
exciting  agent  or  cause  has  been  removed  or  ceased  to  act.  Calcium 
sulphide  mixed  with  small  amounts  of  bismuth  is  used  as  a  luminous 
paint  and  will  continue  to  glow  for  hours  after  exposure  to  sunlight. 
The  hexagonal  zinc  blende,  wurtzite,  will  glow  under  the  emissions 
of  radium.  Diamonds,  willemite,  and  kunzite  phosphoresce  when 
exposed  to  the  Rongten  ray  or  ultra-violet  light.  In  the  case  of 
willemite  the  ultra-violet  light  is  used  to  test  the  completeness 
of  the  mechanical  separation  from  the  gangue  and  other  zinc  min- 
erals, as  every  remaining  particle  of  willemite  will  glow  brilliantly 
when  exposed  to  ultra-violet  light. 

Other  minerals,  as  quartz,  become  luminous  by  friction,  or  when 
fractured,  as  some  micas.  Specimens  of  the  same  species  may  vary 
greatly  in  their  power  to  phosphoresce  and  it  would  seem  not  to  be 
a  property  of  pure  chemical  compounds  or  pure  minerals  but  is 
caused  in  most  cases  by  impurities  and  is  often  restricted  to  local- 
ities, as  all  the  minerals  from  Borax  Lake,  California,  phosphoresce 
under  ultra-violet  light,  which  is  probably  due  to  some  common 
constituent. 


280  MINERALOGY 

Phosphorescence  is  caused  by  the  lengthening  of  the  wave  of  the 
absorbed  energy,  as  the  ultra-violet  wave  and  the  wave  of  the 
Rontgen  ray  are  too  short  to  be  detected  by  the  eye,  but  when 
lengthened  to  .00039  mm.  affect  the  eye  as  light. 

Fluorescence  is  much  like  phosphorescence,  only  the  phenomenon 
continues  during  the  actual  exposure  only,  as  in  the  barium  plati- 
nocyanide  screen,  used  to  visualize  the  Rontgen  rays.  Some  white 
fluorites  fluoresce  in  the  sunlight  with  a  bluish,  milky,  or  hazy 
light.  This  is  a  property  possessed  by  uranium  compounds  and 
some  compounds  of  boron.  Uranium  nitrate  is  used  in  the  manu- 
facture of  fluorescent  glass. 


CHAPTER  IV 
THE  NATIVE  ELEMENTS 

DIAMOND 

Diamond.  —  Carbon,  C;  Isometric;  Type,  Ditesseral  Polar; 
Common  form,  o  (111);  Twinning  planes,  111  and  100;  Cleavage 
octahedral,  perfect;  Brittle;  Fracture,  conchoidal;  H.  =  10; 
G.  =  3.51  —  3.52;  Color,  white,  yellow,  brown  to  black,  rarely  blue 
or  green;  Luster,  adamantine  to  slightly  greasy;  Transparent 
to  opaque;  n  =  2.42;  Dispersion  strong  =.063. 

B.B.  —  Infusible,  insoluble  in  acids.  When  heated  to  a  high 
temperature  for  a  long  time  it  burns  slowly,  forming  CO2. 
Colored  stones  may  change  color  on  heating. 

General  description.  —  Always  crystalline,  usually  simple  octa- 
hedrons or  rounded  hexoctahedrons  which  are  supplementary 
twins,  combinations  of  the  plus  and  minus  hextetrahedrons,  in 
which  the  octahedral  edge  is  replaced  by  a  reentrant  angle. 
Simple  tetrahedral  forms  and  the  cube  are  rare.  Crystal  faces  are 
often  drusy  or  covered  with  triangular  etch-figures,  due  to  corro- 
sion, which  is  also  the  cause  of  the  rounded  appearance  of  many 
diamond  crystals.  Twins  after  the  spinel  law,  where  the  face  of 
the  octahedron  is  the  composition  plane,  are  not  uncommon. 
•  The  perfect  octahedral  cleavage  is  utilized  by  the  cutters  in  the 
rough  preparation  of  the  stones  for  the  grinders.  While  the 
diamond  is  the  hardest  known  substance,  it  is  brittle  and  easily 
broken  or  ground  to  powder,  the  dust  of  which  is  used  on  the 
wheels  or  "  skeifs  "  in  grinding  and  polishing  the  facets  of  the  cut 
stone.  The  inclination  of  all  facets  of  the  brilliant  is  calculated 
so  that  the  greatest  amount  of  light  is  totally  reflected  and 
returned.  Owing  to  the  high  index  of  refraction,  rays  which 
meet  the  lower  facets  at  an  angle  greater  than  24°  13'  are  inter- 
nally totally  reflected,  and  emerge  above  the  girdle,  owing  to  the 
very  strong  dispersion,  yielding  prismatic  color.  The  high  index 
of  refraction  and  strong  dispersion  are  the  two  properties  which 

281 


282  MINERALOGY 

produce  the  brilliancy  and  luster  of  a  well-cut,  perfect  stone. 
Diamond  is  pure  carbon,  yielding  on  combustion  carbon  dioxide 
and  .05  to  .20  per  cent,  of  ash.  This  foreign  matter  is  due  to  impuri- 
ties or  inclusions,  and  it  is  these  impurities  which  cause  the  various 
shades  of  color ;  the  yellow  shades  predominate,  and  while  often 
more  brilliant  are  not  as  valuable  as  those  of  a  steel-white  color; 
blue,  green,  or  red  diamonds  are  the  most  valuable  of  all  gems. 

Cleavage  fragments  and  dark  brown  specimens  are  termed  bort 
and  are  used  in  glass  cutters,  or  reduced  to  dust  for  polishing. 
Another  most  important  use  is  in  core  drilling,  a  most  convenient 
and  economical  method  of  prospecting  mining  properties.  Car- 
bonado, a  black  variety,  from  the  province  of  Bahia,  Brazil,  occur- 
ring in  rounded  masses,  lacks  the  perfect  cleavage  of  the  transpar- 
ent stones.  It  is  slightly  porous  and  therefore  of  a  lower  specific 
gravity,  yet  harder  than  the  well-crystallized  material,  and  is  said 
to  yield  better  results  in  drilling  than  the  diamond  fragments. 

The  commercial  unit  of  weight  used  in  estimating  the  value  of 
diamonds,  as  well  as  other  precious  stones,  is  the  carat.  Like  all 
of  the  old  units  of  measure  the  carat  of  merchants  of  different  coun- 
tries varied  from  188.5  to  254.6  milligrams ;  the  weight  commonly 
used  was  from  205  to  207  milligrams.  The  metric  carat  of  200 
milligrams  is  now  the  legal  carat  in  all  countries  using  the  metric 
system. 

All  diamonds  of  the  ancients  and  of  Europe  until  1727,  when 
diamonds  were  discovered  in  Brazil,  were  from  the  East,  where  they 
were  obtained  from  alluvial  washings  and  in  conglomerates,  espe- 
cially at  Purteal  and  Golconda,  India.  The  noted  diamonds  oi 
these  fields  are  the  Koh-i-noor,  of  186  carats  and  now  recut  to  106  ; 
the  Pitt  or  Regent,  a  yellow  stone  of  137.5  carats,  now  in  the 
Galerie  d'Apollon  in  the  Louvre,  Paris  (this  stone  was  appraised 
in  1791  at  12  million  francs);  the  Orloff,  of  194  carats;  the  Blue 
Hope,  of  44.5;  and  probably  the  Great  Mogul,  of  279  carats. 

In  1727  diamonds  were  discovered  by  the  miners  in  the  gold 
washings  of  Minas  Geraes,  Brazil ;  since  then  these  workings  have 
yielded  continuously  large  quantities  of  good  stones.  Here  also 
the  crystals  are  obtained  in  river  washings  and  prairie  deposits 
and  are  associated  with  a  peculiar  quartz  schist  or  flexible  sand- 
stone, termed  itacolumite.  The  most  famous  diamond  of  the 
Brazilian  field  is  the  "Star  of  the  South,"  weighing  247.5  carats 
uncut  and  125  when  cut. 

The  Vaal  River  locality  of  South  Africa  was  discovered  in  1867, 


THE  NATIVE  ELEMENTS  283 

where  the  first  diamond  was  taken  from  the  sands  of  the  river  by 
some  Boer  children ;  attracted  by  its  brightness,  they  carried  it  home 
to  add  to  their  playthings.  It  weighed  in  the  rough  21.25  carats 
and  sold  for  500  pounds.  In  1869  the  "  Star  of  South  Africa  " 
was  found  by  a  black  shepherd  on  the  Orange  River ;  this  was  a 
magnificent  white  stone  of  83.5  carats  and  cut  to  46.5.  After 
passing  through  several  hands  it  was  sold  to  the  Earl  of  Dudley 
for  125,000  dollars.  In  1870  diamonds  were  found  at  Kimberley, 
for  the  first  time  unmistakably  in  their  primary  position,  contained 
in  a  peculiar  peridotite  in  the  form  of  pipes  and  plugs,  filling  the 
craters  of  ancient  volcanoes.  By  decomposition  this  rock  forms 
the  famous  "  blue  earth  "  from  which  the  South  African  diamonds 
are  obtained,  and  in  which  they  are  associated  with  garnets,  magnet- 
ite, enstatite,  augite,  chromite,  olivine,  corundum,  etc.  Other 
similar  pipes  were  subsequently  discovered,  all  of  which  have  been 
consolidated  in  the  De  Beers  Company  limited,  which  has  produced 
nearly  all  the  world's  supply  of  diamonds  for  the  last  twenty-five 
years.  The  largest  diamond  ever  found  was  the  Cullinan,  weigh- 
ing 3253f  carats,  taken  on  June  6th,  1905,  from  the  walls  of  the 
Premier  mine  near  Pretoria,  South  Africa.  Before  this  discovery, 
the  "  Excelsior  Jubilee,"  weighing  97 If  carats,  discovered  in  the 
Jagersfontein  mine  in  the  Orange  River  colony,  was  the  largest. 
Both  of  these  diamonds,  though  they  were  of  beautiful  color,  owing 
to  internal  flaws  were  cleft  and  cut  into  various  stones.  The  Culli- 
nan was  originally  purchased  by  the  Transvaal  Assembly  for 
1,000,000  dollars  and  presented  to  Edward  VII.  It  is  now  a  part 
of  the  Royal  Regalia  deposited  in  the  Tower  of  London. 

In  the  United  States  diamonds  have  been  found  in  North 
.Carolina,  Georgia,  Virginia,  Colorado,  California,  and  Wisconsin, 
all  of  which  were  loose  in  gravel  or  sand.  In  1906  diamonds  were 
discovered  in  Pike  County,  Arkansas,  in  a  peridotite  resembling, 
in  many  respects,  the  deposits  of  South  Africa.  Several  companies 
have  been  formed  and  some  1200  diamonds  have  resulted,  yield- 
ing, when  cut,  gems  of  good  color.  They  are  small,  very  few 
weighing  over  one  carat.  The  largest  yet  found  was  a  stone  of  6.5 
carats. 

The  origin  of  diamonds  has  not  as  yet  been  satisfactorily  ex- 
plained. They  have  been  crystallized,  probably  from  carbon  dis- 
solved in  a  fused  magma  and  under  high  pressure.  The  source  of 
this  dissolved  carbon  is  in  doubt ;  it  may  have  been  brought  up 
from  depths  with  the  igneous  rock  or  acquired  from  shales  contain- 


284  •  MINERALOGY 

ing  organic  matter,  through  which  the  magma,  while  still  in  a  fused 
condition,  was  forced.  In  most  cases  this  magma  has  been  basic, 
containing  large  quantities  of  magnesium  silicates.  After  solidi- 
fication they  are  of  the  nature  of  peridotites. 

Artificial  diamonds  were  produced  by  Moissan,  by  dissolving 
carbon  in  fused  iron;  upon  cooling  the  fusion  quickly  in  melted 
lead,  the  outer  portions  solidify  first  and  the  resulting  contraction 
subjects  the  still  liquid  interior  to  enormous  pressure.  Under 
these  conditions  the  excess  carbon  was  separated  as  small  diamonds. 
If  the  fusion  was  cooled  slowly  and  without  pressure,  the  more  stable 
crystalline  form  of  carbon,  graphite,  was  formed.  The  diamonds 
contained  in  meteors,  as  the  Canon  Diablo  specimens,  must  be  of 
this  nature.  Diamonds  were  also  produced  by  J.  Friedlander, 
who  dissolved  graphite  in  fused  olivine ;  from  these  experiments 
it  was  found  that  fused  magnesium  and  calcium  silicates  dis- 
solved carbon  and  favored  its  separation  on  cooling  in  the  form  of 
diamond,  a  condition  very  similar  to  that  of  the  natural  deposits. 

GRAPHITE 

Graphite.  —  Carbon,  Black  Lead,  Plumbago ;  C ;  Hexagonal ; 
Type,  Dihexagonal  Alternating;  c  =  1.3859;  J)001A10ll  = 
58°,  r  Ar'  =  94°  31' ;  Common  forms,  c  (0001),  r  (lOfl) ;  Cleavage 
basal,  perfect ;  Laminae,  pliable ;  H.  =  1-2 ;  G.  =  2-2.3 ;  Color, 
black  to  steel-gray;  Streak,  gray  to  black;  Luster,  metallic  to  dull 
and  earthy ;  Opaque,  feels  greasy  and  marks  paper. 

B.B.  —  Infusible,  deflagrates  on  coal  when  mixed  with  nitre 
and  heated  to  a  high  temperature.  Insoluble  in  acids. 

General  description.  —  Occurs  when  crystalline  in  thin  tabular 
crystals  flattened  parallel  to  the  base,  often  in  foliated  masses, 
radiated  scaly,  compact  or  earthy.  Many  specimens  are  impure 
from  oxides  of  iron,  clay,  or  mixture  with  sand. 

Cliftonite  is  an  isometric  form,  harder  than  graphite,  contained 
in  a  meteoric  iron  from  Australia  and  also  from  Cooke  County, 
Tennessee ;  these  crystals  have  been  regarded  as  pseudomorphs  after 
diamond. 

Graphite  is  very  common  in  crystalline  schists,  as  in  the  Adiron- 
dacks  and  at  Ticonderoga,  New  York,  and  High  Bridge,  New  Jersey, 
where  it  has  been  formed  from  the  organic  matter,  contained  in  the 
original  sedimentary  deposits,  by  the  metamorphic  action  of  heat 


THE  NATIVE   ELEMENTS  285 

and  pressure.  It  occurs  in  disseminated  scales  in  crystalline  lime- 
stones, as  the  Laurentians  of  Canada.  The  graphite  of  Colfax 
County,  New  Mexico,  was  formed  by  contact  of  intruded  igneous 
rock  with  coal  beds.  Graphite  occurs  associated  with  diamond  in 
the  Canon  Diablo  meteor,  and  with  native  iron  in  the  basalt  of 
Ovifak,  Greenland.  A  pegmatite  of  Maine  contains  9  per  cent, 
of  graphite,  which  has  separated  from  the  magma  after  the  feld- 
spars and  is  contained  as  inclusions  in  the  quartz.  Graphite  con- 
tained in  fissures  and  veins  at  Ticonderoga  suggests  a  pneumato- 
lytic  origin. 

Commercially  most  of  the  graphite  mining  in  the  United  States 
is  carried  on  at  Ticonderoga ;  this  is  the  crystalline  form  of  graph- 
ite and  is  used  in  the  manufacture  of  crucibles,  for  the  melting  of 
alloys  and  the  refining  of  metals.  The  Ceylon  product  is  consid- 
ered better,  being  more  fibrous,  requiring  less  binder ;  at  the  present 
time  these  mines  supply  most  of  the  world's  product.  Graph- 
ite has  been  used  in  pencils  for  several  hundred  years ;  an  amor- 
phous product  from  Sonora,  Mexico,  is  considered  to  be  the  best 
for  this  purpose.  A*  finely  ground  graphite  is  mixed  with  oil  as  a 
lubricant.  It  is  also  used  for  electrodes  in  the  electrochemical 
industries,  and  the  impure  forms  are  used  as  a  paint  to  protect  iron- 
work from  rusting. 

Artificial.  —  Fused  metals  dissolve  carbon,  iron  as  much  as  four 
per  cent.,  a  large  portion  of  which  separates  as  graphite  on  cooling. 
Thus  cast  iron  contains  carbon  as  graphite,  which  has  been  formed 
under  the  same  conditions  as  that  contained  in  meteoric  iron.  It 
is  common  in  slags  of  blast  furnaces.  Graphite  is  produced  at 
Niagara  Falls  electrolytically,  by  the  Acheson  process,  competing 
commercially  with  the  natural  product. 

SULPHUR 

Sulphur.  —  S ;  Orthorhombic  ;  Type,  Digonal  Holoaxial ;  a  : 
b :  c  =  0.8131  :  1  :  1.9034  ;  100  A  110  =  39°  6',  001  A  101  =  66° 
52',  001  A  Oil  =  62°  17',  111  A  001  =  71°  39';  Common  forms, 
c  (001),  P(lll),p  (111),  e  (101),  s  (113),  n  (Oil) ;  Twinning  plane,  101 ; 
Cleavage,  c  and  m  perfect,  p  imperfect ;  Brittle ;  Fracture,  conchoi- 
dal;  H.=  1.5-2.5;  G.=  2.05-2.09;  Color,  sulphur  yellow  and  va- 
rious shades  of  yellow  ;  Streak,  white  ;  Luster,  resinous  ;  Trans- 
parent to  translucent;  a  =  1.950,  p  =  2.038,  -y  =  2.240 ;  y- 
a  =  .290;  Optically  (+) ;  Plane  of  the  optic  axes  =  010; 
Bxa  =  c,  2V  =  69.5°. 


286  MINERALOGY 

B.B.  —  Fuses  easily  at  114.5°  and  burns  with  a  blue  flame,  form- 
ing sulphur  dioxide.  When  pure,  volatilizes  entirely.  Insoluble 
in  acids.  Dissolves  in  carbon  disulphide. 

General  description.  —  Crystals  are  pyramidal  in  habit,  termi- 
nated by  the  base,  the  two  domes  e  and  n,  and  the  pyramids  s ; 
many  other  forms  have  been  described,  all  of  which  are  rare.  Some 


FIG.  393. — Sulphur  Crystals  from  Girgenti,  Sicily. 

crystals  are  sphenoidal  in  habit,  indicating  the  holoaxial  symmetry. 
The  best  examples  of  sulphur  crystals  are  found  at  Girgenti,  Sicily, 
where  they  occur  associated  with  celestite  and  other  sulphates. 
More  often  the  crystals  are  small  or  the  sulphur  is  incrusted,  massive, 
or  powdery,  mixed  with  clay,  marl,  or  other  impurities.  Sulphur 
is  a  non-conductor  of  heat,  and  a  peculiar  crackling  noise  may  be 
noted  when  a  crystal  is  held  to  the  ear,  in  the  hand,  due  to  the  un- 
even heating ;  in  this  way  crystals  often  fall  to  pieces. 

Sulphur  is  deposited  around  volcanoes  and  solfataras,  where  it 
is  condensed  from  vapors  or  reduced  by  the  interaction  of  S02 
and  H2S,  or  again  by  the  oxidation  of  H2S.  Many  hot  springs  con- 
tain H2S  in  solution  which  on  oxidation  deposits  sulphur.  In  sedi- 
mentary deposits  sulphur  is  formed  in  the  reduction  of  sulphates, 
and  is  often  associated  therefore  with  celestite  and  gypsum.  It 
has  also  been  observed  in  the  cracks  of  galena,  as  at  the  Wheatley 
mine,  Pennsylvania. 

Deposits  of  sulphur  in  the  United  States  are  found  in  the  Yellow- 
stone Park,  in  a  rhyolitic  tuff ;  at  Black  Rock,  Utah ;  at  Cody, 


THE   NATIVE  ELEMENTS  287 

Wyoming.  But  by  far  the  most  important  deposit,  is  at -Bayou 
Choupique,  Lake  Charles,  Louisiana,  where  a  bed  of  almost  pure 
sulphur  100  feet  thick,  lies  at  a  depth  of  440  feet  below  the  sur- 
face. This  deposit  furnishes  nearly  all  the  350,000  tons  annually 
consumed  in  the  United  States,  most  of  which  was  formerly 
imported  from  Sicily. 

Large  quantities  of  sulphur  are  used  in  the  wood  pulp  industry ; 
in  the  manufacture  of  matches ;  in  blasting  powder ;  in  vulcanizing 
rubber,  and  in  bleaching  through  the  chemical  action  of  SO2. 

Artificial  crystals  may  be  formed  by  evaporating  a  saturated 
solution  of  sulphur  in  carbon  disulphide.  There  are  many  allo- 
tropic  forms  of  sulphur ;  a-sulphur  is  stable  at  ordinary  tempera- 
tures, while  monoclinic,  /3-sulphur,  forms  when  melted  sulphur  is 
allowed  to  cool  until  a  crust  forms,  which  is  broken  and  the  still 
liquid  interior  is  poured  off,  when  crystals  of  this  monoclinic  form 
will  cover  the  walls.  On  standing  they  become  opaque  from 
the  formation  of  small  crystals  of  the  more  stable  orthorhombic 
form. 

PLATINUM 

Platinum.  —  Pt ;  Isometric ;  Type,  Ditesseral  Central ;  Common 
forms,  c  (100),  o  (111),  d(110);  Malleable;  Sectile  and  ductile; 
H.  =  4-4.5;  G.  =  14-19,  when  pure,  21.42;  Color  and  streak, 
steel-gray;  Luster,  metallic;  Opaque. 

B.B.  —  Infusible,  fusing  point  1755°.  Soluble  in  hot  aqua  regia. 
For  other  tests  see  page  582. 

General  description.  —  Crystals  are  not  common,  but  the  cube, 
octahedron,  and  rhombic  dodecahedron  occur.  Usually  it  occurs 
as  fine  grains  or  in  irregular  rolled  masses.  Native  platinum  is 
always  alloyed  with  other  metals  of  the  platinum  group,  as 
osmium,  iridium,  paladium,  rhodium,  and  ruthenium,  all  of  which 
occur  only  in  the  native  metallic  state,  with  the  exception  of  plati- 
num, which  occurs  in  sperrylite  (PtAs2)  as  an  arsenide,  and  ruthe- 
nium in  laurite  (RuS2)  as  a  sulphide.  In  addition  platinum  often 
contains  iron,  nickel,  and  gold,  to  which  its  variable  specific  gravity 
and  hardness  are  due. 

Deposits  of  platinum  are  associated  with  basic  rocks,  as  serpen- 
tine and  peridotite.  Its  most  constant  companion  is  chromite. 
Platinum  was  first  discovered  in  the  gold  washings  of  the  Pinto 
River,  Colombia,  South  America,  about  the  year  1720,  and  in  the 


288  MINERALOGY 

alluvial  deposits  of  the  Ural  Mountains,  Russia,  in  1822.  Here 
nuggets  weighing  as  much  as.  18  kilos  were  found.  The  largest, 
weighing  18.57  kilos,  is  in  the  Demidoff  collection  of  minerals  at 
St.  Petersburg.  The  larger  part  of  the  world's  supply  of  the  present 
day,  about  15,000  pounds  annually,  is  derived  from  the  Russian  de- 
posits. A  peculiar  black  sand  left  with  the  gold  in  the  washings  of 
the  Pacific  slope,  particularly  in  British  Columbia  and  the  states  of 
Oregon  and  Washington,  contains  small  amounts  of  platinum;  from 
this  source  a  few  ounces  are  obtained  annually.  Platinum  has  been 
reported  as  contained  in  a  serpentine  of  the  Urals ;  in  a  pegmatite  of 
Copper  Mountain,  British  Columbia;  in  a  decomposed  schist  of 
Broken  Hill,  Australia;  in  limonite  nodules  in  Mexico;  in  an  altered 
limestone  of  Sumatra.  In  addition  it  is  connected  with  certain  sul- 
phides, as  the  pyrrhotite  of  Sudbury,  Canada ;  covellite  in  Wyo- 
ming; and  chalcopyrite  of  the  Key  West  mine  near  Bunker vi lie, 
Nevada,  where  it  is  associated  with  nickel,  as  at  the  Sudbury 
locality.  Possibly  in  these  associations  platinum  may  be  in  the 
form  of  arsenide,  as  sperrylite  has  been  reported  from  both  Sud- 
bury and  the  British  Columbia  localities,  but  has  not  as  yet  been 
reported  from  Nevada. 

Owing  to  the  high  fusing  point  and  its  insolubility  in  single  acids, 
platinum  crucibles  are  used  in  chemical  analyses.  It  is  also  used 
in  thermoelectric  couples  for  the  measurement  of  high  tempera- 
tures ;  as  a  catalyzer  to  oxidize  SO2  to  S03  in  sulphuric  acid  works. 
Having  the  same  coefficient  of  expansion  as  glass,  it  is  used  to  carry 
the  electric  current  through  the  glass  walls  of  physical  apparatus. 
There  are  many  other  minor  uses ;  and  since  the  supply  cannot 
keep  pace  with  the  demand,  the  price  is  constantly  increasing,  until 
at  the  present  time  platinum  is  more  than  double  the  value  of  gold. 

COPPER 

Copper.  — Native  Copper,  Cu;  Isometric;  Type,  Ditesseral 
Central ;  Common  forms,  a  (001),  o  (111),  d  (101),  h  (410) ;  Twin- 
ning plane,  111 ;  Malleable,  ductile;  Fracture,  hackly;  H.  =  2.5-3  ; 
G.  =  8.8-8.9;  Color,  copper  red;  Streak,  shining;  Metallic; 
Opaque. 

B.B.  — Easily  fusible  (1084°).  In  the  blue  cone  of  the  0.  F. 
yields  a  green  flame.  On  coal  becomes  black  after  fusion  from  the 
formation  of  black  oxide.  Dissolves  in  HNO3  or  HC1,  yielding  a 
solution  which  becomes  intensely  blue  on  the  addition  of  an  excess 
of  ammonia. 


THE  NATIVE  ELEMENTS  289 

General  description.  —  Copper  crystallizes  in  cubes,  octahedrons, 
and  tetrahexahedrons;  other  forms  are  rare.  Twins  after  the  spinel 
law  are  not  uncommon.  It  occurs  more  often  in  distorted  forms, 
or  in  arborescent,  reticular,  dendritic,  filiform,  and  irregular  masses. 
Pseudomorphs  after  other  copper  minerals,  as  cuprite,  malachite, 


FIG.  394.  —  Native  Copper  and  Quartz  from  Lake  Superior. 

and  azurite,  are  often  formed  by  double  decomposition  and  reduc- 
tion. Native  copper  is  usually  coated  with  a  coat  of  oxides  or  car- 
bonates ;  many  masses  of  cuprite  still  contain  as  a  central  nucleus 
some  of  the  metallic  copper  from  which  they  were  formed. 

Native  copper  occurs  as  a  secondary  product,  either  formed  by 
precipitation  from  solution,  or  by  the  chemical  action  of  reducing 
agents  upon  minerals  containing  copper.  Percolating  ground 
waters  dissolve  copper  and  especially  under  heat  and  pressure ;  even 
distilled  water  will  dissolve  copper  under  these  conditions,  leaching 
out  the  original  copper  content  of  the  igneous  rocks  and  trans- 
porting it  to  points,  as  veins  and  cavities,  where  it  may  be  pre- 
cipitated by  contact  or  by  intermingling  with  other  solutions  con- 
taining a  reducing  agent,  as  ferrous  iron,  which  is  capable,  either 
as  oxide,  sulphate,  silicate,  or  carbonate,  of  precipitating  copper 
from  its  solutions. 

The  large  deposits  of  the  Lake  Superior  region  have  probably 
been  formed  in  this  way.  Here  metallic  copper,  generally  in  small 
particles,  is  contained  in  the  amygdaloid  cavities  of  a  conglomerate. 
One  mass  of  400  tons  was  found  in  the  Minnesota  mine.  Origi- 
nally the  copper  must  have  been  contained  in  the  adjacent  igneous 


290  MINERALOGY 

rocks,  but  is  now  concentrated  by  solution  and  precipitation  in  the 
conglomerate. 

Native  copper  is  also  found  in  the  Copper  Queen  mine,  Arizona ; 
in  sheets  at  Enid,  Oklahoma,  and  associated  with  fossil  bones  in 
Peru,  evidently  reduced  by  the  organic  matter  of  the  bone.  Small 
amounts  of  copper  are  present,  usually  as  a  secondary  product,  in 
nearly  every  copper  region  and  mine;  but  only  in  the  Lake  Superior 
region  does  it  constitute  nearly  all  the  copper  content  of  the  ore. 

The  United  States  produced,  in  1911,  550,645  tons,  nearly  one 
half  of  the  world's  product ;  of  this  Lake  Superior  region  contrib- 
uted 110.700  tons;  Montana,  141,250  tons,  and  Arizona,  110,500 
tons. 

SILVER 

Silver.  —  Native  Silver,  Ag;  Isometric;  Type,  Ditesseral  Cen- 
tral; Common  forms,  a  (100),  o(lll),  d  (101) ;  Twinning  plane, 
111;  Malleable  and  ductile;  Fracture,  hackly;  H.  =  2.5-3 ; 
G.  =  10.1-11.1 ;  Color  and  streak,  silver- white ;  Luster,  metallic; 
Opaque. 

B.B.  —  Fuses  easily  (955°),  and  in  O.  F.  on  coal  yields  a  brown 
coat  of  silver  oxide.  Soluble  in  nitric  acid.  Other  tests  for  silver 
are  given  on  page  578. 

General  description.  —  Crystals  are  usually  elongated  or  dis- 
torted octahedrons.  All  seven  forms  of  the  type  occur  on  silver 
crystals,  but  others  than  the  octahedron  and  cube  are  comparatively 
rare.  In  occurrence  it  is  more  often,  in  dendritic  or  arborescent 
growths,  due  to  twinning  and  parallel  positions ;  sheets,  wire,  and 
disseminated  scales  are  also  common. 

On  exposure  the  bright  surfaces  become  brown  to  black,  from  the 
formation  of  sulphides.  Owing  to  its  solubility  and  the  easily 
formed  sulphides,  silver  is  not  found  in  placer  deposits,  but  it  is 
associated  with  gold  and  copper  in  vein  deposits,  where  most  of  the 
native  silver  is  of  secondary  origin,  being  reduced  from  sulphides, 
chlorides,  and  other  compounds. 

Masses  of  native  silver  weighing  from  500  to  1000  pounds  have 
been  taken  from  the  deposits  of  Cobalt,  Ontario,  where  it  is  asso- 
ciated with  ores  of  cobalt  and  nickel.  Masses  weighing  as  much  as 
800  pounds  have  been  found  at  Huantaya,  southern  Peru,  while 
beautiful  specimens  of  crystalline  and  wire  silver  are  obtained  in 
the  mines  of  Batopilas,  Mexico ;  and  Kongsberg,  Norway.  In  the 


THE  NATIVE   ELEMENTS  291 

United  States  alloys  of  silver  and  copper  are  found  in  the  copper 
mines  of  the  Lake  Superior  region,  the  Poor  Man's  Lode,  Idaho,  and 
many  localities  through  the  West.  Mexico  is  the  largest  producer 
of  silver,  with  72,000,000 l  ounces,  while  the  United  States 
produced  57,796,117  ounces,  and  Canada,  31,500,000  ounces,  in 
1911. 

GOLD 

Gold.  —  Native  gold,  Au ;  Isometric ;  Type,  Ditesseral  Cen- 
tral;  Common  forms,  o(lll),  d(110);  Malleable  and  ductile; 
Fracture,  hackly ;  H.  =  2.5-3  ;  G.  =  15.8-19.3;  Color  and  streak, 
gold-yellow;  Luster,  metallic;  Opaque. 

B.B.  —  Fuses  easily  (1065°).  Insoluble  in  acids  except  aqua 
regia,  also  in  nascent  chlorine,  and  in  potassium  cyanide  in  pres- 
ence of  oxygen.  For  other  tests  see  page  582. 

General  description.  —  When  crystalline,  usually  in  octahedrons 
elongated  parallel  to  one  axis  or  flattened  parallel  to  a  face.     Also 
in  arborescent  and  reticular 
shapes,    or    sheets,    dissemi- 
nated scales,  and  rolled  water- 
worn    masses,    known    as 
nuggets. 

The  purest  native  gold  is 
said  to  be  that  of  Mount 
Morgan,  Queensland,  99.7 
per  cent.  gold.  Most  gold 

contains       silver;       electrum        FIG.  395.  -  Crystals  of  Gold .    Australia. 

from  Hungary  is  a  pale  yel- 
low natural  alloy  containing  30  per  cent,  silver.     Gold  may  also 
contain  copper,  iron,  lead,  bismuth,  platinum,  or  mercury,  most 
of  which  reduce  its  specific  gravity  and  modify  its  color. 

Gold  is  nearly  world- wide  in  occurrence,  though  in  very  small 
quantities  except  where  it  has  been  concentrated  by  secondary 
causes.  Most  of  the  world's  gold,  until  recent  years,  was  recovered 
from  river  sand  and  alluvial  deposits,  where,  owing  to  its  high 
specific  gravity  and  insolubility,  it  has  remained  behind  unchanged 
for  centuries.  Such  gold  is  recovered  by  the  simplest  of  all  min- 
ing, placer,  which  consists  of  washing  the  lighter  sands  away,  in 
sluices  or  pans.  All  placers  of  the  world  are  worked  by  modifica- 

1 1910. 


292  MINERALOGY 

tions  of  this  simple  method,  which  reduces  the  cost  of  handling 
the  enormous  quantities  of  material  made  necessary.  By  the 
hydraulic  method  in  California  the  cost  has  been  so  reduced  that  a 
cubic  yard  of  earth  can  be  handled  at  a  cost  of  two  cents. 

The  largest  nuggets  have  been  taken  from  the  gravels  of  Australia, 
one  weighing  190  pounds,  another  180  pounds.  Gold  occurs  in 
quartz  veins,  especially  the  veins  of  schists,  porphyry,  and  those 
rocks  high  in  silica.  Less  often  do  the  veins  of  basic  rocks  contain 
gold,  though  it  may  be  found  in  some  limestones  and  slates.  It  is 
probably  more  soluble  in  magmas  high  in  silica. 

Gold  as  a  primary  mineral  has  been  reported  in  granite  from 
Mexico ;  in  pitchstone  from  Chili ;  and  small  amounts  of  gold  are 
shown  by  assay  to  be  contained  in  granites,  syenite,  basalt,  and 
diabases  of  California. 

Gold  is  soluble  in  sodium  or  potassium  silicates,  ferrous  sul- 
phates and  chloride,  and  the  alkali  sulphides.  The  small  amounts 
of  gold  contained  in  the  country  rocks  are  dissolved  by  these  natural 
solvents  and  transported  in  solution  by  the  percolating  waters  to 
veins  or  cavities  where,  through  a  changed  physical  condition,  as  a 
reduction  of  temperature  and  pressure,  or  by  contact  with  a  pre- 
cipitant, they  are  deposited.  As  evidence  of  such  concentration, 
gold  is  being  deposited  with  the  siliceous  sinter  at  the  Steamboat 
Springs  of  Nevada  and  at  the  hot  springs  of  New  Zealand,  and  its 
presence  in  sea  water  has  been  repeatedly  verified. 

Gold  in  solution  is  very  unstable  and  is  easily  precipitated  by 
such  agents  as  organic  matter,  ferrous  salts,  metallic  sulphides, 
especially  those  of  iron,  copper,  zinc,  arsenic,  and  antimony;  with 
the  ores  of  these  metals  gold  is  often  associated. 

Pyrite  is  a  constant  companion  of  gold  in  quartz  veins,  inclosing 
it  within  the  body  of  the  crystals  as  inclusions ;  the  pyrite  on  oxida- 
tion is  mostly  carried  away,  leaving  the  gold  behind,  contained 
in  a  porous,  rusty  quartz,  always  so  pleasing  in  appearance  to  the 
old  prospector. 

The  largest  producing  gold  mines  of  the  world  are  on  the  Rand  in 
South  Africa ;  at  Victoria,  Australia ;  and  in  the  United  States.  The 
United  States  produced,  in  1911,  $96,233,428  of  pure  gold,  the  mint 
value  of  which  is  $20.6718  per  ounce  troy  or  $0.6646  per  gram. 
To  this  production  twenty  states  contributed,  of  which  Colorado, 
Alaska,  California,  Nevada,  and  South  Dakota,  in  order,  were  the 
largest  producers. 

Gold  is  the  basis  of  the  world's  coinage;    that  of  the  United 


THE  NATIVE  ELEMENTS  293 

States  is  9  parts  gold  to  1  of  copper,  or  900  fine.  The  proportion 
in  jewelry  is  designated  by  the  carat,  24  carat  being  pure  gold.  In 
England  the  usual  standard  is  22  carat,  or  916.67  fine. 

MERCURY 

Mercury.  —  Quicksilver,  Hg;  Isometric;  Type,  Ditesseral  Cen- 
tral ;  Crystals,  octahedrons ;  Liquid  at  ordinary  temperatures ; 
Solid  at  —  39°  with  cubic  cleavage ;  G.  =  13.6 ;  Color,  tin- white ; 
Brilliant  metallic  luster;  Opaque. 

B.B.  —  Volatilizes  entirely  when  pure,  yielding  a  gray  coat  on 
coal.  See  tests  on  page  579. 

General  description.  —  Native  mercury  is  not  of  common  occur- 
rence. It  is  found  as  small  metallic  liquid  globules  in  the  gangue 
associated  with  cinnabar,  from  which  it  has  probably  been  reduced 
by  organic  agents.  It  also  occurs  in  shales,  slate,  and  marls,  as  at 
Idria,  Austria,  one  of  the  important  European  localities,  where  it 
occurs  in  a  clay  slate.  The  deposits  of  California  and  Texas  where 
native  mercury  is  found  are  also  associated  with  sedimentary 
rocks,  which  yield  hydrocarbon  gases.  Hot  springs,  as  the  Steam- 
boat Springs  of  Nevada,  bring  mercury  to  the  surface. 

The  mercury  of  commerce  is  obtained  from  the  sulphide,  cinna- 
bar. In  the  United  States,  California  for  a  long  time  was  the  only 
producing  state,  but  since  the  discovery  of  the  mercury  deposits  at 
Terlingua,  Brewster  County,  Texas  has  also  been  a  producer. 
About  twenty-one  thousand  flasks,  of  seventy-five  pounds  each, 
were  produced  in  the  United  States  in  1911,  of  which  California 
produced  seventeen  thousand. 

The  greatest  demand  for  mercury  is  in  the  amalgamation  of  silver 
and  gold  ores.  It  is  also  used  in  the  production  of  vermilion  paint, 
and  in  smaller  quantities  in  the  sciences  and  in  the  construction  of 
electrical  apparatus. 

The  metals  iron,  lead,  bismuth,  arsenic,  and  antimony  are  also 
found  in  nature  in  a  free  state,  but  only  locally  and  in  very  restricted 
quantities.  Iron  occurs  in  meteors  and  as  a  primary  accessory  com- 
ponent in  some  basalts,  as  at  Antrim,  Ireland,  in  the  trap  of  New 
Jersey,  and  in  the  dolerites  of  Mount  Washington,  New  Hampshire. 
The  most  noted  locality,  however,  is  at  Disco  Island,  West  Green- 
land, where  large  masses,  many  tons  in  weight,  have  weathered  out 
of  the  basalt ;  these  masses  were  originally  supposed  to  be  meteoric. 


CHAPTER   V 

SULPHIDES,   ARSENIDES,    ANTIMONIDES 
REALGAR 

Realgar.  —  Sulphide  of  arsenic,  As2S2;  As  =^70.1;  S  =  29.9; 
Monoclinic;  Type,  Digonal  Equatorial;  a:  b  :  c  =  1.4403  :  1 : 
.9729 ;  0  •=  66°  5'  =  001  A  100 ;  100  A  110  =  52°  47',  001  A  101  = 
40°  22',  110 A 110  =105°  34',  001A011  =  41°  39';  Common 
forms,  a  (100),  b  (010),  c  (001),  m  (110),  r  (012)  ;  Cleavage,  b  per- 
fect, c,  a,  and  m  less  so ;  Sectile ;  Fracture,  conchoidal ;  H.  =  1.5-2 ; 
G.  =  3.55 ;  Color,  a  dark  orange-red,  streak  somewhat  lighter  in 
color ;  Luster,  resinous ;  Transparent  to  translucent ;  Optically 
(-);  Plane  of  the  optic  axes,  b  (010) ;  BxaAc  = -f  11°;  2V  = 
92°  58'. 

B.B. —  On  coal  in  O.  F.  burns,  yielding  an  arsenic  odor  and  when 
pure  leaves  no  residue.  In  the  closed  tube  yields  a  cherry-red  sub- 
limate of  arsenic  sulphide,  in  the  open  tube  yields  a  white  subli- 
mate of  As2O3  and  an  S02  odor. 

General  description.  —  Crystals  prismatic,  usually  combina- 
tions of  (001),  (110),  (010)  with  the  prism  zone  striated  lengthwise ; 
also  occurs  as  compact  masses,  granular,  or  in  crusts.  Usually 
coated  with  a  yellow  film  of  orpiment  into  which  it  changes  on  ex- 
posure. Realgar  is  associated  with  antimony,  arsenic,  and  silver 
ores.  It  occurs  as  a  sublimation  product  in  the  lavas  of  Vesuvius, 
and  at  the  present  time  it  is  being  deposited  associated  with  orpiment 
from  the  water  of  the  hot  springs  of  Norris  Geyser  Basin,  Yellow- 
stone Park.  In  the  United  States  it  is  mined  at  Monte  Cristo, 
Washington. 

Uses.  —  Mixed  with  lime,  realgar  is  used  in  tanning,  to  remove 
the  hair  from  the  hides;  it  furnishes  the  white  lights  in  pyro- 
technics. It  is  also  used  as  a  pigment,  but  the  commercial,  or  ruby, 
arsenic  is  an  artificial  product.  When  realgar  is  dissolved  in  sodium 
bicarbonate  at  150°  under  pressure,  upon  cooling  the  solution,  it 
separates  as  crystals. 

294 


SULPHIDES,  ARSENIDES,   ANTIMONIDES  295 

ORPIMENT 

Orpiment.  —  Arsenic  trisulphide,  As2S3 ;  As^  =  61,  S  =  39;  Mon- 
oclinic ;  Type,  Digonal  Equatorial ;  a  :  b  :  c  =  .5962  : 1  :  .665 ; 
J3  =  89°  19':  110  A  110  =  62°  11',  120  A  120  =  96°  23';  Common 
forms,  m  (110),  u  (120),  o  (101) ;  Twinning  plane,  100;  Cleavage,  b 
perfect,  laminae  flexible,  inelastic;  Sectile;  H.  =  1.5-2;  G.  = 
3.4-4.5 ;  Color,  lemon-yellow ;  Streak,  pale  yellow ;  Luster,  resi- 
nous; cleavage  surfaces  pearly ;  Transparent  to  translucent;  Opti- 
cally (  — ) ;  Axial  plane,  001. 

B.B.  —  Fuses  and  volatilizes  entirely  when  pure,  yielding  an 
arsenic  odor. 

General  description.  —  Crystals  are  rare,  but  small  crystals  are 
found  in  the  clays  at  Tajowa,  Hungary.  It  occurs  usually  in  foli- 
ated or  columnar  masses  with  rounded  surfaces.  It  is  associated 
with  and  may  be  formed  as  an  efflorescence  on  realgar. 

STIBNITE 

Stibnite.  —  Antimony  glance,  Sb2S3 ;  Antimony  trisulphide ; 
Sb_=  71.4,  S  =  28.6;  Orthorhombic ;  Type,  Didigonal  Equatorial ; 
a:b:  c  =  .9926:  1:  1.0179;  100 A  110  =  44°  47',  001  A  101  =  45° 
43',  001 A  Oil  =45°  30',  111  A  110  =  34°  41';  Common  forms, 
m  (110),  p  (111),  b(010);  Cleavage,  b  perfect,  a  and  m  imperfect; 
Slightly  sectile  and  pliable;  Fracture,  conchoidal;  H.  =  2;G.  = 
4.52-4.62;  Color,  steel-gray,  tarnishing  to  black;  Streak,  lead- 
gray,  marks  paper;  Luster,  metallic,  splendent  on  fresh  surfaces. 

B.B.  —  Fuses  in  O.  F.  at  once  (630°)  and  on  coal  yields  a  white 
coat  of  Sb20s,  also  an  odor  of  sulphur  dioxide.  When  pure,  vola- 
tilizes entirely,  in  R.  F.,  yielding  a  yellowish  green  antimony  flame. 
Soluble  in  HC1,  but  in  HN03  forms  a  white  insoluble  Sb205. 

General  description.  —  Crystals  are  long  prismatic  or  acicular 
with  striations  and  furrows  parallel  to  the  c  axis,  usually  ter- 
minated with  the  unit  pyramid.  Numerous  forms  have  been  de- 
scribed, especially  on  crystals  from  Ichinokawa,  Island  of  Shikoku, 
Japan,  where  brilliant  crystals  nearly  two  feet  in  length  have  been 
found ;  in  some  cases  these  are  peculiarly  twisted  around  the  vertical 
axis.  Often  in  radiated  groups  and  aggregates  of  acicular  crystals, 
as  at  Felsobanya,  Hungary,  penetrating  the  tabular  crystals  of 
barite  with  which  they  are  associated.  Massive  and  granular 


296  MINERALOGY 

varieties  of  stibnite  often  appear  much  harder  than  2,  from  the  im- 
purities. Crystals  of  stibnite  are  at  times  coated  with  white  crust 
formed  by  oxidation,  and  at  times  the  entire  crystal  has  undergone 


FIG.  396. — Stibnite  from  Lyo  Island,  Japan. 

oxidation,  forming  pseudomorphs  of  cervantite  (Sb204)  after  stib- 
nite, as  at  Charcas,  Mexico. 

In  the  United  States  stibnite  occurs  at  Lovelocks  and  Humboldt 
regions,  Nevada ;  in  Iron  County,  Utah ;  and  in  Buck  County, 
Idaho.  Very  little  is  mined,  as  most  of  the  American  antimony  is 
obtained  in  the  smelting  of  lead  ores,  which  contain  antimony  in 
small  quantities. 

The  metal  is  used  principally  in  the  alloys,  type,  babbett,  and 
britannia  metal.  The  trisulphide  is  used  to  produce  the  "  Bengal 
Fire/'  the  trioxide  in  the  glaze  of  enameled  ironware  and  in  color- 
ing glass  and  porcelain  yellow.  Many  of  its  salts  are  used  in  medi- 
cine. 

BISMUTHINITE 

Bismuthinite.  —  Bi2S3.  The  trisulphide  of  bismuth  is  a  rare 
mineral,  occurring  in  striated,  irregularly  terminated  prisms.  It  is 
found  in  small  quantities  in  Rowan  County  ^  North  Carolina,  asso- 
ciated with  gold,  and  in  the  gold  ores  of  Goldfield,  Nevada.  Lead 
ores  contain  bismuth  and  most  of  the  commercial  metal  is  produced 
in  the  electrolytic  refining  of  lead. 

MOLYBDENITE 

Molybdenite. —  MoS2;  Sulphide  of  molybdenum;  Mo  = 
60,  S  =  40 ;  Hexagonal ;  Type,  Dihexagonal  Equatorial ;  c  =  1.098; 


SULPHIDES,   ARSENIDES,   ANTIMONIDES  297 

0001 A  lOlO  =  65°  35' ;  Crystal  forms,  c  (0001),  m  (lOlO) ;  Cleavage, 
c  perfect,  laminae  flexible  but  inelastic;  H.  =  1-1.5;  G.  =  4.7-4.8; 
Color,  lead-gray  to  bluish  gray  to  bluish  black,  marks  paper,  and 
feels  greasy ;  Streak,  bluish  gray ;  Luster,  metallic ;  Opaque. 

B.B.  —  Infusible  in  the  forceps,  but  yields  a  green  flame.  The 
powdered  mineral  on  coal  in  O.  F.  yields  a  white  coat.  In  the  open 
tube  yields  SO2.  Decomposed  with  HNO2,  moistened  with  H2SO4, 
and  evaporated  in  a  porcelain  crucible,  yields  a  blue  residue  on  cool- 
ing. 

General  description.  —  Crystals  are  tabular,  parallel  to  the  base, 
and  six-sided,  with  the  prism  faces  irregularly  striated  and  furrowed 
horizontally.  Several  prisms  have  been  described,  but  good  faces 
are  rare.  Usually  much  like  graphite  in  appearance.  In  flat  scales 
disseminated  through  granite  and  pegmatites,  as  at  Cooper,  Maine. 
It  occurs  also  in  gneiss,  schist,  gabbro,  granular  limestones,  and  in 
quartz  veins,  as  at  Chelan  County,  Washington,  and  Beaverhead 
County,  Montana.  Crystals  two  'to  three  inches  across  have  been 
found  in  Okanogan  County,  Washington,  and  at  Aldfield,  Pontiac 
County,  Quebec. 

While  molybdenite  occurs  in  many  localities  in  the  United 
States,  they  are  all  small  deposits,  two  of  which  are  at  present  mined, 
that  at  Cooper,  Maine,  and  in  Washington,  even  though  molyb- 
denite is  the  principal  ore  of  the  metal  and  is  quoted  as  being 
valued  at  $1.50  per  pound. 

The  metal  is  used  in  making  tool  steel ;  as  ammonium  molyb- 
date  in  the  determination  and  separation  of  phosphoric  acid  in  iron 
ores ;  in  the  staining  of  leather,  and  as  sodium  molybdate  in  the 
coloring  of  pottery  blue.  Artificial  crystals  of  molybdenite  have 
been  formed  by  fusing  the  oxide  with  sulphur  and  potassium  car- 
bonate. 

ARGENTITE 

Argentite.  —  Silver  glance  ;  Ag2S  ;  Sulphide  of  silver  ; 
Ag  =  87.1;  S  =  12.9;  Isometric;  Type,  Ditesseral  Central; 
Common  forms,  o(lll),  a  (001),  d(110);  Twinning  plane,  111, 
interpenetrating ;  Cleavage,  a  and  d  in  traces ;  Sectile  and  malle- 
able ;  H.  =  2-2.5 ;  G.  =  7.2-7.35 ;  Color,  dark  lead-gray ;  Streak, 
gray,  shining ;  Luster,  metallic ;  Opaque. 

B.B.  —  Fuses  with  intumescence  in  O.  F.  on  coal,  yielding  a 
globule  of  silver  and  a  sulphur  dioxide  odor. 


298 


MINERALOGY 


General  description.  —  Crystals  are  octahedrons,  rhombic 
dodecahedrons,  cubes,  or  combinations  of  these  forms ;  all  forms  of 
the  type  have  been  observed  on  argentite,  but  the  other  four  forms 
are  rare.  Fresh  surfaces  are  bright  and  shiny,  but  like  all  silver 
minerals  become  dark  on  exposure.  Its  occurrence  is  more  often 

dendritic,  granular, 
or  disseminated.  At 
the  Comstock  Lode, 
Nevada,  it  alone 
^^^^  constitutes  a  work- 

^HiftjB  able  silver  ore ;  also 

at  Port  Arthur, 
Lake  Superior.  It 
occurs  in  small 
amounts  in  most 
silver  mines,  and  in 
the  cobalt  region, 
Canada,  in  consid- 
erable quantities. 

It  is  usually  as- 
sociated withsteph- 
anite,  galena,  py- 

rite,  cobalt,  and  nickel  ores,  gold,  and  silver,  the  latter  being  a 
secondary  product  reduced  from  the  sulphide.  Argentite  and 
galena  are  isomorphous,  and  the  latter,  especially  the  fine  granular 
varieties,  contains  small  amounts  of  silver,  and  the  smelting  of 
galena  yields  each  year  a  considerable  amount  of  the  world's  pro- 
duction of  silver. 

Silver  in  solution,  either  as  the  sulphate,  carbonate,  or  nitrate,  is 
precipitated  by  the  natural  sulphides  as  pyrite,  chalcopyrite,  born- 
ite,  or  galena,  a  reaction  which  without  doubt  plays  an  important 
part  in  the  secondary  enrichment  of  silver  ores. 


FIG.  397.  —  Argentite  from  Freiberg,  Saxony. 


GALENA 

Galenite.  —  Galena ;  Lead  glance ;  PbS,  lead  sulphide  ;  Pb  = 
86.6,  S  =  13.4;  Isometric;  Type,  Ditesseral  Central;  Common 
forms,  a  (100),  o  (111),  d  (110) ;  Twinning  plane,  111,  both  contact 
and  interpenetrating ;  Cleavage  cubic,  perfect ;  Brittle  ;  Fracture, 
subconchoidal ;  H.  =  2.5-2.75;  G.  =  7.4-7.6;  Color  and  streak 
lead-gray;  Opaque. 


SULPHIDES,   ARSENIDES,   ANTIMONI.DES  299 

B.B.  —  Fuses  easily  on  coal  in  the  O.  F.,  yielding  SO2  fumes  and 
a  yellow  oxide  of  lead  coat,  which  is  often  quite  white  with  lead 
carbonate  or  sulphate.  In  R.  F.,  especially  when  mixed  with  soda, 
is  reduced  to  malleable  lead.  The  soda  fusion,  when  placed  on  a 
silver  surface  and  moistened,  leaves  a  black  stain  (S).  Dissolves 
in  nitric  acid,  forming  insoluble  white  lead  sulphate  (PbSO4). 

General  description.  —  Crystals  are  usually  cubic  or  combina- 
tion of  the  cube  and  the  octahedron,  less  often  the  rhombic  dodeca- 
hedron; the  simple  octahedral  habit  is  rare.  Other  forms  occur 


FIG.  398.  —  Galena  Crystals.    Bavaria. 

which  at  times  give  the  crystals  a  rounded  appearance;  cube  faces 
are  often  vicinal.  Twins  after  the  spinel  law  art?  flattened,  as  is 
usual  with  twins  of  this  class.  Cleavage  is  cubical  with  brilliant 
surfaces,  and  in  some  cases  striated  from  polysynthetic  twin- 
ning ;  in  rare  exceptions,  as  at  Lancaster,  Pennsylvania,  and  Nord- 
marken,  Sweden,  the  cleavage  is  octahedral ;  such  specimens  have 
always  been  found  to  contain  bismuth.  Massive,  granular,  and 
disseminated  varieties  are  common,  but  fibrous  and  plumose  speci- 
mens are  rare.  Galena  contains  as  impurities  zinc,  copper,  cad- 
mium, bismuth,  arsenic,  and  antimony,  possibly  as  sulphides; 
also  gold  and  silver;  often  the  silver  value  is  greater  than  that 
of  lead,  when  it  constitutes  a  true  silver  ore. 

Galena  occurs  both  as  a  primary  and  secondary  mineral,  but  by 
far  the  most  important  commercially  are  the  secondary  vein  de- 
posits or  those  filling  cavities  in  limestone  formations,  where  it  is 
associated  with  sphalerite  and  chalcopyrite.  The  gangue  of  such 


300  MINERALOGY 

veins  is  usually  calcite,  siderite,  barite,  or  fluorite.  Such  deposits 
have  been  formed  by  the  precipitation  of  lead,  carried  in  solution 
as  the  carbonate,  sulphate,  or  even  the  sulphide  (as  galena  is  depos- 
ited by  some  thermal  springs),  by  water,  percolating  down  from  the 
superficial  areas,  which  tends  to  dissolve  the  oxidized  ores  at  the 
surface  and  again  deposit  them  as  sulphides  at  lower  levels;  de- 
posits formed  in  this  way  are  apt  to  become  poor  at  depths.  Galena 
may  be  deposited  by  ascending  solutions,  in  which  case  the  supply 
is  brought  nearer  the  surface  from  depths;  also  lead  sulphide  is 
volatile  without  decomposition  when  heated  in  an  atmosphere  of 
many  gases,  and  on  cooling  recrystallizes  as  cubes ;  galena  of  this 
nature  has  been  observed  in  the  lavas  of  Vesuvius. 

Galena  is  very  widely  distributed ;  of  the  24  states  commercially 
producing  lead  ore  in  1909,  Missouri,  Idaho,  Utah,  and  Colorado 
produced  more  than  three  quarters  of  the  350  thousand  tons  of 
that  year.  The  deposits  of  Missouri,  Southern  Illinois,  Wisconsin, 
and  the  Mississippi  valley  generally  are  found  in  limestone  and 
dolomite,  Those  of  Idaho  and  Colorado  are  associated  with 
igneous  rocks  as  well  as  dolomite ;  others,  associated  in  veins  with 
gold,  silver,  and  copper  ores,  are  of  a  complex  nature,  and  the 
usual  gangue  mineral  is  quartz. 

Artificial  galena  crystals  have  been  produced  by  the  volatiliza- 
tion of  precipitated  sulphide,  and  by  the  precipitation  of  a  solution 
of  lead  nitrate  containing  free  nitric  acid.  Octahedral  crystals  are 
formed  when  one  part  of  lead  sulphide  is  fused  with  six  parts  each 
of  potash  and  sulphur. 

CHALCOCITE 

Chalcocite.  —  Copper  glance ;  Cu2S;  Cuprous  sulphide;  Cu  = 
79.8,  S  =  20.2 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  a :  b  : 
c  =  . 5822:1:  .9701;  100  A  110  =  30°  12',  001  A  101  =  59°  2',  001  A 
011=44°  8';  Common  forms,  a  (100),  b  (010),  c  (001),  m(110); 
Twinning  plane,  110  and  032;  Cleavage,  m  distinct;  Brittle,  frac- 
ture conchoidal ;  H.  =  2.5-3 ;  G.  =  5.5-5.8 ;  Color  and  streak, 
dark  lead-gray,  tarnishes  to  blue  on  exposure;  Luster,  metallic; 
Opaque. 

B.B.  —  On  coal  fuses  easily  and  boils  with  spirting.  In  0.  F. 
yields  S02  fumes  and  odor.  Powdered  and  roasted  without  fusing, 
then  heated  in  R.  F.  yields  malleable  copper,  also  shows  copper 
with  the  fluxes. 


SULPHIDES,   ARSENIDES,   ANTIMONIDES  301 

General  description.  —  Crystals  are  tabular  parallel  to  the  base, 
combinations  of  the  base,  prism,  and  pinacoid  with  a  pyramid  and 
dome ;  as  the  prism  angle  is  119°  35',  these  combinations  are  pseudo- 
hexagonal  in  symmetry.  This  is  even  more  striking  when  twinned ; 
such  crystals  are  found  at  Bristol,  Connecticut,  and  in  Cornwall, 
England.  The  base  is  often  striated  parallel  to  the  edge  c/d. 
Massive,  granular,  or  disseminated  chalcocite  is  more  common  than 
the  crystalline.  Specimens  are  often  coated  with  a  black  crust  of 
melaconite  or  of  the  green  and  blue  carbonates,  and  less  often  with 
the  blue  sulphide,  covellite ;  all  of  which  are  alteration  products  of 
chalcocite. 

Stromeyerite  (CuAg)2S,  as  it  is  variable  in  composition,  is  prob- 
ably a  mixture  of  acanthite,  Ag2S,  the  orthorhombic  sulphide  of 
silver,  and  chalcocite,  which  in  many  localities  contains  silver. 

Chalcocite  is  a  mineral  of  secondary  origin  associated  with  other 
copper  ores,  with  arsenopyrite,  tetrahedrite,  sphalerite,  pyrite,  and 
galena,  in  veins,  joints,  lenses,  etc.,  in  which  the  gangue  mineral  is 
principally  quartz.  At  Butte,  Montana,  probably  the  largest  cop- 
per camp  of  the  world,  the  sulphide  ore  is  50  per  cent,  chalcocite. 
It  is  also  an  important  mineral  at  Bingham  Canon,  Utah,  asso- 
ciated with  pyrite  and  chalcopyrite,  both  of  which  are  enriched  with 
chalcocite.  Chalcocite  occurs  more  or  less  abundantly  in  all  cop- 
per deposits,  in  the  zone  of  secondary  enrichment,  where  it  has 
been  precipitated  from  the  descending  solutions,  by  contact  with 
pyrite,  sphalerite,  galena,  or  sulphides  of  arsenic,  etc. 

Artificial  chalcocite  may  be  produced  by  heating  a  solution  of 
cuprous  sulphide  in  a  sealed  tube  with  a  solution  of  ammonium 
sulphocyanate. 

SPHALERITE 

Sphalerite.  —  Zinc  blende ;  black  jack,  ZnS,  zinc  sulphide;  Zn  = 
67,  S  =  33  ;  Isometric ;  Type,  Ditesseral  Polar ;  Common  forms, 
±  o  (111),  d  (110),  a  (100);  Twinning  plane,  111;  Cleavage,  dodeca- 
hedral,  perfect ;  Brittle,  fracture  conchoidal ;  H.  =  3.5-4 ;  G.  = 
3.9-4.1 ;  Color,  shades  of  yellow,  brown  to  black,  rarely  red, 
green,  or  white;  Streak,  pale  to  colorless;  Luster,  adamantine; 
Transparent  to  opaque ;  n.  =  2.369 ;  Pyroelectric  and  polar  in  the 
direction  of  the  trigonal  axes. 

B.B.  —  Infusible  or  nearly  so ;  powdered  and  reduced  on  coal 
with  soda  yields  a  white  zinc  oxide  coat,  which  may  be  more  or 


302  MINERALOGY 

less  yellow  when  cold,  from  the  presence  of  cadmium ;  this  coat 
moistened  with  cobalt  solution  and  again  heated  in  the  O.  F.  be- 
comes green ;  the  soda  fusion  yields  a  sulphur  reaction  on  silver. 
Soluble  in  hot  HC1,  evolving  sulphuretted  hydrogen. 

General  description.  —  Crystals  are  usually  combinations  of  the 
plus  and  minus  tetrahedrons  with  the  cube  or  the  dodecahedron, 
less  often  with  the  trigonal  trisoctahedron  (311).  They  are  often 


FIG.  399.  —  Sphalerite  and  Calcite.     Joplin,  Missouri. 

rounded  on  the  tetrahedral  edges  and  striated  parallel  to  the  inter- 
section of  the  two  tetrahedrons ;  on  the  cube  face  these  striations 
are  diagonal,  showing  its  hemihedral  symmetry. 

Twins  after  the  spinel  law  are  common  and  may  be  developed 
polysynthetically.  Granular,  compact,  fibrous,  or  foliated  varie- 
ties are  common.  Pure  sphalerite  is  white  and  occurs  at  Frank- 
lin, New  Jersey,  and  at  Nordmarken,  Sweden.  It  is  usually  colored 
with  iron  or  manganese,  even  though  these  metals  are  present  only 
in  very  small  quantities.  Cadmium  sulphide,  being  isomorphous 
with  zinc  sulphide,  is  usually  present,  in  some  localities  as  high  as 
5  per  cent.  The  very  dark  specimens  contain  small  quantities  of 
indium,  gallium,  or  thallium ;  gallium  was  first  discovered  by  Lecoq 
de  Boisbaudran  in  1875  in  a  specimen  of  blende  from  Pierrefitte, 
Pyrenees.  Indium  was  discovered  in  1863  by  Reich  and  Richter 


SULPHIDES,  ARSENIDES,  ANTIMONIDES  303 

in  a  blende  from  Freiberg,  Saxony.  Specimens  containing  these 
rare  elements  are  very  dark  in  color,  as  that  from  Roxbury,  Con- 
necticut. Sphalerite  is  found  under  the  same  conditions  and  in  the 
same  formations  as  galena,  which  is  its  constant  companion,  with 
the  exception  that  zinc  sulphide  is  more  soluble  than  lead  sulphide 
and  in  many  cases  in  the  oxidized  zones  the  zinc  has  been  carried 
away  in  solution,  leaving  the  galena.  The  oxidation  products  of 
sphalerite  are  calamine  and  smithsonite,  which  are  ores  of  the  su- 
perficial areas  of  zinc  deposits.  In  weathering  these  may  be  redis- 
solved  and  carried  down  by  the  percolating  waters.  This  is  well 
substantiated  by  the  analyses  of  the  mine  waters  at  Freiberg, 
where  it  was  estimated  that  the  discharge  in  the  valley  carried  479 
kilograms  of  zinc  per  day,  or  175,024  kilograms  per  year.  In 
undisturbed  deposits  such  solutions  of  zinc  are  reprecipitated  either 
by  pyrite,  marcasite,  or  organic  matter  as  a  sulphide,  or  by  replace- 
ment in  limestones  as  a  carbonate.  When  sphalerite  is  formed, 
these  reactions  take  place  at  low  temperatures,  as  wurtzite,  the 
hexagonal  zinc  sulphide,  is  the  stable  form  at  high  temperature. 
Simple  crystals  of  sphalerite  occur  in  the  dolomites  of  the  Bin- 
nenthal,  and  beautiful  specimens  are  obtained  at  Santander, 
Spain. 

In  the  United  States  sphalerite  is  widely  distributed  in  the  lime- 
stones of  the  Mississippi  valley,  and  the  regions  of  contact  of  lime- 
stones with  igneous  rocks ;  all  deposits  of  sphalerite  mined  for  the 
zinc  alone  are  of  these  characters.  Southern  Missouri  and  the 
Leadville  district,  Colorado,  both  blende  deposits,  produced  175 
thousand  of  the  280  thousand  tons  of  ore  mined  in  1909.  Sphaler- 
ite also  occurs,  but  of  minor  importance,  in  the  metalliferous  veins 
of  rocks  of  all  ages. 

Zinc  is  used  in  innumerable  ways ;  as  a  metal  it  is  a  component 
of  brass,  white  metal,  and  german  silver.  It  is  used  in  roofing  in 
galvanizing  iron  to  prevent  rusting,  and  in  the  zinc  boxes  as  a  reduc- 
ing agent  to  precipitate  gold  from  the  solutions  in  the  cyanide  pro- 
cess; as  an  oxide  in  paint,  and  at  the  present  time  is  to  a  large 
extent  displacing  white  lead.  Artificial  sphalerite  is  formed  when 
a  solution  of  zinc  is  heated  in  a  sealed  tube  with  hydrogen  sulphide, 
at  higher  temperatures.  In  fusions,  as  of  precipitated  zinc  sul- 
phide with  potassium  carbonate  and  sulphur,  or  with  calcium  fluo- 
ride and  barium  sulphate,  the  hexagonal  form  of  ZnS,  wurtzite,  is 
formed.  Sphalerite  when  heated  to  bright  redness  is  changed  to 
wurtzite. 


304  MINERALOGY 

ALABANDITE 

Alabandite.  —  MnS;  Manganese  sulphide;  Mn  =  63.1,  S  = 
36.9 ;  Isometric ;  Type,  Ditesseral  Polar ;  Common  forms,  a  (100), 
d  (110),  ±  o  (111) ;  Twinning  plane,  111 ;  Cleavage,  cubic,  perfect; 
H.  =  3.5-4 ;  G.  =  3.95-4.04 ;  Brittle,  fracture  uneven ;  Color, 
iron-black;  Streak,  green;  Luster,  dull  submetallic. 

B.B.  —  On  coal  in  0.  F.  yields  sulphur  dioxide  odor;  the  black 
oxide  remaining  reacts  for  manganese  with  the  fluxes.  Soluble 
in  dilute  hot  HC1,  yielding  hydrogen  sulphide. 

General  description.  —  Crystals  are  combinations  of  the  cube 
and  the  rhombic  dodecahedron,  often  repeatedly  twinned  after  the 
spinel  law ;  more  often  massive  or  granular.  On  exposure  weathers 
to  a  brown  color. 

*  Alabandite  is  not  a  common  sulphide,  and  for  that  reason  it  is 
commercially  unimportant.  In  the  United  States  it  occurs  in  the 
Snake  River  region,  Colorado,  where  it  is  associated  in  veins  with 
argentite,  pyrite,  galena,  and  rhodochrosite,  also  at  Tombstone, 
Arizona,  in  large  but  rough  twinned  cubes. 

CINNABAR 

Cinnabar.  —  HgS ;  Mercuric  Sulphide;  Hg  =  86.2,  S  =  13.8; 
Hexagonal;  Type,  Trigonal  Holoaxial;  c  =  1.1453;  0001  A  1011  = 
52°5_4/,  rvr'  =  87°23';  Common  forms,  c  (0001),  m  (1010), 
r(1011);  Twinning  axis  c,  interpenetrating;  Cleavage,  m  perfect; 
H.  =  2-2.5 ;  G.  =  8-8.2 ;  Slightly  sectile,  fracture  conchoidal ; 
Color,  cochineal-red  to  dark  red ;  Streak,  scarlet ;  Luster,  adaman- 
tine; Transparent  to  opaque;  <o  =  2.854,  €  =  3.201  €  -  <o  = 
.447;  Optically  (+);  Circular  polarization. 

B.B.  —  On  coal  in  0.  F.,  when  pure,  volatilizes  entirely,  yielding 
a  sulphur  dioxide  odor  .and  possibly  a  gray  coat.  Heated  in  the 
closed  tube  with  dry  soda  yields  a  sublimate  of  metallic  mercury. 

General  description.  —  A  very  heavy  massive  reddish  brown 
mineral,  at  times  almost  black  from  organic  matter,  or  earthy  and 
disseminated.  Crystals  are  not  common,  but  occur  in  rhombohe- 
dral  habit,  in  short  thick  crystals,  combinations  of  the  unit  prism 
terminated  by  a  rhombohedron,  or  by  the  base,  when  the  habit  is 
tabular.  The  most  general  form  of  the  type,  the  trigonal  trapezo- 
hedron,  is  very  rare,  but  several  have  been  described  on  crystals 


SULPHIDES,   ARSENIDES,   ANTIMONIDES 


305 


from  Mount  Avala,  Servia.  Beautiful  examples  of  penetrating 
rhombohedral  twins  often  repeated  are  obtained  in  the  province  of 
Kweichow,  China.  Twin- 
ning lamellae  parallel  to 
the  base  show  Airy's 
spirals  as  in  quartz.  The 
rotary  polarization  is  very 
strong,  some  fifteen  times 
that  of  quartz  in  sections 
of  the  same  thickness, 
while  the  index  of  refrac- 
tion is  the  highest  of  min- 
erals. 

Mercuric     sulphide    is 

dimorphous;  the  red  cin-       FIG.  400.  —  Cinnabar  Twins.    Hu-Nan,  China. 

nabar  forms  at  a  tempera- 
ture above  45°,  while  the  black  isometric,  tetrahedral  form,  meta- 
cinnabar,    forms    below  45°  C. ;    the  latter  form  was  formerly 
plentiful  at  the  Reddington  mine,  Lake  County,  California. 

Cinnabar  is  associated  more  abundantly  with  sedimentary  de- 
posits, shales,  slates,  and  organic  matter  than  with  quartz  schists 
or  porphyries. 

The  concentrations  of  cinnabar,  forming  impregnations  in  lime- 
stones and  sandstones,  or  filling  fissures,  cracks,  and  cavities  in 
sedimentary  formations  near  regions  of  igneous  action,  owe  their 
origin  in  most  cases  to  ascending  hot  alkaline  solutions  carrying 
mercury  sulphide,  as  in  case  of  the  Steamboat  Springs,  Nevada ; 
from  such  solutions  the  cinnabar  is  either  precipitated  by  dilution 
of  the  solution,  by  a  reduction  of  pressure,  or  by  contact  with  or- 
ganic matter.  Pyrite,  marcasite,  sulphur,  barite,  gypsum,  cal- 
cite  and  quartz  are  the  usual  associates. 

Cinnabar  is  the  principal  ore  of  mercury ;  the  largest  producing 
deposits  are  Idria,  Austria,  and  Almaden,  Spain.  In  the  United 
States  cinnabar  is  mined  in  California  at  New  Idria  and  New  Alma- 
den;  in  southern  Nevada,  and  at  Terlingua,  Texas. 

The  artificial  product  is  used  as  vermilion  paint  and  is  manu- 
factured on  a  large  scale  by  distilling  in  iron  retorts  a  mixture  of 
8  parts  sulphur  and  42  parts  mercury.  In  the  wet  way  cinnabar 
may  be  produced  by  dissolving  a  mixture  of  mercury  and  sulphur 
in  a  caustic  potash  solution  and  heating  above  45°  C.,  as  below  this 
temperature  the  black  phase  will  form. 


306  MINERALOGY 

COVELL1TE 

Covellite.  —  CuS ;  Cupric  sulphide;  Cu  =  66.4,  S  =  (33.6;  Hex- 
agonal; Type,  Trigonal  Holoaxial,  or  Monoclinic ;  _c  =  1.1466, 
0001  A  1011  =  52°  56' ;  Common  forms,  c  (0001),  r  (1011),  m  (1010), 
x(1122);  Cleavage,  basal  perfect,  thin  lamellae,  flexible;  H.  = 
1.5-2;  G.  =  4.59-4.64;  Color,  deep  indigo-blue ;  Streak,  gray  to 
black ;  Opaque. 

B.B.  —  On  coal  in  O.  F.  fuses  to  a  globule  and  yields  S02  fumes. 
Roasted  and  reduced  with  soda  yields  malleable  copper. 

General  description.  —  Usually  in  cleavable  masses  or  in  dis- 
seminated scales,  rarely  in  crystals ;  less  common  in  occurrence  than 
the  cuprous  sulphide,  chalcocite,  of  which  it  is  at  times  an  alteration 
product. 

Covellite  is  a  sulphide  of  secondary  origin,  probably  produced  by 
the  interaction  of  chalcocite  and  copper  sulphide  in  solution,  result- 
ing in  covellite  and  metallic  copper.  It  is  associated  with  other 
copper  minerals,  as  at  Butte,  Montana;  at  the  Copper  Queen 
mine,  Bisbee,  Arizona;  and  at  the  Rambler  mine,  Wyoming. 
While  its  percentage  of  copper  is  high,  owing  to  its  rare  occurrence 
in  quantity  it  is  only  a  minor  ore  of  copper. 

GREENOCKITE 

Greenockite.  —  CdS ;  Cadmium  sulphide ;  Cd  =  77.7,  S  A  =  22.3  ; 
Hexagonal;  Type,  Dihexagonal  Polar;  c  =J).8109;  0001  A  1011  = 
43°  7';  Common  forms,  c  (0010),  M  (1010),  p(1011);  H.  = 
3-3.5;  G.  =  4.9-5.0;  Cleavage,  prismatic  distinct,  c  less  so; 
Brittle,  fracture  conchoidal;  Color,  shades  of  yellow;  Streak, 
orange-yellow  to  red;  Luster,  adamantine  to  resinous;  Trans- 
parent to  translucent ;  co  =  2.688;  Optically  (+). 

B.B.  —  On  coal  yields  a  sulphur  dioxide  odor  and  a  reddish 
brown  coat  of  cadmium  oxide.  Reduced  with  soda  yields  malle- 
able buttons  of  cadmium.  Soluble  in  hot  HC1,  yielding  hydrogen 
sulphide. 

General  description.  —  Crystals  of  greenockite  are  interesting 
from  their  polar  development,  being  combinations  of  the  lower 
base  with  the  hexagonal  prism  of  the  second  order,  terminated 
above  with  a  series  of  hexagonal  pyramids  of  the  second  order  and  a 


SULPHIDES,   ARSENIDES,  ANTIMONIDES  307 

small  basal  plane.  It  is  isomorphous  with  wurtzite,  the  fibrous 
zinc  sulphide ;  used  in  the  spintharoscope  to  demonstrate  the  radia- 
tions of  radium. 

It  occurs  in  the  Joplin  region  of  Missouri  as  a  yellow  coating  on 
crystals  of  calcite  and  sphalerite.  The  yellow  smithsonite  of 
Arkansas,  or  "  turkey-fat  ore,"  owes  its  color  to  an  admixture  of 
greenockite ;  it  also  occurs  in  small  quantities  with  zinc  minerals 
at  Friedensville,  Pennsylvania,  and  as  a  furnace  product. 

Artificial  crystals  of  greenockite  have  been  obtained  by  fusing 
cadmium  sulphide,  sulphur,  and  potassium  carbonate,  or  from  a  hot 
solution  of  cadmium  sulphide  in  ammonium  sulphydrate,  which 
deposits  crystals  of  greenockite  on  cooling. 

MILLERITE 

Millerite.  —  Capillary  pyrite  ;  Hair  sulphide ;  NiS,  sulphide 
of  nickel ;  Ni  =  64.6,  S  =  35.3 ;  Hexagonal ;  Type,  Ditrig- 
onal  Polar  (?),  c  =  .9883  ;  0001  A  1121  =  48°  46';  H.  =  3-3.5; 
G.  =  5.3-5.65 ;  Color,  bronze-yellow ;  Streak,  greenish  black  ; 
Luster,  metallic. 

B.B.  —  On  coal  in  the  0.  F.  blackens,  fuses  to  a  globule,  and 
yields  a  sulphur  dioxide  odor.  Well  roasted,  the  black  oxide 
reacts  for  nickel  with  the  fluxes ;  it  may  contain  iron,  cobalt,  or 
copper,  all  of  which  will  interfere  with  the  bead  reactions. 

General  description.  —  In  very  slender  elongated  crystals  or 
hairlike  tufts,  often  interwoven  and  matted,  as  at  Antwerp,  New 
York,  where  it  occurs  in  cavities  of  hematite.  It  often  forms 
botryoidal  crusts  with  a  radiated  structure,  as  at  the  Gap  mine, 
Lancaster  County,  Pennsylvania;  here  it  is  associated  with  a 
granular  pyrrhotite.  Very  fine  tufts  of  the  hairlike  variety  are 
found  in  cavities  of  calcite  and  dolomite  near  St.  Louis,  Missouri. 

Nickel,  as  is  shown  by  the  analyses  of  rocks  made  by  the  United 
States  Geological  Survey,  is  a  very  common  constituent  of  igneous 
rocks,  though  as  a  very  minor  accessory  mineral,  amounting  to  .027 
per  cent.  It  is  to  the  concentration  of  this  trace  of  nickel  that  the 
occurrence  of  millerite  in  veins  and  cavities  is  due. 

Beryrichite,  Ni3S4,  and  polydymite,  Ni4S5  are  rare,  though  they 
may  be  formed  under  nearly  the  same  conditions  as  millerite. 

Millerite,  from  its  restricted  occurrence,  though  rich  in  nickel,  is 
of  minor  importance  as  a  nickel  ore. 


308  MINERALOGY 

PYRRHOTITE 

Pyrrhotite.  —  FeS,  FenSn+i ;  Magnetic  sulphide  of  iron ;  Fe  = 
61.6,  S  =  38.4;  Hexagonal;  Type,  Ditrigonal  or  Dihexagonal 
Polar  (?)  c  =0.7402;  0001  A  1120  =  45°  7';  Common  forms, 
c(0001),  m(lOlO),  s(1011);  Twinning  plane  s;  Cleavage,  c  dis- 
tinct, a  less  so;  Brittle,  fracture  uneven;  H.  =3.5-4.5;  G.  = 
4.58-4.64 ;  Color,  bronze-yellow ;  Streak,  grayish  black ;  Luster, 
metallic;  Magnetic. 

B.B.  —  In  O.  F.  on  coal  yields  sulphur  dioxide  and  in  R.  F.  black- 
ens and  becomes  strongly  magnetic.  Yields  an  iron  reaction  with 
the  fluxes;  some  may  show  cobalt,  nickel,  or  copper.  Dissolves  in 
hot  HC1,  yielding  hydrogen  sulphide. 

General  description.  —  Usually  massive  with  a  distinct  part- 
ing, granular,  or  disseminated.  Crystals  are  rare,  generally  small 
six-sided  tablets,  combinations  of  the  base  and  a  prism ;  the  prism 
faces  are  horizontally  striated.  Other  forms  have  been  described 
as  a  series  of  pyramids,  but  are  unusual  in  occurrence.  Artificial 
crystals  show  a  polar  development.  Pyrrhotite  darkens  on  expo- 
sure, often  iridescent,  easily  oxidizing  to  hematite  or  limonite. 

Chemical  analyses  show  a  great  variation  in  the  proportions  of 
iron  and  sulphur,  there  being  more  sulphur  than  is  required  for  the 
formula  FeS  by  an  amount  equal  to  an  atom  of  sulphur  in  excess ; 
from  this  it  has  been  suggested  that  the  series  may  be  represented 
by  the  formula  FenSn+i,  commonly  FenSi2,  though  analyses  show 
a  variation  from  Fe6Se  to  Fei6Si7  in  the  natural  mineral.  Both 
nickel  and  cobalt  may  form  an  analogous  series  of  sulphides,  as  in 
each  case  minerals  related  in  the  same  manner  have  been  described, 
while  pyrrhotite  contains  small  quantities  of  these  metals,  and  in 
fact  the  larger  portion  of  the  nickel  of  commerce  is  derived  from 
pyrrhotite.  When  the  percentage  of  nickel  is  above  6  per  cent.,  the 
mineral  is  known  as  pentlandite  (FeNi)S. 

Troilite,  FeS,  is  the  massive  ferrous  sulphide  occurring  in  me- 
teorites. Pyrrhotite  may  occur  as  a  magmatic  segregation,  as  is 
the  case  of  the  Norway  deposits,  or  as  a  minor  accessory,  connected 
with  gabbro,  diorite,  basalts,  and  norites,  or  with  other  ferromagne- 
sian  rocks.  Such  magmas  dissolve  sulphides  which  on  solidifica- 
tion separate  very  early,  a  process  somewhat  analogous  to  the 
formation  of  a  matte  in  the  smelting  furnace.  The  large  deposits 
at  Ducktown,  Tennessee,  and  at  Sudbury,  Ontario,  are  impregna- 


SULPHIDES,  ARSENIDES,  ANTIMONIDES  309 

tions,  though  it  may  be  said  there  is  great  doubt  of  this  in  case  of  the 
Sudbury  deposit.  In  these  localities  pyrrhotite  is  associated  with 
pyrite,  chalcopyrite,  and  other  minerals,  which  are  the  results  of  oxi- 
dation. Pyrrhotite  may  also  result  from  pneumatolytic  action,  as 
by  the  interaction  of  hydrogen  sulphide  and  ferric  chloride. 

In  addition  to  the  localities  mentioned  above,  pyrrhotite  occurs 
at  Standish,  Maine,  in  crystals ;  in  Essex  County,  New  York ;  at 
the  Gap  mine,  Pennsylvania;  in  North  Carolina  and  Virginia; 
large  tabular  crystals  are  obtained  from  Minas  Geraes,  Brazil. 

Artificially  pyrrhotite  may  be  formed  by  the  reaction  of  H2S 
on  iron  at  a  high  temperature  and  under  pressure ;  in  the  wet  way 
by  heating  a  solution  of  FeCl3,  Na2C03,  and  H2S,  for  a  considerable 
time  at  200°  C.  in  a  closed  tube  with  all  air  excluded,  otherwise 
pyrite  will  form ;  also  by  the  action  of  H2S  on  slightly  acid  solu- 
tions of  ferrous  salts  containing  some  ferric  salts  between  the 
temperatures  of  80°  and  225°. 

NICCOLITE 

Niccolite.  —  Copper  nickel;  NiAs,  Arsenide  of  nickel;  Ni  = 
43.9,  As  =  56.1;  0001 A  lOll  =  43°  25'.  Hexagonal;  Type, 
Hexagonal  Polar  (?)  c  =  0.8194 ;  Common  forms,  c  (0001), m  (1011)  ; 
Cleavage,  doubtful ;  Brittle,  fracture  uneven ;  H.  =  5-5.5 ;  G.  = 
7.33-7.67 ;  Color,  pale  copper-red ;  Streak,  brownish  black ; 
Luster,  metallic. 

B.B.  —  Fuses  easily  on  coal  to  a  brittle  globule  and  yields  an 
arsenical  odor,  with  fumes  of  As2O3  and  possibly  a  white  coat  of 
oxide.  Well  roasted  and  treated  with  borax  on  coal  yields  a  per- 
sistent nickel  reaction.  Soluble  in  hot  nitric  acid. 

General  description.  —  Crystals  very  rare,  usually  massive  or 
disseminated ;  when  fresh,  the  color  is  very  characteristic  but 
tarnishes  to  black.  Isomorphous  with  CoAs,  NiSb,  and  FeAs,  all 
of  which  it  may  contain  in  small  quantities,  and  these  metals  will 
show  in  the  first  portions  of  borax  when  testing  on  coal  for  the  bead 
colorations. 

Breithauptite,  NiSb,  is  the  very  similar  antimony  compound 
associated  with  niccolite  at  the  mines  at  Freiberg,  Saxony;  An- 
dreasberg  in  the  Harz ;  and  at  Cobalt,  Ontario.  At  the  latter 
locality  there  is  a  remarkable  deposit  of  cobalt,  nickel,  and  silver 
ores,  filling  veins,  fissures,  and  the  joints  in  a  metamorphosed  rock 


310  MINERALOGY 

and  in  diabase,  with  a  gangue  of  calcite.  The  associated  minerals 
are  niccolite,  chloanthite,  millerite,  cobaltite,  smaltite,  argentite, 
dyscrasite,  pyrargyrite,  arsenopyrite,  with  other  oxidized  ores  and 
native  silver  in  the  superficial  areas.  These  minerals  have  either 
been  concentrated  from  the  near-by  diabase  or  carried  up  from 
depths  by  warm  solutions.  In  addition  to  the  localities  mentioned, 
niccolite  occurs  at  Chatham,  Connecticut ;  Franklin,  New  Jersey  ; 
Silver  Cliff,  Colorado;  and  Tilt  Cove,  Newfoundland. 

BORNITE 

Bornite.  —  Purple  copper  ore ;  Horseflesh  ore;  CusFeSs;  Cu  = 
55.5,  Fe  =  16.4,  S  =  28.1;  Isometric;  Type,  Ditesseral  Central; 
Common  forms,  o  (111),  d  (110),  n  (211) ;  Twinning  plane,  111; 
Cleavage,  octahedral  in  traces,  slightly  sectile;  Fracture,  con- 
choidal ;  H.  =  3  ;  G.  =  4.9-5.4 ;  Color,  copper-red  to  brown,  irides- 
cent from  tarnish ;  Streak,  grayish  black ;  Luster,  metallic,  opaque. 

B.B.  —  Fuses  easily  on  coal  in  the  R.  F.  to  a  magnetic  globule. 
In  the  O.  F.  yields  a  sulphur  dioxide  odor;  well  roasted  and  reduced 
with  soda  yields  malleable  copper  buttons.  Soluble  in  hot  nitric 
acid. 

General  description.  —  Crystals  are  very  rare,  cubic  or  octahe- 
dral in  habit,  but  are  rough  and  irregular ;  the  usual  occurrence  is 
massive,  granular,  or  disseminated. 

Bornite  occurs  as  a  primary  magmatic  constituent  of  igneous 
rocks,  but  more  often  in  veins  and  fissures  of  contact  zones,  as  well 
as  impregnations  and  replacements,  associated  with  other  metallic 
sulphides.  It  oxidizes  easily  in  the  superficial  areas,  forming  car- 
bonates, etc.,  or  is  carried  down  in  solution  as  sulphates. 

Bornite  next  to  chalcopyrite  and  chalcocite  is  the  most  important 
copper  ore ;  it  forms  the  greater  bulk  of  the  ore  in  the  Copper 
River  region,  Alaska ;  also  in  quantity  at  Butte,  Montana,  and  in 
nearly  all  copper  mining  districts. 

CHALCOPYRITE 

Chalcopyrite.  —  Copper    pyrite,    CuFeS;    sulphide    of    copper 
and  iron ;    Cu  =  34.5,  Fe  =  30.5,  S  =  35.0 ;    Tetragonal ;    Type, 
Ditetragonal  Alternating ;   c  =  .9852;   Common  forms,  ±P(111) 
c(001),  m(110)  r(332),  z  (201) ;  Twinning  plane,  111  and  110; 
Cleavage,  at  times  distinct ;  Brittle,  fracture  uneven ;  H.  =  3.5-4 ; 


SULPHIDES,  ARSENIDES,  ANTIMONIDES 


311 


G.  =  4.1-4.3;    Color,  brass-yellow,  often  tarnished  and  iridescent; 
Streak,  greenish  black ;  Luster,  metallic. 

B.B.  —  In  R.  F.  on  coal  fuses  to  a  magnetic  globule  and  in  O.  F. 
yields  an  odor  of  sulphur  dioxide.  Well  roasted  and  reduced  with 
soda,  yields  malleable  copper  buttons.  Soluble  in  hot  nitric  acid. 

General  description.  —  Crystals  are  not  uncommon  as  simple 
sphenoids  or  combinations  of  the  sphenoid,  base,  and  prism.  The 
axial  ratio  being  so  near  unity  these  sphenoids  have  the  appearance 
of  tetrahedrons  ;  such  simple  forms  are  found  in  the  Joplin  region, 


FIG.  401.  —  Chalcopyrite  and  Sphalerite  on  Dolomite.    Aurora,  Missouri. 

Missouri,  on  dolomite  associated  with  sphalerite  and  galena. 
Other  sphenoids  both  obtuse  and  acute  have  been  described,  and  the 
appearance  of  the  crystal  will  depend  upon  which  is  the  predominat- 
ing form ;  the  tetragonal  trapezohedron  co  (576)  occurs  on  crystals 
from  French  Creek,  Pennsylvania.  Twins  in  which  the  twinning 
axis  is  P,  both  contact  and  penetrating,  occur;  other  twins  are 
rare. 

Chalcopyrite  is  present  in  all  copper  mines,  especially  in  the  zone 
below  oxidation,  and  is  very  widely  distributed  in  other  mines; 


312  MINERALOGY 

it  is  the  most  important  of  the  copper  ores.     Its  copper  content, 
however,  varies  greatly  from  an  admixture  of  pyrite. 

Some  deposits  have  arisen  from  magmatic  segregation,  and  then 
it  is  associated  with  other  sulphides,  as  pyrite,  bornite,  or  pyrrho- 
tite ;  as  a  primary  mineral  it  is  disseminated  through  the  ferro- 
magnesian  igneous  rocks,  especially  those  containing  pyroxene 
or  hypersthene.  It  is  these  primary  sulphides  which  furnish  the 
copper  of  the  secondary  deposits,  in  veins,  etc.  Of  the  products 
of  oxidation,  derived  from  chalcopyrite,  the  sulphates  of  iron  and 
copper  are  very  soluble  and  may  be  transported  in  solution  to  be 
reprecipitated  again,  either  close  at  hand  or  at  a  distance  according 
to  local  conditions,  as  chalcopyrite  or  in  some  other  form.  These 
solutions  may  flow  down  along  the  veins  from  which  the  sulphates 
originated,  to  be  precipitated  at  lower  levels  by  hydrogen  sulphide, 
pyrite,  or  other  sulphides,  yielding  the  more  valuable  ores  of  the  area 
of  secondary  enrichment.  In  this  way  pyrite  used  in  the  Copper 
Queen  mine,  in  the  early  days,  to  fill  stopes  is  being  mined  at  the 
present  time,  it  having  accumulated  nearly  12  per  cent  copper. 
Ghalcopyrite  is  present  in  nearly  all  ore  veins  contained  in  meta- 
morphic  or  sedimentary  rocks  and  limestones ;  also  reported  in  fur- 
nace slags,  indicating  its  formation  by  dry  fusion  at  ordinary 
pressures,  analogous  to  its  artificial  formation  .in  a  simple  fusion  of 
pyrite  and  copper  sulphide. 


STANNITE 

Stannite.  —  Cu2FeSnS4 ;  Tin  pyrite ;  Cu  =  29.5,  Sn  =  27.5 ; 
Fe  =  13.1,  S  =  29.9 ;  Tetragonal ;  Type,  Ditetragonal  Alternating ; 
c  =  .986 ;  Crystals  sphenoidal,  very  rare ;  Cleavage,  indistinct ; 
Brittle,  fracture  uneven ;  H.  =  4 ;  G.  =  4.3-4.52 ;  Color,  steel- 
gray  to  iron-black ;  Streak,  black ;  Luster,  metallic. 

B.B.  —  On  coal  fuses  and  yields  a  white  coat  (SnO2)  and  a  sul- 
phur dioxide  odor.  Roasted  and  reduced  with  soda,  yields  malle- 
able buttons  which  react  with  the  fluxes  for  copper.  Soluble  in 
hot  nitric  acid,  yielding  a  blue  solution  and  a  white  residue  of  tin 
oxide  and  sulphur. 

General  description.  —  Crystals  have  been  described  from 
Bolivia ;  its  usual  occurrence  is  massive,  granular,  or  disseminated, 
variable  in  composition  owing  to  the  admixture  of  other  sulphides  as 
chalcopyrite.  Stannite  is  associated  with  cassiterite  in  the  Corn- 


SULPHIDES,   ARSENIDES,  ANTIMONIDES  313 

wall  mines,  in  Bolivia,  Zinnwald,  and  the  Black  Hills,  South 
Dakota.  It  is  not  a  common  mineral,  and,  while  it  contains  con- 
siderable tin  and  copper,  owing  to  its  restricted  occurrence  it  is  of 
minor  importance  as  an  ore. 

PYRITE 

Pyrite.  —  FeS2 ;  Bisulphide  of  iron;  Fe  =  46.6,  S  =  53.4; 
Isometric;  Type,  Tesseral  Central;  Common  forms,  a  (100),  o 
(111),  e  (120),  s  (312) ;  Twinning  supplementary  and  interpene- 
trating ;  Cleavage,  a  and  o  distinct ;  Brittle,  fracture  conchoidal 
to  uneven;  H.  =  6-6.5;  G.  =  4.5-5.1 ;  Color,  light  brass  yellow; 
Streak,  greenish  to  brownish  black;  Luster,  metallic,  often 
brilliant. 

B.B.  — On  coal  in  0.  F.,  yields  a  sulphur  dioxide  odor,  darkens, 
and  in  R.  F.  becomes  magnetic.  With  the  fluxes  reacts  for  iron. 
In  the  closed  tube  yields  a  sublimate  of  sulphur.  -Soluble  in  hot 
nitric  acid. 

General  description.  —  Crystals  are  very  common  of  cubic, 
octahedral,  or  pentagonal  dodecahedral  habit,  or  combinations  of 
these  three  forms.  Numerous  other  forms,  including  all  seven  forms 
of  the  type,  have  been  described,  as  on  crystals  from  Gilpin  County, 
Colorado.  The  cube  and  pyritohedral  faces  are  usually  striated 
parallel  to  their  intersection ;  on  the  cube  the  striations  are  parallel 
to  the  opposite  sides  and  at  right  angles  to  the  striations  on  adja- 
cent faces,  indicating  the  hemihedral  symmetry  of  the  pyrite  cube. 
Penetrating  supplementary  twins  of  the  plus  and  minus  pyritohe- 
drons  are  obtained  at  Minden,  Prussia ;  complex  stellate  and 
parallel  growths  are  common  in  clays,  slates,  and  argillites.  A 
peculiar  elongated  octahedral  form,  possibly  a  tetragonal  phase, 
occurs  at  French  Creek,  Pennsylvania.  Beautiful  crystals  are 
obtained  from  the  Isle  of  Elba,  from  Peru,  and  from  French  Creek, 
Pennsylvania.  Pyrite  also  occurs  massive,  granular,  or  in  nodules. 

Chemically  pyrite  may  contain  copper,  nickel,  and  cobalt  sul- 
phides as  mixtures,  as  well  as  arsenic  and  metallic  gold.  It  is  the 
most  widely  distributed  of  all  the  sulphides,  occurring  under  the 
most  varied  conditions;  as  primary  constituent  of  igneous  rocks 
and  secondary  impregnations,  in  sedimentary  rocks,  clays,  and  coal 
formations,  as  well  as  filling  veins,  fissures,  and  joints,  where  it  is 
associated  with  other  sulphides,  carbonates,  sulphates,  and  oxides. 
By  oxidation  it  yields  sulphates,  acid  sulphites,  and  sulphuric  acid, 


314  MINERALOGY 

all  of  which  are  soluble  in  water  and  when  in  solution  react  both  as 
solvents  and  reducing  agents,  attacking  other  minerals.  The  ulti- 
mate result  or  products  of  oxidation  are  the  hydroxides  of  iron, 
limonite  and  gothite,  and  the  oxide  hematite,  all  of  which  often 


FIG.  402.  —  Pyrite  Crystals. 

occur  as  pseudomorphs  after  pyrite.  Secondary  pyrite  is  formed 
from  the  sulphates  of  iron  carried  in  the  circulating  ground  waters ; 
the  organic  matter  contained  in  shales,  coal,  and  fossils  reduces 
these  solutions  and  the  iron  is  precipitated  as  pyrite. 

Pyrite  is  mined  commercially  at  Davis,  in  the  Berkshire  Hills, 
Massachusetts,  where  it  is  found  in  a  crystalline  schist;  in  St. 
Lawrence  County,  New  York ;  these  deposits  are  associated  with 
limestone  and  schist.  In  Eastern  Virginia  large  lenses  of  pyrite 
occur  associated  with  chalcopyrite,  sphalerite,  galena,  and  pyrrho- 
tite ;  these  deposits  yield  one  half  of  the  quarter  of  a  million  tons 
produced  in  the  United  States  annually.  It  is  also  mined  in  smaller 
quantities  in  Georgia,  Alabama,  and  California.  All  the  pyrite 
mined  is  used  in  the  production  of  sulphuric  acid.  Pyrite  may  be 


SULPHIDES,   ARSENIDES,   ANTIMONIDES  315 

formed  artificially  by  gently  heating  FeS  with  sulphur,  or  by  pass- 
ing H2S  over  oxides  and  chlorides  of  iron,  heated  to  redness ;  also 
by  heating  the  mixture  of  ferric  oxide,  sulphur,  and  ammonium 
chloride  slowly  to  a  temperature  above  which  the  latter  volatilizes, 
when  cubes  and  octahedrons  of  pyrite  will  form.  Pyrite  is  also 
formed  by  the  action  of  H2S  on  ferric  sulphate,  as  in  marcasite, 
but  from  neutral  or  slightly  acid  solutions. 

SMALTITE 

Smaltite.  —  Diarsenide  of  cobalt,  CoAs2 ;  Co  =  28.2,  As  =  71.8 ; 
Isometric;  Type.  Tesseral  Central;  Common  forms,  a  (100), 
o  (111),  d  (110),  e  (210) ;  Twinning  plane,  111,  composition  face, 
211 ;  Cleavage,  111  distinct,  a  in  traces;  Brittle,  fracture  uneven; 
H  =  5.5-6 ;  G.  =  6.4-6.6 ;  Color,  tin-white ;  Streak,  gray-black  ; 
Luster,  metallic. 

B.B.  —  On  coal  easily  fusible,  yielding  a  magnetic  globule  and  an 
arsenic  odor,  may  yield  a  white  coat  of  As2O3.  With  borax  shows 
persistent  cobalt  reactions.  Soluble  in  hot  nitric  acid. 

General  Description.  —  Habit  like  pyrite,  but  well-developed 
crystals  are  rare ;  more  often  massive  with  the  surface  darkened 
from  tarnish.  It  may  contain  both  iron  and  nickel,  as  the  corre- 
sponding diarsenide  of  nickel,  chloanthite,  is  isomorphous  with 
cobaltite ;  the  two  species  grade  into  each  other.  Safflorite  is  an 
orthorhombic  form  of  CoAs2  occurring  at  Tunaberg,  Sweden. 

Skutterudite,  CoAs3,  is  a  triarsenide  from  Modun,  Norway,  also 
isometric.  Smaltite  occurs  in  veins  associated  with  other  ores  of 
cobalt,  nickel,  iron,  and  silver ;  the  most  noted  locality  of  America 
is  at  Cobalt,  Ontario.  The  historic  European  localities  are  Frei- 
berg, Saxony ;  Joachimsthal,  Bohemia ;  and  Tunaberg,  in  Sweden. 

CHLOANTHITE 

Chloanthite.  —  Diarsenide  of  nickel;  NiAs2;  Ni  =  28.1,  As  = 
71.9;  Common  forms,  c  (100),  o  (111),  d  (110),  e  (210) ;  Twinning 
plane,  111;  Cleavage,  111  distinct,  a  in  traces;  Brittle,  fracture 
uneven ;  H.  =  5.5-6 ;  G.  =  6.4-6.6 ;  Color,  tin-white ;  Streak, 
gray-black ;  Luster,  metallic. 

B.B.  —  Fuses  easily  on  coal  to  a  globule  and  yields  an  arsenic 
odor,  with  possibly  a  white  coat  of  As2O3.  Roasted  and  treated 
with  borax  yields  a  persistent  nickel  reaction.  It  may  contain 
considerable  cobalt. 


316  MINERALOGY 

General  Description.  —  Crystalline  habit,  occurrence,  and  asso- 
ciation like  smaltite,  often  coated  with  the  green  nickel  arsenate, 
annabergite,  Ni2(As04)3,  an  oxidation  product. 

Rammelsbergite  is  an  orthorhombic  form  of  NiAs2  occurring  very 
sparingly  at  Schneeberg,  Saxony. 

COBALTITE 

Cobaltite.  —  Sulpharsenite  of  cobalt,  CoAsS ;  Co  =  35.5, 
As  =  45.2,  S  =  19.3;  Isometric;  Type, Tesseral  Central ;  Common 
forms,  c  (100),  o  (111),  e  (120) ;  Cleavage,  cubic ;  Brittle,  fracture 
uneven ;  H.  =  5.5-6 ;  G.  =  6-6.3 ;  Color,  reddish  to  silver- white ; 
Streak,  grayish  black ;  Luster,  metallic. 

B.B.  —  Fuses  easily  on  coal  to  a  magnetic  globule  and  yields  an 
arsenic  odor  and  sulphur  dioxide  fumes.  Well  roasted  and  treated 
with  borax  yields  a  cobalt  reaction.  Soluble  in  hot  nitric  acid, 
which  after  diluting  yields  a  white  precipitate  with  BaCl2  (BaSOJ . 

General  description.  —  Crystals  are  cubes  and  pyritohedrons,  or 
combinations  of  these  two  forms.  In  habit  very  much  like  pyrite, 
with  similar  striations  on  the  crystal  faces ;  crystals  are  much  more 
common  than  those  of  the  two  preceding  species ;  often  massive, 
granular,  and  disseminated.  Occurs  under  similar  conditions  and 
associations  as  smaltite. 

Gersdorffite,  sulpharsenide  of  nickel,  NiAsS,  is  similar  in 
habit,  occurrence,  and  association  to  cobaltite,  with  which  it  is 
isomorphous. 

ULLMANNITE 

•  Ullmannite.  —  Sulph-antimonite  of  nickel,  NiSbS ;  Ni  =  27.8, 
Sb  =  57.0,  S  =  15.2 ;  Isometric ;  Type,  Tesseral  Polar ;  Forms, 
a  (100),  ±  e  (120),  d  (110),  ±  o  (111) ;  Twins,  interpenetrating  tetra- 
hedrons ;  Cleavage,  cubic  perfect ;  Brittle,  fracture  uneven ;  H.  = 
5-5.5 ;  G.  =  6.2-6.7 ;  Color,  steel-gray  to  silver-white ;  Streak, 
grayish  black;  Luster,  metallic. 

B.B.  —  On  coal  in  R.  F.  fuses  easily  to  a  globule,  boils,  and  yields 
an  antimony  coat.  After  roasting  yields  a  nickel  reaction  with  the 
fluxes.  Decomposed  with  hot  nitric  acid  with  the  separation  of 
sulphur  and  white  oxide  of  antimony.  It  may  contain  some  ar- 
senic. 


SULPHIDES,   ARSENIDES,  ANTIMONIDES  317 

General  Description.  —  Crystals  from  Sarrabus",  Sardinia,  are 
combinations  of  the  cube,  rhombic  dodecahedron,  and  the  pyri- 
tohedron ;  while  crystals  from  Lolling  in  Carinthia  are  tetrahedral 
inhabit,  combinations  of  the  tetragonal  and  trigonal  tristetrahe- 
drons  with  the  plus  and  minus  tetrahedrons  and  the  rhombic  do- 
decahedron. Chemically  the  two  are  alike  and  therefore  must  be 
tetartohedral  in  symmetry,  as  in  that  type  only  can  all  these  forms 
occur. 

Ullmannite  is  a  rare  mineral  and  its  principal  interest  is  as  a  repre- 
sentative of  tetartohedral  symmetry  in  the  isometric  system. 

MARCASITE 

Marcasite.  —  Bisulphide  of  iron,  FeS2 ;  Fe  =_46.6,  S  =  53.4 ; 
Orthorhombic  ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c  =  .7662  :  1 : 
1.2342;  100  A  110  =  37°  27',  001  A  101  =  58°  10',  001 A  Oil  =  50° 
59',  111  A  111  =  66°  7';  Common  forms,  c  (001),  1(011),  e  (101), 
m  (110) ;  Twinning  planes,  110  and  101 ;  Cleavage,  m  distinct,  1 
less  so ;  Brittle,  fracture  uneven ;  H.  =  6-6.5 ;  G.  =  4.85-4.9 ; 
Color,  light  bronze  yellow ;  Streak,  gray ;  Luster  metallic. 

B.B.  —  Fuses  easily  on  coal  and  blackens,  yielding  a  sulphur 
dioxide  odor  in  O.  F.  In  R.  F.  becomes  magnetic  and  reacts  for 
iron  with  the  fluxes. 

General  Description.  —  Crystals  when  simple  are  tabular  paral- 
lel to  the  base ;  it  is  more  often  twinned,  with  the  prism  face  the 
composition  plane,  which  twinning  is  often  repeated  to  fivelings  with 
a  pentagonal  outline  and  concentric  striations;  this  method  of 
twinning  forms  the  spearhead  pyrites  of  the  Dover  Cliffs,  England. 
Twins  with  the  macrodome  as  the  composition  face  are  not  so  com- 
mon ;  also,  radiated,  encrusted,  stalactitic,  and  reniform. 

Marcasite  is  dimorphous  with  pyrite.  It  is  always  a  secondary 
mineral  found  in  sedimentary  rocks,  or  associated  with  coal  beds 
and  organic  matter ;  how  its  molecule  differs  from  that  of  pyrite 

has  not  been  determined ;  marcasite  has  been  represented  as  a 

o a 

ferrous  sulphide,  Fe<^o <o Fe,  and  pyrite  as  a  ferriferrous  sul- 

/s-    -s\ 

phide,  Fe\— S  —  Fe  —  S— -/Fe.    This  conclusion  has  been  reached 

\S-      -S/ 
by  a  consideration  of  the  amount  of   iron  yielded  by  their  de- 


318  MINERALOGY 

composition,  in  "the  ferrous  and  ferric  states.  Marcasite  oxidizes 
more  readily  than  pyrite,  yielding  ferrous  sulphate  and  sul- 
phuric acid;  the  ferrous  sulphate  appears  as  a  white  efflores- 
cence, and  the  specimen  swells,  cracks,  and  falls  to  pieces.  Marca- 
site is  the  less  stable  form  and  passes  into  pyrite.  Their 
ultimate  oxidation  products  are  the  same.  At  450°  marcasite 
passes  over  to  pyrite  with  an  evolution  of  heat ;  this  probably 
explains  the  absence  of  marcasite  as  a  primary  constituent  of 
igneous  rocks. 

Marcasite  has  been  formed  artificially  by  the  slow  action  of 
H2S  on  ferric  sulphate  or  chloride  at  several  temperatures  up  to 
300°.  Acidity  favors  the  formation  of  marcasite,  for  when  the 
solution  is  neutral,  pyrite  is  formed.  The  amount  of  pyrite  mixed 
with  marcasite  formed  under  these  conditions  decreases  with  the 
acidity.  The  favorable  conditions  for  the  formation  of  nearly 
pure  marcasite  is  100°  and  1.18  per  cent,  of  free  H2SO4. 

ARSENOPYRITE 

Arsenopyrite. — Mispickel,  FeSAs;  Sulpharsenide  of  iron; 
Fe  =  34.3,  As  =  46,  S  =  19.7 ;  Orthorhombic ;  Type,  Didigonal 
Equatorial ;  a  :  b  :  c  =  .6773  :  1 : 1.1882 ;  100  A  110  =  34°  6',  001  A 
101  =  60°  19',  001A011  =  49°  54';  Common  forms,  m  (110)  A 
u  (014),  e  (101)  q  (Oil) ;  Twinning  plane,  110,  also  101,  both  contact 
and  interpenetrating ;  Cleavage,  m  distinct,  c  faint  traces ;  Brittle, 
fracture  uneven ;  H.  =  5.5-6  ;  G.  =  5.9-6.2 ;  Color,  silver-white 
to  steel-gray ;  Streak,  grayish  black ;  Luster,  metallic. 

B.B.  —  On  coal  fuses  easily  to  a  brittle  globule  and  yields  an 
arsenic  odor  and  sulphur  dioxide  fumes ;  roasted  and  treated  in  the 
borax  bead  yields  an  iron  reaction ;  in  the  closed  tube  yields  first 
an  arsenious  sulphide  sublimate  and  then  a  metallic  mirror  of 
arsenic. 

General  description.  —  Crystals  are  prismatic  combinations  of 
the  unit  prism  with  the  unit  domes,  the  prism  zone  striated ;  at 
times  the  prism  is  very  short  when  the  crystal  is  pyramidal  in  ap- 
pearance. Twinning  is  of  two  classes ,  those  in  which  the  composition 
face  is  m,  often  repeated,  yielding  pseudo-hexagonal  forms,  as  in 
marcasite,  or  the  dome  e  is  the  composition  face ;  as  the  angle  e  A  e' 
is  120°  38',  the  individuals  when  repeated  form  a  six-armed  star. 
Arsenopyrite  is  very  often  massive,  granular,  or  compact;  some 


SULPHIDES,   ARSENIDES,   ANTIMONIDES 


319 


iron  is  usually  replaced  by  cobalt  as  the*  corresponding  sulphar- 
senide  of  cobalt,  glaucodot  (CoFe)SAs,  is  an  isomorphous  com- 
pound. 

Arsenopyrite  occurs  as  a  constituent  of  pegmatites,  in  the  deep- 
seated  and  intermediate  veins,  especially  those  in  metamorphic 
schists  and  serpentine,  where  it  is  associated  with  chalcopyrite, 
pyrite,  sphalerite,  and  the  arsenides  of  cobalt  and  nickel,  as  well  as 
with  gold  or  silver  minerals.  By  oxidation  it  yields  arsenates  which 


FIG.  403.  —  Arsenopyrite  from  Freiberg,  Saxony. 

\ 

form  a  series  of  minerals  characteristic  of  the  zone  of  oxidation ;  or, 
being  soluble  in  this  form,  the  arsenic  may  be  carried  away  in  solu- 
tion; some  mine  waters  and  indeed  the  waters  of  some  natural 
springs  contain  arsenic  in  such  quantities  as  to  be  highly  poisonous. 
Complex  crystals  occur  at  Franconia,  New  Hampshire;  simple 
combinations  at  Tavistock,  Devonshire.  Arsenopyrite  is  mined 
commercially,  for  the  arsenic  it  contains,  at  several  places  in  Vir- 
ginia and  in  the  state  of  Washington ;  but  as  it  is  associated  with 
gold,  silver,  and  copper  ores,  most  of  the  commercial  arsenic  is 
recovered  from  the  flue  dust  of  smelters  using  such  ores. 


CHAPTER   VI 

SULPHO    COMPOUNDS 

ZINKENITE 

Zinkenite.  —  Sulphantimonide  of  lead,  PbSb2S4 ;  Pb  =  35.9, 
Sb  =  41.8,  S  =  22.3;  Orthorhombic ;  Type,  Didigonal  Equato- 
rial ;  &  :  b  :  c  =  .5575  :  1 :  .6353 ;  100 ,110  =  29°  8',  001  A  101  = 
48°  44',  001 A  Oil  =32°  25';  Crystal  forms,  e  (102),  k  (061) ; 
Cleavage,  none ;  Brittle,  fracture  uneven ;  H.  =  3-3.5 ;  G.  = 
5.3-5.35;  Color  and  streak,  steel-gray ;  Luster,  metallic. 

B.B.  —  Fuses  easily  in  R.  F.,  yielding  an  antimony  coat,  and  with 
von  Kobell's  flux  shows  lead ;  also  yields  a  sulphur  reaction  when 
fused  with  soda.  Dissolves  in  hot  HC1,  yielding  H2S  and  white  lead 
chloride.  Crystals  are  columnar  with  faces  indistinct  and  striated 
lengthwise,  or  fibrous,  also  massive. 

General  description.  —  Zinkenite  is  a  member  of  a  series  of  min- 
erals which  may  be  represented  by  the  general  formula  R"S, 
IV'Sa,  in  which  R"  may  be  lead,  copper,  or  iron  and  R'"  may  be 
antimony,  arsenic,  or  bismuth.  They  are  all  very  similar  in  their 
physical  properties  and  crystalline  habit,  and  as  they  are  isomor- 
phous,  grade  into  each  other. 

Sartorite,  PbAs2S4,  and  galenobismuthite,  PbBi2S4,  are  the  lead 
members.  Emplectite,  Cu2Bi2S4,  and  chalcostibite,  CuSb2S4,  are  the 
copper  members ;  and  berthierite,  FeSb2S4,  is  the  iron  member  of 
the  series.  Berthierite  is  the  most  common  of  the  group.  On 
coal  it  fuses  easily,  yielding  an  antimony  coat,  sulphur  dioxide 
odor,  and  a  magnetic  residue.  It  occurs  fibrous,  plumose,  or  mas- 
sive with  cleavage. 

JAMESONITE 

Jamesonite.  —  Pb2Sb2S6 ;  Pb  =  50.8,  Sb  =  29.5,  S  =  19  .  7  ; 
Orthorhombic ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c  =  .819  : 1 
:  ? ;  Cleavage,  basal  perfect  ;  Brittle,  fracture  uneven  ;  H.  = 
2-3;  G.  =  5.5-6;  Color,  steel-gray;  Streak,  grayish  black; 
Luster,  metallic. 

B.B.  —  Like  zinkenite. 

320 


SULPHO  COMPOUNDS  321 

General  description.  —  Jamesonite  is  a  representative  of  a  series 
of  isomorphous  lead  and  silver  minerals ;  they  are  generally  not  well 
crystallized,  and  resemble  each  other  so  closely  that  in  many  cases 
they  are  to  be  distinguished  only  by  chemical  analysis.  They  are 
not  common  in  occurrence,  and  are  associated  with  other  sulphides 
and  arsenides  in  ore  deposits. 

BOURNONITE 

Bournonite.  —  (Cu2 .  Pb)3Sb2S6 ;  Cu  =  13,  Pb  =  42.5,  Sb  = 
24.7,  S  =  19.8 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ; 
a  :  b  :  c  =  .9379  :  1 :  .8968  ;  100  A  110  =  43°  10',  001  A  101  = 
43°  43',  001  A  Oil  =41°  53';  Forms,  a  (100),  b  (010),  c  (001), 
n  (Oil),  o  (101),  m  (110) ;  Twinning-plane,  m ;  Cleavage  b  distinct, 
a  and  c  less  so ;  H.  =  2.5-3 ;  G.  =  5.7-5.9 ;  Brittle,  fracture 
uneven;  Color  and  streak  gray  to  iron-black;  Luster,  metallic. 

B.B.  —  Fuses  easily  on  coal,  yielding  a  white  coat  of  antimony 
trioxide;  on  continued  heating  yields  a  yellow  coat  (lead).  The 
residue  reduced  with  soda  yields  copper  buttons  or  reacts  for  copper 
with  the  fluxes. 

General  description.  —  Crystals  tabular  parallel  to  the  base, 
with  the  short  prism  faces  striated  parallel  to  the  vertical  axis. 
Often  twinned  with  the  prism  as  the  composition  face,  when  repeated 
forming  the  characteristic  cogwheel-shaped  aggregates,  as  at  Kop- 
nik,  Hungary,  where  it  is  known  as  wheel  ore.  The  best  crystals 
are  from  the  Harz,  Germany.  Also  massive,  granular,  or  compact. 

Occurs  in  the  United  States  in  Yavapai  County,  Arizona ;  Mont- 
gomery County,  Arkansas ;  and  in  several  localities  in  Colorado. 
It  is  an  important  ore  of  lead  and  copper,  but  only  locally. 

BOULANGERITE 

Boulangerite.  —  Pb3Sb2S6;  Pb  =  58.9,  Sb  =_22.8,  S  =  18.3  ; 
Orthorhombic ;  Type,  Didigonal  Equatorial ;  a :  b  :  c  =  .5527  :  1 : 
.7478  ;  Crystal  forms,  rare  ;  Brittle,  fracture  uneven ;  H.  = 
2.5-3;  G.  =  5.75-6;  Color,  bluish  lead-gray;  Streak,  black; 
Luster,  metallic,  dull. 

>     B.B.  —  The  same  as  zinkenite. 
i 

General    description.  —  Crystals    very    rare,    usually    fibrous, 

plumose,  granular,  or  compact.     Often  with  yellow  spots  of  oxide 
of  antimony  on  the  surface. 


322  MINERALOGY 

Occurs  at  Pribram,  Bohemia ;  in  the  Harz ;  and  in  the  Echo 
district,  Nevada.  It  is  an  ore  of  lead,  but  too  rare  to  be  of  much 
importance. 

PYRARGYRITE 

Pyrargyrite.  —  Dark  ruby  silver ;  Sulphantimonide  of  silver, 
Ag2Sb2S3 ;  Ag  =  59.9,  Sb  =  22.3,  S  =  17.8 ;  Hexagonal ;  Type, 
Ditrigonal  Polar ;  c  =  0.7891 ;  0001  A  lOTl  =  42°  20^,  e  A  e'  =_42° 
5',  r_Ar'  =  71°_22',  VAV'  =  74°  25';  Forms,  aai20),  mJlOlO), 
e  (0112),  r  (1011),  v_(2131) ;  Twinning  plane,  1120  also  1014  quite 
common,  1011  or  1012  rare ;  Cleavage,  r  distinct,  e  less  so ;  Brittle, 
fracture  conchoidal ;  H.  =  2.5 ;  G.  =  5.77-5.86 ;  Color,  grayish 
black  to  black ;  Streak,  purplish  red ;  Luster,  adamantine ; 
Nearly  opaque,  deep  red  in  thin  splinters;  <o  =  3.084,  e  =  2.881, 
o>-€  =  .203;  Optically  (-). 

B.B.  —  Fuses  easily  on  coal  to  a  globule  and  yields  a  white 
coat  of  antimony  trioxide.  In  R.  F.  or  with  soda  yields  a  malle- 
able button  of  silver.  Soluble  in  nitric  acid  with  the  separation  of 
sulphur  and  insoluble  oxide  of  antimony.  In  the  closed  tube 
yields  a  red  sublimate. 

General  description.  —  Crystals  usually  quite  complicated  both 
through  twinning  and  the  complexity  of  the  forms.  A  large  num- 
ber of  forms  have  been  described,  but  as  formerly  pyrargyrite  and 
proustite  were  considered  one  and  the  same  species,  their  individual 
forms  have  not  as  yet  been  entirely  separated.  Doubly  terminated 
crystals  are  not  common,  but  when  they  do  occur  they  are  generally 
supplementary  twins  with  the  basal  plane  as  the  composition  face, 
in  which  case  the  two  terminations  are  similar.  The  polarity  of 
its  symmetry  is  shown  by  the  striations  on  the  prism  faces,  which 
are  not  symmetrical  to  the  basal  plane.  Also  occurs  compact, 
granular,  or  disseminated. 

Pyrargyrite  occurs  in  veins  associated  with  other  silver  minerals, 
as  argentite,  proustite,  or  native  silver,  with  sulphides  and  arsen- 
ides ;  calcite,  barite,  fluorite,  or  quartz  are  the  gangue  minerals. 
In  such  cases  it  has  been  produced  by  the  interaction  of  antimonides 
on  silver,  in  solution,  either  by  precipitation  or  replacement  at 
comparatively  low  temperatures,  which  also  its  synthesis  in  the 
laboratory  would  indicate.  Often  alters  to  argentite  and  forms 
pseudomorphs  after  native  silver. 


SULPHO   COMPOUNDS 


323 


Pyrargyrite  is  an  important  ore  of  silver  occurring  in  most  silver 
mines,  as  at  Andreasberg  and  Freiberg,  Saxony;  at  Pribram, 
Bohemia;  Kongsberg,  Norway;  at  various  localities  in  Chili  and 
Mexico.  In  the  United  States  in  the  Ruby  district,  Colorado, 
associated  with  tetrahedrite ;  in  the  Poorman's  Lode,  Idaho,  in 
large  masses ;  also  in  Arizona  and  New  Mexico.  Artificially  formed 
by  precipitating  a  solution  of  silver  with  potassium  sulph  antimo- 
nate ;  this  precipitate  is  amorphous,  but  if  mixed  with  sodium  car- 
bonate and  heated  in  a  sealed  tube  above  80°  the  product  becomes 
crystalline. 

PROUSTITE 

Proustite.  —  Light  ruby  silver ;  AgaAsSs ;  Ag  =  65.4,  As  = 
15.2,  S  =^19.4;  Hexagonal;  Type,  Ditrigonal  Polar;  c  =  .8039, 
0001  A  lOll  =  42°  51',  eA  e'  =  42°  46',  v^v'  =  74°  _39  ,  rArj  = 
72°  12';  Forms,  a  (1120),  e(0112),  _r  (1011),  m  (1010),  (2131); 
Twinning  plane,  u  (1014)  and  r  (1011)  common,  e  and  c  rare; 
Cleavage,  r  distinct;  Brittle,  fracture  conchoidal;  H.  =  2-2.5; 
G.  =  5.57-5.64 ;  Color  and  streak,  scarlet ;  Transparent  to  trans- 
lucent, becoming  black 
on  exposure ;  <o  =  3.087, 
€  =  2.792,  <o-€  =0.295; 
Optically  (-). 

B.B.  —  On  coal  in 
R.  F.  fuses  easily  and 
yields  an  arsenic  odor 
and  sulphur  fumes,  may 
yield  a  white  coat  of  ar- 
senic trioxide.  Reduced 
with  soda  yields  a  glob- 
ule of  malleable  silver. 
Soluble  in  nitric  acid 
with  the  separation  of 
sulphur. 

General  Description. 
—Crystals  elongated 

and  terminated  by  the  scalenohedron  v,  or  the  rhombohedron  e. 
Proustite  is  isomorphous  with  pyrargyrite  and  resembles  it  in 
every  respect  except  in  its  light  color  and  in  its  streak.  Fresh 
specimens  are  very  beautiful,  but  soon  lose  their  ruby  color  and 
become  dark  and  opaque  unless  protected  from  the  light. 


FIG.  404.  —  Proustite.     Freiberg,  Saxony. 


324 


MINERALOGY 


It  occurs  with  pyrargyrite  in  the  same  localities  and  under  the 
same  conditions. 

TETRAHEDRITE 

Tetrahedrite.  —  Cu3SbS3 ;  Sulphantimonide  of  copper ;  Cu  = 
46.8,  Sb  =  29.6,  S  =  23.6 ;  Isometric ;  Type,  Ditesseral  Polar ; 
Common  forms,  ±  o  (111),  d  (110),  n  (211) ;  Twinning  plane,  111 ; 
Cleavage,  none;  Brittle,  fracture  subconchoidal ;  H.  =  3-4.5; 
G.  =4.4-5.1;  Color,  lead-gray  to  iron-black;  Streak,  brown 
to  iron-black ;  Luster,  metallic ;  Opaque. 

Tennantite,  Cu3AsS3;  Sulpharsenide  of  copper.  The  isomor- 
phous  arsenide  is  similar  in  crystallization  and  appearance  and 
often  mixed  with  the  antimonide. 

B.B.  —  Fuses  easily  in  R.  F.  on  coal,  yielding  a  white  coat  of 
either  Sb203  or  As2O3  as  the  case  may  be ;  in  O.  F.  yields  a  sulphur 
dioxide  odor.  The  coat  may  contain  either  lead  or  zinc  oxide  if 
either  of  these  metals  is  present.  Well  roasted  and  reduced  with 
soda  yields  malleable  copper  buttons  which  may  be  quite  white 
in  color  from  the  presence  of  lead  or  silver.  When  mercury  is 
present,  the  powdered  mineral  when  heated  in  the  closed  tube  with 
dry  soda  will  yield  a  sublimate  of  small  globules  of  metallic  mercury. 
Decomposed  in  hot  nitric  acid  with  the  separation  of  sulphur  and 
oxide  of  antimony. 

General  description.  —  Crystals  are  well-developed  combina- 
tions of  the  plus  and  minus  tetrahedrons,  modified  by  the  trigonal 

and  tetragonal 
trisoctahedrons, 
usually  striated 
or  furrowed  par- 
allel to  the  tet- 
rahedral  edges ; 
such  crystals  are 
found  at  Kapnik, 
Hungary,  asso- 
ciated with  chal- 
copyrite,  pyrite, 
and  sphalerite. 
At  Clausthal, 
Harz,  the  simple 
trigonal  tristet- 
rahedron,  '  the 


FIG.  405.  —  Tetrahedrite  on  Dolomite.    Clausthal,  Harz. 


SULPHO  COMPOUNDS  325 

tetrahedron,  or  combinations  of  these  two  forms,  also  penetrating 
supplemental  tetrahedral  twins,  with  the  perpendicular  to  111  as 
the  twinning  axis,  occur,  in  veins  in  a  dolomite  associated  with 
chalcopyrite  crystals  and  quartz.  In  crystalline  habit  tennantite 
is  more  apt  to  be  rhombic  dodecahedral,  with  small  modifications 
of  the  tetrahedrons,  as  at  Redruth  in  Cornwall,  England.  Both 
occur  massive,  granular,  or  compact. 

Chemically  tetrahedrite  may  be  represented  by  the  formula 
R"3R'"S3,  in  which  R"  is  mostly  copper,  but  often  replaced  in  part 
by  silver,  lead,  mercury,  zinc,  or  iron,  and  R'"  represents  antimony, 
arsenic,  or  bismuth.  These  substitutions  lead  to  numerous  local 
variations  and  possibly  when  not  well  crystallized  to  confusion 
with  other  minerals  of  the  same  nature.  It  is  often  an  important 
silver  ore,  as  the  variety  freibergite,  while  schwartzite  is  the  mer- 
curial form. 

Tetrahedrite  occurs  as  a  vein  mineral  associated  with  sulphides, 
arsenides,  and  antimonides  in  the  silver  districts  of  Mexico,  in 
Colorado,  and  Nevada.  It  has  probably  been  formed  by  precipi- 
tation or  replacement  at  comparatively  low  temperature. 

Alteration  products  are  bournonite,  covellite,  malachite,  azurite, 
and  oxidized  antimony  or  arsenic  minerals.  Chalcopyrite  often 
forms  a  crust  enveloping  the  mother  crystal  of  tetrahedrite,  as  at 
Liskeard,  Cornwall.  Tetrahedrite  is  an  important'  ore  of  copper 
and  silver. 

STEPHANITE 

Stephanite.  —  Ag5SbS3;  Sulphantimonide  of  silver;  Ag  = 
68.5,  Sb  =  15.2,  S  =  16.3,  Orthorhombic ;  Type,  Didigonal  Polar ; 
a:b:c  =  .6291:  1 :  .6851,  100  A  110  =  32°  10',  001  A  101  =  47° 
26',  001  A  Oil  =  34°  25',  111  A  110  =  37°  51';  Common  forms, 
c  (001),  b  (010),  m  (110),  d  (Oil),  P  (111) ;  Twinning  plane  m  quite 
common,  others  as  a  and  b  less  so ;  Cleavage,  b  and  d  imperfect ; 
Brittle,  fracture  uneven ;  H.  =  2-2.5 ;  G.  =  6.2-6.5 ;  Color  and 
streak,  iron-black;  Luster,  metallic;  Opaque. 

B.B.  —  On  coal  fuses  easily  with  spirting,  yielding  a  white  coat 
of  antimony  trioxide,  which  may  become  red  with  oxidized  silver. 
In  O.  F.  yields  an  odor  of  sulphur  dioxide.  Reduced  with  soda  yields 
a  malleable  button  of  silver. 

General  Description.  —  Crystals  are  usually  tabular  parallel 
to  the  base,  or  short  stout  prisms,  at  times  elongated  parallel  to  the 
brachyaxis.  Twins  with  m  as  the  composition  face  are  often 


326 


MINERALOGY 


repeated,  yielding  a  pseudo-hexagonal  symmetry.  The  polar  nature 
of  the  crystals  is  shown  by  the  diagonal  striations  on  the  prism 

faces ;    also   granular   or  mas- 
sive. 

Fine  crystals  are  obtained  at 
Freiberg,  Saxony,  and  in  the 
Ophir  mine,  Nevada. 

It  is  an  important  ore  of  sil- 
ver and  is  found  at  all  the 
noted  silver  localities  of  Europe, 
as  in  the  Harz ;  Pribram,  Bo- 
hemia ;  Kremnitz,  Hungary ; 
Kongsberg,  Norway.  In  the 
United  States  it  is  the  princi- 
pal ore  of  silver  in  the  noted 
Comstock  lode  in  Nevada; 
also  occurs  at  other  points  in 
Nevada  'and  Idaho. 


FIG.  406.  —  Stephanite.    Freiberg, 
Saxony. 


Polybasite,  9Ag2S,Sb2S3,  and  Polyargyrite,  12  Ag2S,  Sb2S3, 
are  two  sulphantimonides  in  the  same  series  as  stephanite,  but 
higher  in  their  silver  content.  The  former  usually  contains  some 
copper  and  has  a  good  basal  cleavage.  The  latter  is  isometric, 
crystallizes  in  cubo-octahedrons  with  a  cubic  cleavage. 


ENARGITE 

Enargite.  —  3  Cu2S,  As2S5 ;  Sulpharsenate  of  copper ;  Cu  = 
48.3,  As  =  19.1,  S  =  32.6;  Orthorhombic ;  Type,  Didigonal  Equa- 
torial; a:b:c  =  .8711:1:.  8248;  100  A  110  =  41°  3' ;  001  A  101 
=  43°  26' ;  001  A  Oil  =  39°  31' ;  Forms,  a  (100),  b  (010),  c  (001), 
m  (110),  d  (Oil) ;  Twinning  plane,  x  (320) ;  Cleavage,  m  perfect,  a 
and  b  distinct ;  Brittle,  fracture  uneven ;  H.  =  3 ;  G.  =  4.4 ; 
Color  and  streak,  grayish  to  iron-black;  Luster,  metallic;  Opaque. 

B.B.  —  Fuses  easily  in  R.  F.  on  coal  and  yields  an  arsenic  odor. 
In  the  0.  F.  a  sulphur  dioxide  odor.  Roasted  and  reduced  with 
soda  yields  malleable  copper  buttons.  It  may  also  contain  anti- 
mony, zinc,  or  iron. 

General  Description.  —  Crystals  are  small,  prismatic,  and 
striated  lengthwise;  it  usually  occurs  massive  or  disseminated. 
Not  a  common  mineral,  but  at  Butte,  Montana,  it  is  an  important 
copper  ore.  Found  also  in  several  localities  in  Colorado,  Utah, 
and  in  the  Brewster  gold  mine,  South  Carolina.  • 


CHAPTER  VII 

THE  HALOID  COMPOUNDS 

HALITE 

Halite.  —  Common  salt ;  NaCl,  Sodium  chloride ;  Na  = 
39.4,  Cl  =  60.6 ;  Isometric ;  Type,  Ditesseral  Central ;  Common 
forms,  a  (100),  o(lll);  Cleavage,  cubic  perfect;  Twins  rare; 
Brittle,  fracture  conchoidal;  H.  =  2.5;  G.  =  2.1-2.6;  Color, 
white,  gray,  blue,  yellowish,  or  red ;  Streak,  white ;  Luster,  vitre- 
ous; Transparent  to  translucent;  n  =  1.542. 

B.B.  —  Fuses  at  two  (815°),  often  with  decrepitation,  and  colors 
the  flame  intense  yellow  (Na).  With  S.  Ph.  bead  saturated  with 
copper  oxide  yields  an  azure  blue  flame  (Cl).  Dissolves  in  water 
easily  and  has  a  salty  taste. 

General  Description.  —  Crystals  are  cubes,  while  other  forms 
in  combination  are  rare.  When  crystallized  from  solution  con- 
taining caustic  soda  it  separates  in  octahedrons,  or  in  combinations 
of  the  cube  and  octahedron,  according  to  the  alkalinity  of  the  solu- 
tion. Natural  crystals  showing  the  cube  and  octahedron  are  rare. 
Twins  are  not  known,  except  in  some  cases  thin  lamellae  have  been 
noted  which  have  probably  been  formed  by  pressure.  Pure  salt 
is  white ;  the  yellow  and  reddish  colors  are  caused  by  such  impuri- 
ties as  oxide  of  iron  or  clay.  Calcium  and  magnesium  sulphates 
and  chlorides  are  often  present,  and  these  chlorides  cause  the  deli- 
quescence of  many  specimens  of  rock  salt.  The  beautiful  blue  of 
some  specimens  from  Stassfurt  has  been  attributed  to  the  presence 
of  the  subchloride  of  sodium  or  to  small  amounts  of  metallic  sodium, 
as  the  blue  color  may  be  produced  by  exposing  crystals  to  the  vapors 
of  sodium,  also  to  the  cathode  ray,  or  to  the  radiations  of  radium. 

Both  halite  and  sylvite  are  diathermous  and  allow  the  non- 
luminous  heat  rays  to  pass  with  very  little  absorption. 

Salt  deposits  are  found  in  the  sediments  of  all  ages;  those  of 
New  York  are  Upper  Silurian,  while  those  of  Michigan  are  Carbon- 
iferous. They  have  been  formed  and  deposited  from  solution,  the 

327 


328  MINERALOGY 

mother  brines  having  become  saturated  by  the  evaporation  of  an 
enclosed  arm  of  the  sea.  In  some  cases  the  supply  of  salt  has  been 
added  to  by  a  periodical  breaking  in  of  the  sea,  or  by  the  waves  wash- 
ing over  the  inclosing  bar ;  through  these  additions  of  salt,  carried 
in  by  each  fresh  supply  of  water  which  in  turn  is  evaporated,  salt 
beds  of  enormous  thickness  have  been  made  possible.  Those  at 
Sperenberg,  Germany,  reach  a  thickness  of  4000  feet,  while  the 
average  thickness  of  the  New  York  beds  is  75  feet,  and  those  of 
Michigan  are  in  places  400  feet  thick.  Such  beds  of  salt  are  in 
course  of  formation  at  the  present  time,  under  practically  the  above 
conditions,  in  some  of  the  bays  of  the  eastern  shore  of  the  Caspian 
Sea.  Other  deposits  may  form  by  the  simple  concentration  of  an 
inland  sea,  as  the  Salton  Sea  deposits  of  California.  Both  the 
Great  Salt  Lake  and  the  Dead  Sea  are  highly  concentrated  brines. 
The  water  of  the  Dead  Sea  contains  22.86  per  cent,  salt,  while  the 
average  sea  water  contains  but  3.5  per  cent.  Such  lakes  are  noth- 
ing more  than  large  evaporating  dishes  which  deposit  their  salts 
in  more  or  less  definite  order  when  concentration  has  reached  satura- 
tion (see  anhydrite),  as  the  salt  deposits  of  Stassfurt,  Germany. 
When  such  deposits  of  salt  are  protected  from  the  solvent  action  of 
percolating  ground  waters  by  an  impervious  stratum  of  clay,  they 
have  been  preserved  through  the  ages ;  however,  many  salt  springs 
exist  which  derive  their  salt  from  such  embedded  deposits.  All 
natural  waters  contain  from  one  to  ten  parts  of  salt  in  a  million 
in  solution.  The  origin  of  the  sodium  contained  in  all  these 
natural  solutions  and  enormous  deposits  of  salt  have  in  large  part 
resulted  from  the  decomposition  of  feldspars;  but  the  source  of 
such  quantities  of  chlorine  is  more  difficult  of  explanation,  since 
few  primary  minerals,  as  soldalite  and  apatite,  contain  chlorine, 
and  they  only  in  small  amounts.  The  two  states,  Michigan  and 
New  York,  produce  three  quarters  of  the  thirty  million  barrels  of 
salt  used  annually  in  the  United  States,  while  fourteen  states  con- 
tribute to  the  remaining  quarter. 

SYLVITE 

Sylvite.  —  KC1 ;  Potassium  chloride ;  K  =  52.4,  Cl  =  47.6  ; 
Isometric;  Type,  Tesseral  Holoaxial;  Common  forms,  c  (100), 
o(lll);  Twins  rare;  Cleavage,  cubic  perfect;  Brittle,  fracture 
uneven ;  H.  =  2;  G.  =  1.97 ;  Color,  white,  yellow,  reddish,  or  blue ; 
Streak,  white ;  Luster,  vitreous ;  Transparent  to  translucent : 
n  =  1.490. 


THE  HALOID   COMPOUNDS  329 

B.B.  —  Fuses  at  two  (790°)  and  colors  the  flame  violet  (K).  If 
much  sodium  is  present,  the  blue  glass  should  be  used.  With 
S.  Ph.  bead  saturated  with  copper  oxide  yields  an  azure  blue  flame 
(Cl).  Dissolves  easily  in  water,  yielding  a  saline  taste. 

General  Description.  —  Crystals  are  usually  small  cubes  and  less 
often  in  combination  with  the  octahedron;  other  forms  are  rare. 
The  variation  in  color  of  specimens  is  due  to  impurities,  as  in  the 


FIG.  407.  —  Sylvite  from  Stassf urt,  Prussia. 

case  of  halite,  as  pure  potassium  chloride  is'  white.  Its  holoaxial 
symmetry  is  clearly  shown  by  the  asymmetrical  position  of  the 
square  etch  figures  in  relation  to  the  edges  and  diagonals  of  the 
cube  faces. 

Sylvite  is  more  soluble  than  halite,  and  is  therefore  deposited  at  a 
later  stage  in  the  concentration  of  complex  salt  solutions,  or,  in  the 
sequence  of  the  deposit,  sylvite  will  lie  above  the  stratum  of  halite 
and  will  be  mixed  with  the  more  soluble  magnesium  compounds, 
from  which  at  times  sylvite  may  be  formed  as  a  secondary  mineral. 

All  rock  salt  contains  potassium  chloride  in  small  amounts,  but 
the  most  noted  deposit  of  potassium  salts  is  that  of  Stassf  urt  in 
Prussia;  smaller  deposits  of  the  same  general  nature  are  also 
found  in  Austria,  It  is  from  these  European  localities  that  the 


330  MINERALOGY 

commercial  supply  of  the  potassium  salts  of  the  world  is  at  present 
obtained ;  for  this  reason  they  are  most  important,  as  potassium 
is  one  of  the  few  elements  required  by  all  soils  to  produce  good 
crops,  and  must  be  imported  by  all  nations,  as  a  fertilizer,  to  add 
to  the  soil  when  the  original  supply  of  potassium  has  been  exhausted. 
It  is  also  used  in  the  production  of  niter  (KNO3)  from  Chili 
saltpeter,  which  is  the  oxidizing  agent  in  gun  and  blasting  powder. 


CERARGYRITE 

Cerargyrite.  —  Horn  silver ;  Silver  chloride,  AgCl,  Ag  =  75.3,  Cl 
=  24.7;  Isometric;  Type,  Ditesseral  Central;  Forms,  a  (100), 
d(110),  o(lll);  Twinning  plane  o;  Cleavage,  none;  Highly 
sectile;  H.  =  1-1.5;  G.  =  5.55;  Color,  white  gray  to  brown; 
Darkens  on  exposure  to  light;  Luster,  resinous  to  adamantine; 
Transparent  to  opaque;  n  =  2.61. 

B.B.  —  On  coal  fuses  easily,  yielding  globules  of  metallic  silver. 
In  the  S.  Ph.  bead  saturated  with  copper  oxide  yields  an  azure 
blue  flame  (Cl).  Insoluble  in  acids,  but  soluble  in  ammonia. 

General  description.  —  Crystals  are  rare,  but  cubic  in  habit ; 
it  occurs  more  often  massive,  disseminated,  or  in  crusts  and 
dendritic. 

The  best  specimens  have  been  obtained  from  Peru,  though  good 
crystals  are  found  in  the  Poor  Man's  lode  in  Idaho.  Cerargyrite 
is  an  important  ore  of  silver  at  the  Comstock  lode,  Nevada; 
Leadville,  Colorado,  and  Cobalt,  Ontario.  It  is  common  in  veins, 
associated  with  galena,  native  silver,  barite,  calcite,  and  quartz, 
especially  in  the  superficial  portions,  where  it  is  formed  as  a  second- 
ary mineral.  Soluble  silver  sulphate  is  formed  by  the  oxidation 
of  sulphides,  arsenides,  or  antimonides,  and  is  then  precipitated  by 
contact  with  chlorides  or  by  the  intermixing  with  other  solutions 
containing  chlorides. 

Cerargyrite  has  been  produced  artificially  by  the  slow  diffusion, 
through  a  diaphragm  of  asbestos,  of  solutions  of  sulphate  and 
chloride  of  silver. 

Bromyrite,  AgBr,  iodyrite,  Agl,  and  embolite,  Ag(ClBr),  are  very 
similar  to  Cerargyrite,  both  in  appearance  and  association.  Iodyrite 
is  interesting  as  an  example  of  dihexagonal  polar  symmetry.  They 
are  all  valuable  ores  of  silver. 


THE  HALOID   COMPOUNDS  331 

FLUORITE 

Fluorite.  —  Calcium  fluoride,  CaF2;  Ca  =  51.1,  F  =  48.9; 
Isometric;  Type,  Ditesseral  Central;  Common  forms,  c(100), 
o  (111),  d  (110),  e  (210),  t  (421) ;  Twinning  plane  ill,  interpene- 
trating ;  Cleavage,  octahedral  perfect ;  Brittle,  fracture  conchoidal 
to  splintery ;  H.  =  4 ;  G.  =  3.01-3.25 ;  Color,  white,  various 
shades  of  yellow,  green,  blue,  brown,  and  red;  Streak,  white; 
Luster,  vitreous ;  Transparent  to  nearly  opaque ;  n  =  1.434. 

B.B.  —  Fuses  at  1387°C.  to  an  enamel,  yielding  a  reddish  flame 
(Ca).  The  ignited  fragment  reacts  alkaline  with  turmeric  paper. 
With  potassium  bisulphate  in  the  closed  tube  shows  fluorine. 

General  description.  —  Crystals  are  beautifully  developed  and 
very  common,  usually  cubic  in  habit :  simple, octahedrons  are  less 


FIG.  408.  —  Fluorite  and  Calcite  from  Weardale,  England. 

common,  but  occur  at  Chamonix  and  St.  Gothard,  Switzerland; 
at  Spitzenberg,  and  in  Colorado.  They  are  usually  pink  or  rose- 
colored,  with  drusy  faces,  caused  by  repeated  growths  of  small  cubes. 


MINERALOGY 


FIG.  409.  —  Fluorite  Twins,   showing   the 
.    Hexoctahedron.    Weardale,  England. 


Combinations  of  the  cube,  octahedron,  and  rhombic  dodecahedron 
occur  at  St.  Gothard,  Switzerland;  at  Redruth  in  Cornwall, 
England;  while  cubes  with  the  corners  modified  by  the  hex- 
octahedron  (421)  occur  at  Alston  Moor  and  Weardale,  England. 

Fluorite  presents  a  good  example  of  a  perfect  octahedral  cleavage  ; 
with  care,  perfect  octahedral  cleavage  pieces  may  be  split  out,  but 

as  there  are  four  cleavage  di- 
rections the  ultimate  particles 
are  tetrahedrons,  in  color, 
fluorite  is  the  most  variable 
of  minerals;  commonly  it  is 
white,  yellow,  purple,  or  green. 
The  green  specimens,  termed 
chlorophane,  and  occurring 
at  Trumbull,  Conn.,  yield 
phosphorescent  light  when 
heated.  The  phosphorescence 
is,  however,  not  confined  to 
the  green  varieties.  The 
various  colors  have  been  at- 
tributed to  organic  matter, 
as  many  specimens,  on  heating,  lose  their  color,  and  particularly 
if  heated  in  oil.  Transparent  white  specimens  are  very  valuable, 
as,  owing  to  the  low  index  of  refraction  of  fluorite,  it  is  used 
in  the  construction  of  physical  apparatus.  Some  colored  crystals 
exhibit  a  slight  birefringency  caused  by  laminae  parallel  to  the 
cube  or  octahedral  faces;  colorless  specimens  do  not  show  this 
phenomenon. 

Fluorite  occurs  more  often  as  a  vein  mineral  associated  with 
sulphides  or  sulphates,  as  galena,  sphalerite,  and  barite,  and  as  a 
gangue  in  many  metalliferous  deposits.  It  is  also  associated  with 
the  deposits  of  cassiterite,  for  the  formation  of  which,  as  a  pneu- 
matolytic  agent,  it  is  in  large  part  responsible.  Veins  containing 
fluorite  may  be  contained  in  both  acid  and  basic  igneous  rocks  as 
well  as  in  the  schists  and  sedimentary  formations ;  it  is  of  common 
occurrence  lining  the  cavities  of  limestones,  where  it  has  been  de- 
posited from  solution. 

Less  frequently  it  is  found  as  a  primary  mineral  in  granites, 
syenites,  and  quartz  porphyry ;  in  which  occurrences  the  escape  of 
the  volatile  fluorides  must  have  been  prevented  by  the  superim- 
posed formations,  as  determined  in  the  syenites  of  Norway.  Fluor- 


THE  HALOID  COMPOUNDS  333 

ite  has  also  been  produced  by  the  condensation  of  vapors  or  by  the 
interaction  of  volatile  fluorides  with  other  minerals,  as  in  the  lavas 
of  Vesuvius. 

Crystalline  specimens  of  great  beauty  are  obtained  in  Cornwall 
and  Cumberland,  England;  at  St.  Gothard,  Switzerland;  at  Amity 
and  Brewster,  and  in  St.  Lawrence  County,  New  York;  and  at 
Castle  Dome,  Arizona.  Fluorite  is  mined  in  large  quantities  along 
the  Ohio  River  in  Illinois  and  Kentucky,  where  there  are  large 
deposits,  filling  veins  in  limestone,  associated  with  galena,  sphal- 
erite, chalcopyrite,  and  pyrite.  They  were  probably  formed  by 
precipitation  or  replacement  of  the  calcium  carbonate.  Both  in 
Virginia  and  Colorado  fluorite  is  produced  commercially,  but  as  a 
by-product  resulting  from  the  mining  of  lead,  zinc,  or  silver  ores. 

In  the  United  States  60,000  tons  are  used  annually,  mostly  as  a 
flux  in  the  metallurgical  industries.  By  its  use  a  very  fluid  slag, 
melting  at  a  low  temperature,  is  formed,  at  the  same  time  lowering 
the  sulphur  and  phosphorus  content  of  the  resulting  iron  or  steel. 
It  is  used  in  minor  quantities  in  the  production  of  hydrofluoric 
acid,  in  enameling  works,  in  glazes  and  opalescent  glass.  The 
famous  Blue  John  mine  of  Derbyshire,  England,  produces  a  banded 
purple  spar  used  for  vases  and  other  ornaments. 

CRYOLITE 

Cryolite.  —  Sodium  aluminium  fluoride ;  Na3AlF6 ;  Na  = 
32.8,  Al  =  12.8,  F  =  54.4 ;  Monoclinic  ;  Type,  Digonal  Equatorial ; 
a:b:c  =  .9663:  1:1.3882;  p  =  89°  49'  =  001,100;  100  A 
110  =  44°  1';  001  A  101  =  55°  2';  001  A Oil  =  54°  14';  Com- 
mon forms,  a  (100),  c(001),  m(110),  d  (101),  r  (Oil) ;  Twining 
plane  010 ;  Cleavage,  c  quite  perfect,  m  less  so ;  Brittle,  fracture 
uneven ;  H.  =  2.5 ;  G.  =  2.95-3 ;  Color,  white,  reddish,  brown 
to  nearly  black;  Streak,  white;  Luster,  vitreous  to-  greasy, 
Transparent  to  opaque;  n  =  1.36;  Optically  (+).  Axial  plane 
perpendicular  to  010 ;  Bxa  A  normal  to  001  =  44°  5' ;  2  E  =  59° 
24'. 

B.B.  —  Fuses  easily  to  a  white  enamel,  which  becomes  blue  with 
cobalt  solution  (Al),  yields  a  yellow  flame,  and  the  ignited  frag- 
ment reacts  alkaline  with  turmeric  paper.  Fused  with  potassium 
bisulphate  in  the  closed  tube  shows  fluorine. 

General  description.  —  Crystals  are  rare  and  exist  only  in  the 
cracks  of  the  massive  mineral ;  as  its  name  indicates,  the  massive 


334 


MINERALOGY 


mineral  when  wet  resembles  ice  in  appearance.     Crystals  are 

cubelike  in  shape,  with  the  corners  and  edges  modified,  while  the 

massive  mineral  shows  striations  due  to  polysynthetic  twinning. 

There  is  but  one  large  deposit  of  cryolite  in  the  world,  that  of 


FIG.  410.  —  Cryolite  Crystals  from  Ivigtut,  Greenland. 

Ivigtut,  West  Greenland.  This  deposit  is  from  500  to  1000  feet 
thick,  occurring  as  a  residual  secretion  in  the  granite ;  associated 
and  contained  in  the  cryolite  are  siderite,  sphalerite,  chalcopyrite, 
galena,  and  other  rare  minerals,  such  as  columbite,  wolframite, 
molybdite,  and  cassiterite. 

At  Miask  in  the  Urals  and  at  the  base  of  Pike's  Peak  in  Colorado 
other  deposits  occur,  but  these  are  small  and  of  no  commercial  im- 
portance. The  product  of  the  Greenland  mine,  some  six  or  seven 
thousand  tons,  is  used  in  the  manufacture  of  glass,  porcelain,  and 
enamel  ware.  Cryolite  forms  the  basis  of  the  fused  bath  used  as  a 
solvent  for  the  oxide  of  aluminium  in  the  Hall  electrolytic  process 
for  the  reduction  of  the  metal  aluminium. 

Associated  with  cryolite,  as  a  secondary  product  formed  by  the 
substitution  of  calcium  for  some  of  the  sodium,  is  pachnolite 
(Na,Ca)3AlF6,H2O. 

ATACAMITE 

Atacamite.  —  CuCl2, 3  Cu(OH)2;  Basic  chloride  of  copper; 
Cu  =  14.9,  Cl  =  16.6,  H2O  =  12.7 ;  Orthorhombic ;  Type,  Didig- 
onal  Equatorial ;  a :  b  :  c  =  .6612  :  1 :  .7515 ;  100  A  110  =  33° 
28';  001 A 101  =48°  49';  001  A  01 1=36°  58';  Common  forms, 


THE  HALOID  COMPOUNDS  x       335 

b  (010),  c  (001),  m  (110),  e  (Oil),  p  (1H),  d  (110) ;  Twinning  plane 
110;  Cleavage,  b  perfect,  101  imperfect;  Brittle,  fracture  con- 
choidal;  H.  =  3-3.5;  G.  =  3.75-3.77;  Color,  various  shades  of 
green ;  Streak,  apple-green ;  Luster  vitreous ;  Transparent  to  trans- 
lucent; Optically  (-);  Plane  of  the  optic  axes  parallel  to  100; 
Bxa  parallel  to  a. 

B.B.  —  Fuses  on  coal,  yielding  an  azure-blue  flame,  and  in  R.  F. 
yields  malleable  copper  buttons.  In  the  closed  tube  blackens 
and  yields  water.  Easily  soluble  in  acids. 

General  description.  —  Crystals  are  slender  prisms  striated 
lengthwise  and  usually  terminated  by  the  unit  pyramid  and  brachy- 
dome ;  more  often  massive,  fibrous,  or  granular.  First  discovered 
as  a  sand  associated  with  the  copper  ores  in  the  province  of  Ata- 
cama,  Chili.  Occurs  in  dry  regions  associated  with  other  copper 
minerals  in  Tarapaca,  Bolivia;  Wallaroo,  Australia ;  and  at  Jerome, 
Arizona;  but  only  in  Chili  does  it  occur  in  quantities  sufficient  to 
constitute  an  important  copper  ore. 

CARNALLITE 

Carnallite.  —  Potassium  and  magnesium  chlorides ;  KC1. 
MgCl2.6H2O;  K  =  14.1,  Mg  =  8.7,  Cl  =  38.3,  H20  =  39.0; 
Orthorhombic  ;  Type,  Didigonal  Equatorial ;  & :  b  :  c  =  .5935 :  1 : 
.6906;  100  A 110 -30°  41',  001 A 101  =  49°  19',  001  A  011  = 
34°  37',  001  A  111  =  53°  32';  Common  forms,  c(001),  m(110), 
p  (111) ;  Cleavage,  none:  Fracture  conchoidal,  brittle;  H.  =1; 
G.  =  1.60;  Color,  white  or  reddish  ;  Streak,  white ;  Luster,  greasy ; 
Transparent  to  translucent;  Deliquescent;  Has  a  bitter  taste; 
Optically  (+) ;  Plane  of  the  optic  axes  parallel  to  a;  2E  =  115  1. 

B.B.  —  Easily  fusible,  yielding  a  violet  flame  (K).  With  S.  Ph. 
bead  saturated  with  copper  oxide  an  azure-blue  flame  (Cl).  After 
ignition  on  coal  leaves  a  residue  which  becomes  flesh-colored  when 
treated  with  cobalt  solution.  Easily  soluble  in  water,  the  solution 
having  a  bitter  taste. 

General  description.  —  Usually  massive,  deposited  in  layers 
interbedded  with  salt  and  kieserite  (MgS04 .  H20) ,  and  associated 
with  sylvite  and  kainite,  both  of  which  may  be  derived  from  the 
carnallite,  as  percolating  waters  containing  magnesium  sulphate 


336  MINERALOGY 

dissolve  the  carnallite ;    on  recrystallization  the  salts  separate  as 
sylvite  and  kainite. 

The  carnallite  beds  are  at  the  top  of  the  Stassfurt  deposits  and 
represent  the  last  products  of  deposition  from  the  brines.  Here 
it  is  mined  in  large  quantities,  and  together  with  kainite  is  the  prin- 
cipal source  of  the  quantities  of  potassium  used  as  fertilizers. 


CHAPTER  VIII 

OXIDES,    INCLUDING    THE   ALUMINITES,    FERRITES, 
CHROMITES 

OXIDES   OF  THE  RO  TYPE 

WATER 

Water. —  Ice;  Snow;  Frost;  H2O ;  H  =  11.1,  0  =  88.9; 
Hexagonal;  Type,  Dihexagonal  Alternating;  c  =  1.4026; 
0001 A  1011  =  58°  18' ;  Common  forms,  c  (0001),  m  (10lO),  r  (1012) ; 
H.  =  1.5;  G.  =  .9167;  Luster,  vitreous;  Transparent;  Color, 
white  when  pure,  but  blue  in  large  masses;  co  =  1.3090; 
c  =  1.3133;  Optically  (+). 

B.B.  —  Fuses  at  0°,  volatilizes  entirely,  leaving  no  coat  and  yield- 
ing no  odor. 

General  description.  —  Crystals  with  individual  faces  rare ; 
sometimes  they  occur  on  hailstones.  Usually  complex  stellate  aggre- 
gates produced  by  twinning;  occurs  during  low  temperature  as 
frost,  snow,  and  sheets  of  ice  covering  lakes  and  streams.  In  the 
high  latitudes  it  forms  the  permanent  polar  ice  cap.  In  Alaska  sheets 
of  geological  ice  occur  interbedded  with  sedimentary  deposits. 
Glaciers  are  streams  of  ice  moving  from  the  perpetual  snow-covered 
mountain  ranges  to  lower  altitudes. 

CUPRITE 

Cuprite.  —  Red  oxide  of  copper ;  Cuprous  oxide,  Cu20 ;  Cu  = 
88.8;  O  =  11.2;  Isometric;  Type,  Tesseral  Holoaxial;  Common 
forms,  a  (001) ;  o(lll);  d(110);  Cleavage,  octahedral  imperfect; 
Brittle,  fracture  conchoidal ;  H.  =  3.5-^t;  G.  =  5.85-6.15;  Luster, 
adamantine  to  earthy;  Color,  cochineal-red  to  nearly  black; 
Streak,  shades  of  red  or  brown;  Transparent  on  thin  edges  to 
opaque;  n  =  2.849. 

B.B.  —  Fuses  and  in  R.  F.  yields  malleable  copper,  and  an  emer- 
ald green  flame.  With  the  fluxes  shows  copper.  Easily  soluble 
in  acids. 

z  337 


338 


MINERALOGY 


General  description.  —  Crystals  are  octahedrons,  rhombic 
dodecahedrons,  and  cubes,  or  combinations  of  these  three  forms. 
Other  forms  occur,  but  are  rare.  The  pentagonal  didodecahedron 


FIG.  411.  —  Crystalline  Cuprite.    Cornwall,  England. 

(986)  has  been  described  as  occurring  on  crystals  from  Cornwall, 
England.     It  is  never  twinned.     Occurs   in   compact,  granular, 

massive,  and  also 
earthy  forms  .  Tile  ore 
is  an  impure  cuprite 
mixed  wTith  limonite 
and  hematite. 

Cuprite  is  found  in 
connection  with  all 
copper  deposits,  as  a 
secondary  oxidation 
product  in  the  surface 
or  upper  levels  of  the 
workings.  It  is  formed 
as  the  result  of  oxi- 
dation of  the  sulphur 
of  sulphides,  reducing 
some  of  the  copper  to 
the  metallic  state, 


' 


dized  to  cuprite.  Quite 
often  masses  of  cuprite  are  found  which  still  inclose  a  core  of 
metallic  copper  that  has  as  yet  not  been  oxidized  to  cuprite. 


OXIDES 


339 


Cuprite  may  oxidize  to  CuO,  the  black  oxide  of  copper,  tenorite, 
or  by  the  action  of  percolating  waters  containing  carbon  dioxide 
be  oxidized  and  transformed  to  the  carbonates,  either  malachite 
or  azurite,  as  is  illustrated  by  the  pseudomorphs  of  malachite 
after  cuprite  of  Chessy,  France,  some  of  which  still  contain  a 
central  nucleus  of  cuprite. 

It  occurs  in  good  crystals  at  Bisbee,  Arizona;  in  the  Lake 
Superior  region ;  and  along  the  border  of  the  trap  of  the  Atlantic 
coast,  as  at  Somerville,  New  Jersey ;  at  New  Haven,  Connecticut. 
Elongated,  capillary,  distorted  cubes  occur  at  Morenci,  Arizona, 
in  plushlike  mats,  forming  the  variety  known  as  chalcotrichite. 
Noted  European  localities  are  Cornwall,  England,  where  beautiful 
crystals  with  complicated  and  rare  forms  are  found ;  also  Chessy, 
France,  in  individual  crystals,  combinations  of  the  octahedrons 
and  the  rhombic  dodecahedron. 

Cuprite  is  an  important  ore  of  copper. 

ZINCITE 

Zincite.  —  Oxide  of  zinc,  ZnO ;  Zn  =  80.3,  0  =  19.7 ;  Hexag- 
onal ;  Type,  Dihexagonal  Polar ;  c  =  1.621 ;  0001  A  1011  =  61° 
54';  Forms,  p  (lOll),  m  (lOlO), 
c  (0001) ;  Cleavage,  basal  and  pris- 
matic ;  Brittle,  fracture  subconchoi- 
dal ;  H.  =  4-4.5 ;  G.  =  5.43 ;  Color, 
deep  red  to  orange ;  Streak,  orange ; 
Luster,  adamantine  ;  Transparent  to 
nearly  opaque;  Optically  (+). 

B.B.  —  Infusible.  In  R.  F.  with 
soda  and  borax  on  coal  yields  a  zinc 
coat.  Easily  soluble  in  hot  acids  with- 
out effervescing.  Generally  shows 
manganese  with  the  fluxes. 

General  Description.  —  Crystals 
are  very  rare,  usually  simple  hemi- 
pyramids,  as  both  the  base  and  prism 
are  rare  faces  on  natural  crystals. 
Crystals  of  zincite  appearing  as  flue 
or  slag  products  are  much  more  com- 
plex than  the  natural  crystals. 

Pure  oxide  of  zinc  is  white,  but 


FIG.   413.  —  Zincite  Crystal  from 
Franklin,  New  Jersey. 


340  MINERALOGY 

the  natural  oxide,  occurring  as  it  does  with  other  manganese  min- 
erals, is  colored  by  that  element. 

The  only  deposit  of  zincite  in  the  world  of  any  importance  is 
at  Franklin  and  Sterling,  Sussex  County,  New  Jersey,  where  it 
forms  an  important  zinc  ore ;  otherwise  a  very  rare  mineral.  Here 
it  is  intimately  mixed  with  franklinite,  willemite,  and  embedded 
in  calcite.  Cleavable  masses  are  common,  which  may  also  have  a 
lamellar  structure.  The  crystals  occur  in  isolated  side  stringers, 
where  the  conditions  of  formation  have  differed  from  that  of  the  gen- 
eral body  of  the  deposit,  as  these  crystals  are  associated  with  such 
rare  minerals  as  leucophcenicite,  pyrochroite  and  gageite. 

TENORITE 

Tenorite.  —  Melaconite ;  Black  oxide  of  copper,  CuO ;  Cu  = 
79.8,  O  =  20.2 ;  Triclinic  ;  a  :  b  :  c  =  1.4902  :  1 :  1.3604  ;  p  =  80° 
28';  100 A 110  =  55°  46',  001  A  101 ;  =  38°  2',  001  A  Oil  =53° 
18';  Forms,  a  (100),  c(001),  p(lll);  Twinning  plane,  001; 
Cleavage,  basal  easy;  Brittle,  fracture  conchoidal ;  H.  =  3-4; 
G.  =  5.85 ;  Color,  iron-gray  to  black ;  Streak,  black ;  Opaque  ; 
Metallic. 

B.B.  —  Fuses  and  in  the  R.  F.  on  coal  is  reduced  to  malleable 
copper.  Other  reactions  as  in  cuprite. 

General  Description.  —  Crystals  are  rare ;  small  plates  hexag- 
onal in  outline  occur  in  the  lavas  of  Vesuvius.  Mostly  occurs  in 
the  black  massive  form,  melaconite,  or  granular,  earthy,  and  dis- 
seminated. It  is  a  common  mineral  occurring  usually  in  small 
quantities  in  the  upper  oxidation  zone  of  most  all  copper  deposits, 
though  not  as  common  as  the  red  oxide,  cuprite.  It  is  the  ultimate 
product  formed  in  the  oxidation  of  other  copper  minerals,  as  the 
sulphides,  native  copper,  and  cuprite. 

Tenorite  fuses  at  1050°  C.,  when  it  loses  oxygen  and  forms  cu- 
prite. 

Other  anhydrous  protoxides  of  the  formula  of  R"O,  in  nature, 
are  very  rare  and  occur  in  localities  in  which  there  has  been  a  pecu- 
liar combination  of  physical  and  chemical  conditions  which  has 
favored  their  formation. 

Periclase,  MgO,  is  an  isometric  mineral  found  in  cubo-octahedrons 
in  the  lavas  of  Vesuvius  and  also  at  Nordmark,  Sweden. 

Manganosite,  MnO,  occurring  at  Langban,  Sweden,  and  bun- 


OXIDES 


341 


senite,  NiO,  occurring  at  Johanngeorgenstadt,  Saxony,  are  both 
isometric  minerals  crystallizing  in  small  cubes. 

Massicot,  PbO,  is  a  massive  yellow  mineral  found  in  several 
localities  in  Chihuahua,  Mexico. 

OXIDES  OF  THE  R203  TYPE 

CORUNDUM 

Corundum.  —  Sesquioxide   of    aluminium,     A1203 ;    Al  =  52.9, 

0  =  47.1 ;  Hexagonal ;  Type,  Dihexagonal  Alternating ;  c  =  1.363  ; 

0001  A  1011  =  57°  34';    rAr'  =  93°56' ;    Common  forms,  a  (1120), 
r  (1011),  c  (0001),  v  (4483),  n  (2243) ;    Twinning  plane  r,  polysyn- 
thetic ;    Cleavage,  parting  parallel  to  r  and  c  at  times  perfect ; 
H.  =  9 ;    G.  =  3.95-4.1;    Color,  gray,  red,  blue,  green,  white,  or 
nearly  black;    Streak,  white;    Luster,  adamantine;    Transparent 
to  opaque;   to  =  1.768;   c  =  1.76;  <o  -  €  =  .008;   Optically  (-). 

B.B.  —  Infusible,  fusing  point  1880°;  the  very  fine  powder  heated 
on  coal  in  O.  F.,  moistened  with  cobalt  solution,  and  again  heated 
becomes  blue.  Insoluble  in  acids,  but  dissolves  slowly  both  in 
borax  and  the  S.  Ph.  beads. 


FIG.  414.  —  Corundum  Crystals  from  Montana. 

General  description.  —  Crystals  when  large  are  coarse,  rough 
prisms,  swelling  in  the  middle  and  drawn  in  at  the  ends  or  barrel- 
shaped,  terminated  with  the  base  ;  such  large  crystals  are  opaque 


342  MINERALOGY 

and  colored  brown  or  flesh-colored  from  the  iron  content,  Fe2C>3 
being  isomorphous  with  A1203.  Clear  corundum  is  variable  in 
color  and  runs  through  the  entire  list  of  colored  precious  stones. 
When  it  is  wished  to  convey  the  idea  that  the  gem  in  question  is 
corundum,  the  term  "oriental"  is  prefixed,  as  "oriental  topaz," 
"  oriental  amethyst,"  or  "  oriental  emerald,"  etc.  The  true  ruby  is 
corundum;  and  when  of  that  dark  "  pigeon  blood  "  red,  so  much 
sought  after  and  of  the  right  transparency,  it  is  the  most  expensive 
of  gems,  even  surpassing  the  diamond  of  the  first  water  in  value. 

The  ruby  is  colored  with  small  quantities  of  chromium,  while  the 
sapphire  and  other  colors  are  due  to  cobalt,  titanium,  or  iron. 

Its  physical  properties,  together  with  its  wide  range  of  colors, 
render  the  transparent  varieties  of  corundum  an  ideal  gem  stone. 
It  is  the  third  hardest  substance  known,  being  surpassed  only  by 
the  diamond  and  silicon  carbide,  a  product  of  the  electric  furnace. 

Corundum  is  a  primary  mineral  of  such  igneous  rocks  as  granites, 
syenites,  rhyolites,  and  rocks  rich  in  alumina. 

In  thin  sections  it  appears  transparent  and  nearly  colorless,  with 
or  without  crystalline  outline,  and  there  are  no  characteristic  in- 
clusions. Relief  is  strong,  and  the  interference  colors  are  of  the 
first  order  yellow  or  gray. 

In  addition  to  being  a  primary  mineral  of  igneous  rocks,  it  is  also 
a  characteristic  mineral  of  the  belt  of  metamorphism,  and  is  there 
associated  with  tourmaline,  spinels,  cyanites,  garnets ;  while  in  its 
decomposition  by  weathering  it  forms  a  whole  series  of  aluminous 
minerals,  as  gibbsite,  diaspore,  margarite,  muscovite,  etc. 

Occurrence.  —  Most  of  the  gem  material  is  of  Eastern  origin; 
the  best  rubies  are  found  in  the  gravels  of  Irrawaddy  River,  near 
Mandalay,  Burma,  and  in  the  crystallized  limestone  on  its  eastern 
bank ;  the  crystals  are  tabular  in  habit  and  associated  with  spinels 
and  garnets.  Those  recovered  from  the  river  gravels  are  rounded 
and  water-worn,  but  owing  to  their  excessive  hardness  some  still 
retain  their  crystalline  faces.  In  Ceylon  the  gemstones  are  also  re- 
covered mostly  from  the  gravels  of  the  Ratnapura  and  Rakwena 
districts.  In  the  United  States  sapphires  are  found  in  the  gravel 
bars  of  the  upper  Missouri  River  in  Montana,  and  stones  of  gem 
value  are  mined  in  the  Judith  River  valley,  Montana.  These  are 
contained  in  a  dyke  cutting  through  a  crystalline  limestone.  The 
dyke  having  weathered  faster  than  the  limestone  may  be  traced  by 
a  depression  across  the  country  for  five  miles,  and  many  sapphires 
have  been  taken  from  the  piles  of  dirt  at  the  entrance  to  the  gopher 


OXIDES  343 

holes.  The  decomposed  dyke  furnished  much  easier  digging  for 
these  little  animals  than  the  hard  crystalline  limestone  adjacent. 
Corundum  is  also  found  all  along  the  Blue  Ridge  in  Virginia,  North 
and  South  Carolinas,  and  Georgia.  Very  fine  blue  specimens  have 
been  obtained  at  Sparta,  New  Jersey,  and  at  various  points  in 
Sussex  County.  Most  of  the  "  adamantine  spar  "  used  in  the 
United  States  is  mined  in  Renfrew  County,  Ontario,  where  it  occurs 
in  a  coarse  pink  syenite.  Emery  is  a  compact  or  granular  variety  of 
corundum  which  is  mixed  with  a  large  proportion  of  oxides  of  iron. 
It  is  mined  at  Chester,  Massachusetts,  and  at  Peekskill,  New  York. 
In  the  latter  locality  it  is  associated  with  hercynite,  magnetite,  and 
garnet,  and  occurs  as  a  segregation  product  from  the  norite  of  the 
Courtland  series.  Abroad  emery  is  mined  in  Asia  Minor,  Tur- 
key, and  Greece ;  60  per  cent,  of  this  product  is  exported  to  the 
United  States. 

Corundum  and  emery  are  commercially  used  as  an  abrasive,  espe- 
cially in  emery  paper  for  polishing  and  cleaning  metals ;  in  wheels 
for  sharpening  steel  tools,  and  in  the  cutting  of  glassware,  though 
artificial  corundum  made  from  bauxite  by  heating  it  to  a  high  tem- 
perature in  an  electric  furnace  is  fast  replacing  the  natural  mineral. 

Artificial  crystallized  A12O3  may  be  produced  by  fusing  equal 
parts  of  A1203  and  lead  oxide,  and  allowing  the  fusion  to  cool 
slowly,  when  tabular  crystals  of  corundum  separate;  if  a  little 
oxide  of  cobalt  is  added,  they  will  be  sapphire-blue ;  if  a  little  po- 
tassium dichromate  is  added  they  will  be  ruby-red.  The  so-called 
reconstructed  rubies  are  formed  in  the  oxyhydrogen  flame;  by 
rotating  a  small  crystal  rapidly  in  the  flame  with  the  temperature 
near  the  fusing  point,  it  is  then  built  up  with  fine  particles  of  natural 
ruby,  the  color  being  regulated  by  the  amount  of  chromium 
present.  These  artificial  stones  in  their  physical  properties  differ 
in  no  way  from  the  natural  ruby,  and  they  puzzle  even  the  expert 
to  recognize  them.  They  have  however  peculiar  circular  markings 
or  pores,  due  to  the  rotation  in  the  flame  while  in  a  semifused  con- 
dition, which  are  a  material  aid  in  their  identification. 

HEMATITE 

Hematite.  —  Red  oxide  of  iron,  Fe203 ;  Fe  =  70,  0  =  30 ; 
Hexagonal;  Type,  Dihexagonal  Alternating ;  c  =  1.3656;  0001  A 
1011  =  57°  37';  rAr'  =  94°;  Common  forms,  c  (0001),  r(1011), 
e  (01  II),  u  (10H) ;  n  (2243) ;  m  (10TO) ;  Twinning  plane  r,  both 


344 


MINERALOGY 


interpenetrating  and  polysynthetic ;  Cleavage,  none,  parting  caused 
by  twinning;  Brittle,  fracture  uneven;  H.  =  5.5-6.5;  G.  = 
4.9-5.3,  varies  with  the  structure ;  Luster,  metallic,  splendent  in 
crystals,  dull  in  massive  form ;  Color,  dark  iron-gray,  red,  reddish 
brown,  to  black;  Streak,  cherry-red;  Opaque  except  in  thin 
scales  when  blood-red  by  transmitted  light ;  €  =  2.94 ;  <o  =  3.22 ; 
co-6=  0.28;  Optically  (-). 

B.B.  —  Nearly  infusible  (1350°).     In  R.  F.  on  coal  blackens  and 
becomes  magnetic,  with  the  fluxes  shows  iron.      Soluble  in  HC1 
at  least  in  fine  powder  on  heat- 
ing, when  impure  there  will  be 
a  considerable  residue  of  quartz. 

General  Description.  —  When 
crystalline,  usually  in  tabular 
crystals  with  a  splendent  luster 
or  iridescent,  combinations  of  m, 
r,  e,  n,  and  c,  with  striations  on 
the  base;  a  large  number  of 
other  forms  have  been  de- 
scribed. Micaceous  hematite  is 
formed  by  thin  scales  in  parallel 
position.  All  highly  lustrous  crystalline  hematites  are  known  as 
specular  iron  ore.  Another  peculiar  complex  aggregate  in  which 
the  individual  crystals  are  arranged  radially  around  the  center  like 
the  petals  of  a  rose,  "  eisenrosen,"  occurs  at  St.  Gothard,  Fig.  416. 

Amorphous  hematite  occurs  mas- 
sive, fibrous,  radiated,  botryoidal, 
concretionary,  or  apparently  soft 
and  unctuous,  "  clay  iron-stone," 
which  soils  the  fingers;  all  such 
varieties  are  red-,  brown,  or  even 
slightly  yellowish  in  color,  as  they 
grade  into  limonite;  but  all  true 
hematites  yield  a  cherry-red  streak. 
Hematite  is  connected  with 
igneous  rocks  in  which  it  occurs  as 
a  primary  constituent,  as  black 
scales,  easily  mistaken  for  magnet- 
ite. Many  feldspar  phenocrysts,  especially  orthoclase,  which 
seems  to  have  a  predisposition  to  collect  these  fine  hematite  scales 


FIG. 


415.  — Hematite 
Elba. 


Crystals    from 


FIG.  416. —  Eisenrosen.   St.  Gothard, 
Switzerland. 


OXIDES  345 

as  inclusions,  are  colored  red  by  them.  Indeed  most  of  the  red 
sedimentary  rocks  are  colored  with  hematite,  as  jaspers,  sand- 
stones, shales,  and  clays. 

The  most  important  deposits  of  hematite  are  associated  with 
sedimentary  rocks,  where  the  layers  of  hematite  are  interbedded 
with  chert,  jasper,  shales,  and  sandstones.  These  deposits  have 
been  derived  from  the  chemical  precipitation  of  Fe203  resulting  from 
the  oxidation  of  ferrous  carbonate  in  solution.  These  precipitates 
gather  in  layers  at  the  bottom  of  drainage  basins,  their  physical 
condition  depending  upon  future  metamorphism  to  hematite. 
Such  are  the  fossil  and  oolitic  hematites  connected  with  the 
Clinton  formation,  extending  from  New  York  to  Alabama. 

In  the  Lake  Superior  region  the  oxidation  of  silicates,  pyrite, 
and  carbonates  has  filled  an  important  r61e  in  the  formation  and 
concentration  of  enormous  deposits,  which  have  yielded  millions 
of  tons  of  ore,  but  like  the  Clinton  ores  they  are  connected  with 
sedimentary  formations.  In  the  oxidation  and  replacement  pro- 
cesses hematite  may  form  pseudomorphs  after  pyrite,  calcite,  sider- 
ite,  magnetite,  quartz,  and  fossils,  especially  shells.  A  variety  of 
Fe203,  martite,  occurring  in  large  octahedrons  at  Twin  Peaks, 
Mallard  County,  Utah,  is  possibly  magnetite  oxidized  to  hematite. 

Eighty  per  cent,  of  all  iron  ore  mined  in  the  United  States  is 
hematite,  most  of  which  is  taken  from  the  Lake  Superior  region, 
where  it  is  mined  in  large  open  quarries,  as  it  lies  near  the  surface. 
The  principal  localities  are  Marquette,  Menominee,  and  Gogebic, 
Wisconsin ;  Vermilion  and  Mesaba  districts  of  Mirinesota ;  and  the 
Northern  Peninsula  of  Michigan.  In  the  East  hematite  is  mined 
at  various  points  along  the  Clinton  formation.  Large  deposits  also 
occur  at  Iron  Mountain,  Missouri;  in  Wyoming;  while  Nova 
Scotia  and  Newfoundland  supply  the  Canadian  industries.  The 
best  crystalline  specimens  are  obtained  from  the  Isle  of  Elba; 
from  Cumberland,  England,  where  it  is  associated  with  quartz  and 
dolomite ;  at  St.  Gothard,  Switzerland,  in  rosettes  and  in  flat  tabular 
scales  associated  with  rutile  crystals.  Large  rosettes  five  or  six 
inches  across  are  obtained  from  Brazil. 

Artificial.  —  Micaceous  hematite  crystals  may  be  formed  by 
heating  a  concentrated  solution  of  ferrous  sulphate  with  copper 
sulphate  in  a  sealed  tube  for  10  hours  at  a  temperature  of  210°. 

Fe2O3  when  fused  loses  oxygen  and  on  cooling  forms  magnetite. 
On  fusing  ferric  oxide  and  borax  and  dissolving  the  melt  in  hot  dilute 
HC1  crystals  of  hematite  will  be  left. 


346  MINERALOGY 

Hematite  is  also  formed,  as  in  the  lavas  of  Vesuvius,  by  decom- 
posing the  vapors  of  ferric  chloride  with  steam. 

Arsenic,  antimony,  and  bismuth  form  anhydrous  sesquioxides 
which  are  found  as  minerals,  but  only  in  small  quantities  and 
restricted  in  distribution.  Those  of  arsenic  and  antimony  are 
isomorphous  and  dimorphic.  Arsenolite,  As2O3,  occurs  at  the  Ophir 
mine,  Nevada,  and  appears  as  white  crusts  on  native  arsenic. 
Senarmontite,  Sb2O3,  is  formed  as  a  coating  on  stibnite.  Both  are 
isometric,  crystallizing  in  octahedrons. 

Claudetite,  As2O3,  and  valentinite,  Sb2O3,  are  both  formed  by 
oxidation  of  other  minerals;  the  former  is  monoclinic,  the  latter 
orthorhombic. 

Bismite,  Bi2O3,  is  orthorhombic  in  artificial  crystals,  in  nature  it 
is  not  crystalline.  It  occurs  at  Schneeberg,  Bohemia,  as  a  white 
to  yellowish  powder. 

ILMENITE 

Ilmenite.  —  Menaccanite ;  Titanic  iron  ore ;  Ferrous  metatitan- 
ate,  FeTiO3;  FeO  =  47.3,  TiO2  =  52.7 ;_  Hexagonal;  Type, 
Hexagonal  Alternating,  c  =  1.3845 ;  1011  A  1101  =  94°  29' ;  0001 A 
1014  =  21°  47';  0001A01l2  =  38°  38';  0001 A  2243  =  61°  33'; 
Usual_  forms,  _c  (0001),  a  (1120),  m  (10TO),  _r  (lOll),  e  (0112), 
u  (1014),  n  (2243) ;  Twinning  plane,  0001  or  1011,  usually  lamellar  ; 
Cleavage,  none;  Brittle,  fracture  conchoidal;  H.  =  5-6;  G.  = 
4.5-5;  Color,  black  to  brownish;  Streak,  black  to  brownish; 
Submetallic ;  Opaque ;  At  times  slightly  magnetic. 

B.B.  —  Fuses  on  the  thin  edges  in  a  strong  R.  F.,  and  becomes 
magnetic;  otherwise  infusible.  Well  powdered  and  fused  with 
soda,  the  fusion  boiled  in  strong  HCI  and  reduced  with  powdered 
tin,  yields  a  violet  colored  solution  (titanium).  Reacts  for  iron 
with  the  fluxes, 

General  Description.  —  Crystals  are  tabular  parallel  to  the  base, 
usually  combinations  of  the  prism  a,  the  rhombohedron  r,  and  the 
base  or  combinations  of  the  base  and  the  rhombohedrons  r,  s,  and  n. 
When  tabular  the  base  is  often  marked  with  striations,  forming 
triangular  areas  in  outline,  as  the  striations  are  parallel  to  the 
rhombohedr^il  faces.  The  tabular  crystals  are  often  grouped  in 
rosette  aggregates,  or  "  eisenrosen  "  as  is  hematite.  Bright  and 
well-formed  crystals  are  not  common.  It  is  usually  massive,  com- 
pact, or  in  disseminated,  rounded  grains,  or  as  sand. 


OXIDES  347 

Ilmenite  has  been  grouped  with  the  sesquioxides,  as  Ti203  is  iso- 
morphous  with  Fe2C>3,  but  there  is  no  doubt  but  that  ilmenite  is  a 
ferrous  metatitanate  and  is  out  of  place  here  as  placed  by  Dana's 
classification.  Magnesium  and  manganese  are  both  isomorphous 
with  ilmenite,  and  pyrophane  is  the  manganous  metatitanate, 
MnTi03. 

Ilmenite  occurs  associated  with  magnetite  and  under  the  same 
conditions,  as  a  primary  constituent  of  igneous  rocks ;  as  such  it  is 
one  of  the  first  minerals  to  separate  from  the  magma.  It  is  more 
abundant  in  the  basic  rocks,  as  the  diorites/ diabases,  and  basalts. 
It  also  occurs  in  schists,  gneisses,  metamorphic  rocks,  argillites, 
and  slates. 

In  rock  sections  it  is  opaque  and  appears  brownish  by  reflected 
light.  When  in  crystalline  outline  it  is  elongated,  but  occurs  more 
often  as  rounded  grains  and  irregular  masses,  not  to  be  distinguished 
from  magnetite  or  chromite  but  by  chemical  tests.  Ilmenite 
is  often  altered,  resulting  in  a  clear  or  translucent  boundary,  or  area 
surrounding  the  opaque  masses,  composed  of  a  highly  doubly  re- 
fracting substance  termed  leucoxene,  formed  by  the  decomposition 
of  the  ilmenite,  and  which  has  been  identified  as  perovskite 
(CaTiO3),  as  titanite,  and  again  as  anatase.  Ilmenite  occurs  in 
large  masses  at  Bay  St.  Paul,  Quebec,  also  in  Orange  Co.,  New 
York,  associated  with  serpentine,  spinel,  rutile,  and  chondrodite; 
at  Litchfield,  Connecticut;  at  Chester  and  South  Royalston, 
Massachusetts.  The  largest  crystals  of  ilmenite,  some  of  which 
weigh  sixteen  pounds  or  more,  have  been  found  in  a  diorite  at 
Kragero,  Norway.  As  an  accessory  in  igneous  rocks  ilmenite  is 
very  widely  distributed. 

Ilmenite  finds  but  little  use  in  commerce;  it  is  used  as  linings  in 
puddling  furnaces,  but  owing  to  the  difficulty  of  handling  it  in  the 
blast  furnaces  it  is  not  used  as  an  iron  ore,  though  at  the  present 
time  titanium  steel  is  being  tried  for  rails  with  encouraging  results. 
Artificially  ilmenite  has  been  formed  by  heating  a  mixture  of  me- 
tallic iron,  ferric  oxide,  and  amorphous  titanic  oxide  in  a  sealed 
tube  to  270°-300°  C. 

OXIDES  OF  THE  R02  TYPE 
CASSITERITE 

Cassiterite.  —  Stream  tin ;  Tin  binoxide,  SnO2 ;  Sn  =  78.6, 
O  =  21.4;  Tetragonal;  Type,  Ditetragonal  Equatorial ;  c  =  .6723; 
001 A 101  =33?  54';  110,111=46°  27';  Common  forms, 


348 


.MINERALOGY 


s  (111),  e  (101),  m  (110)  a  (100).  Twinning  plane  101,  both  geniculate 
and  cyclic;  Cleavage,  110  imperfect;  Brittle,  fracture  uneven; 
H.  =  6-7;  G.  =  6.8-7.1;  Luster,  splendent  adamantine ;  Color, 
various  shades  of  brown,  red,  and  gray  to  almost  black ;  Streak, 
pale;  n  =  1.997. 

B.B.  —  Infusible,  reduced  with  soda  and  borax  on  coal  yields 
malleable  tin  buttons.  Insoluble  in  acids. 

General  description.  —  Crystals  are  short  stout  prisms  with  the 
prism  faces  striated  parallel  to  the  c  axis,  usually  terminated  with 
the  two  unit  pyramids.  Acicular  crystals  terminated  by  the  pyra- 
mids (321)  and  (521)  occur  at  Cornwall,  England,  also  massive  or 


FIG.  417.  —  Cassiterite  from  Bohemia.    The  Upper  Figures  are  Stream  Tin 
from  Mexico. 

granular.  While  cassiterite  is  found  as  a  granular  or  disseminated 
primary  accessory  mineral  in  some  igneous  rocks,  it  is  more  often 
connected  with  the  cavities  and  pegmatitic  veins  in  the  region  of 
granitic  masses  which  have  been  intruded  in  sedimentary  forma- 
tions. Here  its  origin  is  the  result  of  pneumatolytic  agencies 
which  have  concentrated  the  tin  on  the  border  of  the  granitic  mass, 
where  it  has  been  deposited  in  the  veins,  close  at  hand,  of  the  dis- 
turbed area.  In  such  veins  it  is  associated  with  fluorite,  tourma- 


OXIDES 


349 


line,  topaz,  and  other  rarer  minerals,  as  wolframite,  scheelite,  or 
unraninite,  which  have  been  concentrated  by  the  same  agents. 

Owing  to  its  high  specific  gravity  and  not  being  affected  by 
weathering,  cassiterite  is  left  behind  after  most  of  the  other  minerals 
forming  the  rock  mass  have  been  decomposed  and  carried  away  ; 
it  is  thus  mechanically  concentrated  in  the  bottom  of  streams  as 
rolled,  rounded,  and  water-  worn  pebbles  (stream  tin).  It  is  from 
these  alluvial  deposits  that  a  large  amount  of  the  tin  of  commerce 
is  recovered. 

Little  cassiterite  is  produced  or  mined  in  the  United  States  ;  small 
deposits  are  found  in  Lincoln  County,  North  Carolina  ;  at  Harney's 
Peak,  South  Dakota;  near  El  Paso,  Texas;  and  in  the  Seward 
Peninsula,  Alaska.  The  world's  supply  is  derived  from  the  Malay 
Peninsula,  Bolivia,  Australia,  and  Cornwall,  England. 

RUTILE 

Rutile.  —  Dioxide  of  titanium,  Ti02  ;  Ti  =  60,  O  =  40  ; 
Tetragonal  ;  Type,  Ditetragonal  Equatorial  ;  c  =  .644  ;  001  A 
101  =  32°  47'  ;  Common  forms,  s  (111),  e  (101),  m  (110),  a  (100), 
Twinning  as  in  cassiterite;  Cleavage,  110  and  001  good;  H.  = 


FIG.  418.  —  Rutile  Crystals  from  Lynchburg,  Virginia. 

6-6.5  ;G.  =  4.18-4.25;  Brittle,  fracture  uneven;  Color,  shades 
of  brown  to  nearly  black ;  Streak,  pale  brown  or  reddish ;  Luster, 
adamantine,  metallic  in  appearance;  Translucent  to  opaque; 
<o  =  2.615;  €-o>  =  .287;  Optically  (+). 


350 


MINERALOGY 


B.B.  —  Infusible.  In  the  S.  Ph.  bead  beside  tin  on  coal  yields 
a  violet  color  when  cold;  the  bead  powdered  and  dissolved  in 
concentrated  HC1,  then  reduced  with  powdered  tin,  yields  a  violet 
solution.  Insoluble  in  acids. 

General  Description.  —  Rutile  occurs  in  all  varieties  of  rocks, 
igneous,  metamorphic,  and  sedimentary,  either  as  short  stout,  or 
elongated  and  acicular  prisms,  with  striations  on  the  prism  zone 
parallel  to  the  c  axis ;  very  often  these  long,  hairlike  crystals  are 

found  penetrating 
clear  quartz,  as  at 
St.  Gothard,  Switz- 
erland, when  the 
specimens  are  pol- 
ished and  cut  as 
ornaments.  At  this 
same  locality  small 
prismatic  rutile 
crystals  are  found 
placed  in  parallel 
position  on  hex- 
agonal plates  of 
hematite.  At 
Tavetsch,  Switzer- 
land, reticulated, 
platelike  masses  of 
elongated  crystals, 
interlocking  at  the 
twinning  angle  of 
65°  35',  occur  and 
are  known  as  sage- 
nite. 

Rutile  in  the 
United  States  has 
been  mined  in  Vir- 
ginia, where  it  is 

,,^^  found  near  Arring- 

ton  in  a  pegmatite;  at  Nelson  it  is  associated,  in  dykes,  with 
apatite;  at  Lynchburg  beautifully  formed  crystals  with  a  steel- 
like  luster,  both  simple  and  twinned,  are  common,  as  also  in  Alex- 
ander County,  North  Carolina,  and  at  Graves's  Mountain,  Georgia. 


FIG.  419.  —  Acicular  Crystals  of  Rutile  included  in 
Quartz.    Japan. 


OXIDES 


351 


As  a  secondary  mineral  rutile  is  derived  from  octahedrite  and 
brookite,  forming  pseudomorphs   after  the  latter,  as  at  Magnet 


FIG.  420. — Rutile  Crystals  from  Graves's  Mountain,  Georgia.    The  Upper  Twins 

are  from  the  Tyrol. 


Cove,  Arkansas.  These  three  minerals  are  all  dioxides  of  titanium, 
in  different  phases,  and  are  quite  often  associated;  rutile  has  the 
highest  specific  gravity  and  is  the  stable  form  at  high  tem- 
peratures. Octahedrite  is  also  of  tetragonal  symmetry,  while 
brookite  is  orthorhombic. 
Rutile  may  also  be  formed 
in  the  alteration  of  ilmenite 
and  titanite,  which  process 
is  reversed  in  the  alteration 
of  rutile. 

Chemically  rutile  gener- 
ally contains  iron,  to  which 
the  red  or  brown  color  is 
due ;  the  iron  content  is 
considerable  in  some  cases 
and  massive  rutile  may 
grade  gradually  into  ilme- 
nite. 

Commercially  rutile  is  the  source  of  titanium  salts,  which  are 
used  as  a  yellow  coloring  agent  in  porcelain  and  to  obtain  the  ivory- 


FIG.   421.  —  Brookite    from 
Arkansas. 


Magnet    Cove, 


352  MINERALOGY 

like  color  in  artificial  teeth.  Titanium  trichloride  is  replacing 
stannous  chloride  as  a  mordant  in  cloth  printing.  Ferro-titanium 
is  used  to  produce  a  hard  steel,  with  a  high  transverse  and  tensile 
strength;  this  more  recent  use  may  in  the  near  future  increase 
the  demand  for  titanium  minerals. 

PYROLUSITE 

Pyrolusite.  —  Dioxide  of  manganese,  MnO2 ;  Mn  =  63.2, 
O  =  36.8 ;  Amorphous  but  often  in  pseudomorphs ;  H.  =  2-2.5, 
soils  paper ;  G.  =  4.82 ;  Color  and  streak,  black ;  Luster,  metallic 
to  dull,  opaque. 

B.B.  —  Infusible,  shows  manganese  with  the  fluxes.  In  the 
closed  tube  yields  little  or  no  water.  .  Dissolves  in  HC1  with  the 
liberation  of  chlorine. 

General  Description.  —  Massive,  compact,  fibrous,  staiactitic, 
or  dendritic.  Oxides  of  manganese  are  usually  associated  with  iron 
ores,  having  been  formed  in  many  cases  by  the  interaction  of  the 
same  agents.  Pyrolusite  is  formed  from  hydrated  oxides  of  man- 
ganese by  loss  of  water.  It  has  been  thought  that  pyrolusite  may 
be  orthorhombic  in  symmetry,  but  it  is  probable  that  such  crystals 
are  pseudomorphs  derived  from  manganite  by  the  expulsion  of  its 
water. 

Oxides  of  manganese  are  concentrated  at  many  localities  either 
by  replacement  or  by  precipitation  from  solutions  resulting  from 
the  decomposition  of  silicates  or  carbonates  containing  manganese. 
Such  deposits  are  associated  with  limestone  and  sedimentary  forma- 
tions, or  form  irregular  pockets  and  nodules  in  clays.  Such  deposits 
extend  all  along  the  Appalachian  and  Piedmont  regions,  where  they 
are  mined  commercially  in  the  Blue  Ridge  Mountains  of  Virginia, 
and  at  Cartersville,  Georgia.  Pyrolusite  is  also  mined  at  Bates- 
ville,  Arkansas ;  and  Livermore,  California.  Cabinet  specimens  are 
obtained  at  Salisbury,  Connecticut,  and  Stockbridge,  Massachu- 
setts. 

QUARTZ 

Quartz.  —  Silicon  dioxide,  Si02;  Si  =  46.7,  O  =  53.3;  Hexag- 
onal;_  Type,  Trigonal  Holoaxial;  c  =  1.100;  Common  forms, 
m  (1010),  r  (1011),  z  (0111),  x  (5161) ,_  s  (1121) ;  0111  A  0110  =  38° 
13';  1010,5161  =  12°  1';  10TOAlI01  =  38°  18'  Cleavage,  r 
perfect  but  difficult;  Brittle,  fracture  conchoids! ;  H.  =  7;  G.  = 


OXIDES  353 

2.65 ;  Color,  when  pure,  colorless  or  white,  when  impure,  all  shades ; 
Luster,  vitreous,  splendent  to  dull  or  greasy ;  Streak,  white,  or  in 
colored  specimens  very  pale;  Transparent  to  opaque;  €  = 
1.5532;  (0  =  1.5441;  €-a>=.0091;  Optically  (+) ;  Rotary 
polarization  in  thick  sections. 

B.B.  —  Infusible  (1600°) ;  yields  little  or  no  water  in  the  closed 
tube;  when  finely  ground  and  fused  with  two  volumes  of  soda  on 
the  platinum  wire  yields  a  clear  bead  when  cold.  Insoluble  in 
acids  except  hydrofluoric. 

General  description.  —  Crystals  are  very  common  and  well 
developed,  usually  elongated  parallel  to  the  c  axis,  combinations 
of  the  plus  and  minus  rhombohedrons  and  the  unit  prism.  The 
prism  faces  are  often  striated  horizontally,  which  serves  to  identify 
the  prism  faces  on  distorted  specimens.  The  trigonal  pyramid  s 


FIG.  422.  —  Quartz  Var.  Rock  Crystal.    Hot  Springs,  Arkansas. 

and  the  trapezohedron  x  are  common  in  certain  localities,  but 
are,  like  the  large  number  of  other  forms  that  have  been  described, 
rare  except  for  the  few  favored  localities.  When  the  two  rhom- 
bohedrons are  equally  developed,  the  crystals  have  the  appearance 
of  being  terminated  by  a  hexagonal  pyramid,  but  this  is  not  pos- 

2A 


354 


MINERALOGY 


• 

sible  in  the  type  in  which  quartz  crystallizes.  When  the  rhom- 
bohedrons  are  unequally  developed,  the  plus  rhombohedron  r  is 
usually  the  larger  and  has  a  high  luster,  in  fact  is  more  perfect 
than  the  minus  rhombohedron  z,  which  may  be  dull,  and  faces 
occurring  in  the  same  zone  with  it  and  the  prism  face  beneath  it  are 
also  apt  to  be  dull,  while  the  faces  appearing  under  r  will  be  bright. 
Since  the  right  and  left  trigonal  trapezohedrons  are  rare  forms  or 
restricted  to  noted  localities,  it  is  not  always  possible  to  determine 


(6)  (a) 

FIG.  423.  —  Right-  (a)  and  Left-  (&)  handed  Smoky  Quartz  Crystals  from  St.  Goth- 

ard,  Switzerland. 

whether  any  given  specimen  is  a  right-  or  left-handed  crystal;  but 
when  the  trigonal  trapezohedron  is  present,  the  crystal  is  a  right- 
handed  one  when  the  face  appears  in  the  upper  right-hand  corner 
of  the  prism  face,  or  just  below  the  right-hand  corner  of  r,  and 
left-handed  when  in  a  similar  position  in  the  left-hand  corner. 

The  trigonal  pyramid  s  is  a  member  of  the  same  zone  as  m,  x,  and 
z,  and  will  appear  between  z  and  x;  striations  on  s  parallel  to  the 
intersection  of  s  and  r  are  characteristic,  and  will  determine  the 
character  of  the  crystal  when  x  is  absent.  Holding  the  crystal  in 
the  usual  position,  when  these  striae  run  from  northeast  to  south- 
west, as  directions  are  considered  on  a  map,  the  crystal  is  right- 


OXIDES 


355 


handed ;  if  from  northwest  to  southeast  the  crystal  is  left-handed. 
Again  if  the  edges  of  the  zone  mxsz  ascend  to  the  right  around  the 
crystal,  like  a  right-handed  screw,  the  crystal  is  right-handed; 
if  to  the  left,  left-handed.  Still  a  fourth  method  of  distinguishing 
right  and  left  crystals  is  by  the  corrosion  figures  which  often  appear 
on  crystals  as  the  result 
of  some  solvent,  or  are 
produced  artificially  by 
treatment  with  hydro- 
fluoric acid;  these  fig- 
ures are  pointed  at  one 
end  and  broad  at  the 
other ;  those  on  the  right 
are  minor  images  of  those 
on  the  left. 

As  the  trapezohedral 
faces  are  a  key  to  both 
the  twinning  of  quartz 
and  its  right-  or  left- 
handedness,  it  should  be 
remembered  that  four  of 
these  forms  are  possible. 
The  positive  forms  occur 
the  more  often,  and  are 
situated  below  r,  the 
positive  right  under  the  right-hand  corner,  and  the  positive  left 
directly  under  the  left-hand  corner.  The  left-handed  forms  hold 
the  same  relation  to  the  rhombohedral  faces  z. 

Twinning  in  quartz  is  very  frequent,  though  it  is  not  always  to  be 
recognized  unless  the  trapezohedral  faces  are  present. 

1.  Interpenetrating  twins  occur,  where  the  twinning  axis  is 
normal  to  the  prism  edge.  In  such  twins  x  will  occur  on  adjacent 
prism  faces,  modifying  the  upper  right-hand  corner ;  they  may  not 
appear  on  all  six  prism  faces,  but  if  they  do  appear  on  any  two  ad- 
jacent faces  it  is  sufficient  to  establish  the  twinned  nature  of  the 
crystal.  In  all  such  twins  the  rhombohedral  faces  are  complex  in 
nature,  portions  which  are  bright  are  r  and  portions  which  may  be 
dull  are  z ;  these  areas  are  quite  irregular  and  separated  by  curved 
and  jagged  boundaries,  plainly  shown  in  the  photographs.  Whether 
the  twin  consists  of  two  right-  or  two  left-handed  individuals  may  be 
determined  from  the  relation  of  x  to  the  bright  patches  of  the  rhom- 


FIG.  424.  —  Quartz  Twinned,  composed  of  Right- 
hand  Individuals.    St.  Gothard,  Switzerland. 


356 


MINERALOGY 


bohedral  faces,  as  x  will  be  under  the  bright  areas  in  right-  and  under 
the  dull  areas  in  left-handed  crystals. 

2.  Twins  occur  in  which  the  face  x  modifies  both  upper  or  both 
lower  corners  of  the  prism  face;   these  are  twinned  by  reflection 

over  a  plane  perpendicular  to  the 
prism  face  and  parallel  to  the  verti- 
cal axis.  These  are  termed  Bra- 
zilian twins  and  are  often  repeated, 
interpenetrating  and  quite  irregu- 
lar, but  the  twins  are  always 
formed  by  the  union  of  right  and 
left  individuals.  In  some  cases 
the  plane  of  reflection  may  pass 
through  opposite  prism  edges,  when 
four  x  faces  will  lie  adjacent  to  a 
single  and  alternate  prism  edge. 

3.  Twins  occur  in  which  the 
twinning  axis  is  perpendicular  to 
the  rhombohedral  face  1122;  after 
a  revolution  of  180°  around  this 

axis  the  vertical  crystallographical  axes  of  the  two  individuals  will 
lie  at  84°  33'.  This  type  of  twins  is  usually  flattened  parallel  to  the 
prism  face  and  they  are  known  as  the  Japanese  twins,  as  the  most 
beautiful  specimens  are  ob- 
tained from  that  country. 

Quartz  is  the  most  com- 
mon of  all  minerals.  It  is 
distributed  universally  and 
occurs  under  the  most  varied 
conditions.  It  is  one  of  the 
essential  minerals  of  granite, 
mica  schist,  and  gneiss,  while 
quartzite  and  sandstones 
may  be  almost  pure  quartz. 
In  rock  magmas  Si02  takes 


FIG.  425.  —  Quartz  Crystals  from  near 
Rome,  Italy. 


FIG.  426.  —  Quartz  twinned  on  1122. 
Alaska. 


the  part  of  an  acid,  and  for 
this  reason  quartz  is  never 
found  in  the  basic  and  dark- 
colored  igneous  rocks,  as  quartz  is  formed  only  in  those  cases  where 
there  is  an  excess  of  SiO2  over  the  basic  oxides.  It  is  usually  near 
the  last  to  crystallize  or  separate  in  the  solidification  of  a  rock 


OXIDES 


357 


magma.  It  may  therefore  include  within  its  mass  all  those  min- 
erals which  have  separated  previous  to  it,  as  the  oxides  of  iron, 
rutile,  apatite,  zircon,  mica,  amphibole,  and  pyroxene;  and  in 
some  instances  it  forms  a  ground  mass  in  which  the  individual 
crystals  of  other  minerals  are  imbedded. 

In  the  final  solidification  of  acid  igneous  rocks,  quartz  usually 
preserves  a  balanced  equilibrium  with  orthoclase,  separating  as  a 
eutectic,  which  is  well 
illustrated  in  the  struc- 
ture of  the  micropeg- 
matites,  where  the 
crystals  are  so  fine  and 
intimately  mixed  as  to 
be  revealed  only  by 
the  microscope. 

In  thin  sections 
quartz  is  colorless  and 
transparent.  Its  index 
of  refraction  is  so  near 
that  of  Canada  balsam 
that  there  is  scarcely 
any  relief,  and  the 
quartz  grains  when  free 
of  inclusions  have  the 
appearance  of  holes  in 

the  rock  sections.  In  sections  less  than  .04  mm.  in  thickness  the 
interference  is  gray  or  yellow  of  the  first  order.  It  is  often  filled 
with  small  inclusions  of  liquids  or  gas. 

As  a  secondary  mineral  quartz  may  be  formed  by  the  solvent 
action  of  percolating  waters  containing  carbon  dioxide,  decompos- 
ing complex  silicates  by  carrying  out  the  bases  in  solution,  forming 
carbonates  of  the  bases  and  quartz.  The  SiO2  may  dissolve  also, 
to  be  later  deposited  in  veins,  cracks,  and  cavities,  either  in  the 
form  of  quartz  or  as  hydra  ted  silica  (opal).  Such  quartz  is  asso- 
ciated with  many  ore  deposits  as  the  gangue  mineral  or  vein  filler ; 
it  has  a  peculiar  greasy  luster  and  splintery  fracture  and  is  often 
termed  vein  quartz. 

The  quartz  grains  of  granites  resist  atmospheric  weathering  ;  and 
when  changes  and  decomposition  of  the  other  minerals  are  in  prog- 
ress at  the  surface,  these  grains  remain  unchanged  in  the  residue, 
forming  the  sands  of  the  soil ;  or  when  washed  clean  and  redeposited 


FIG.  427.  —  Enlarged  Micropegmatite  between 
Crossed  Nicols.  The  Light  Areas  are  Quartz,  the 
Dark  Orthoclase. 


358  MINERALOGY 

by  running  water  form  the  stratified  sandstones  in  which  silica 
deposited  from  solution  may  be  the  cementing  agent. 

Varieties.  —  Rock  crystal  is  that  clear,  colorless,  crystallized 
variety  to  which  the  sciences  of  mineralogy  and  crystallography  owe 
so  much.  It  has  furnished  convenient  material  to  the  scientist 
and  physicist  for  experimentation  and  for  apparatus  since  historic 
times,  and  it  stands  in  its  relation  to  crystallography  as  the  frog 
does  to  the  biological  sciences.  It  has  indeed  furnished  the  name 
crystal,  as  the  ancients  believed  it  to  be  water  which  had  been  sub- 
jected to  such  a  low  temperature  as  to  be  no  longer  capable  of  re- 
turning to  the  liquid  state.  Nicolaus  Steno  in  1669  noted  the 
similarity  of  the  angles  between  crystal  faces  on  quartz  while  cut- 
ting sections. 

Beautiful  clear  crystals  of  quartz  occur  in  the  calcareous  sand- 
stone of  Herkimer  County,  New  York,  known  from  their  brightness 
as  Herkimer  County  diamonds.  These  are  in  some  cases  chemi- 
cally pure  SiO2,  others  are  colored  dark  with  carbonaceous  inclu- 
sions, while  others  have  cavities  containing  liquids  in  which  bubbles 
may  be  rolled  back  and  forth  like  the  bubble  of  a  spirit  level.  Evi- 
dently all  these  crystals  have,  from  the  character  of  the  inclusions, 
been  formed  from  solution  and  at  a  low  temperature.  This  variety 
of  quartz  is  in  all  cases  considered  to  have  been  formed  at  a  tempera- 
ture below  that  of  575°  C.,  for  when  quartz  is  heated  to  a  tempera- 
ture of  575°  C.,  it  passes  over  to  another  phase,  /?-quartz,  in  which 
the  physical  properties  are  different  from  a-quartz,  the  phase  stable 
below  575  C. 

Large  crystals  and  thick  sections  of  quartz  in  passing  this 
inversion  temperature  are  shattered,  crack  and  fall  in  pieces; 
/3-quartz  is  hexagonal  holoaxial,  while  a-quartz  is  trigonal  holoaxial; 
and  whenever  the  trigonal  trapezohedral  face  x  appears,  that  crystal 
must  have  been  formed  at  a  temperature  below  the  inversion  point 
and  as  a-quartz,  since  the  trigonal  trapezohedron  is  not  a  possible 
form  in  the  hexagonal  holoaxial  type ;  also  the  occurrence  of  two 
rhombohedrons  r  and  z  unequal  in  development  and  luster  would 
indicate  that  the  crystal  had  separated  as  a-quartz,  since  these  faces 
in  the  hexagonal  holoaxial  type  should  be  similar  in  all  respects. 
Quartz  of  granite  micropegmatites  and  some  macropegmatites  has 
been  formed  at  a  temperature  above  575°,  as  is  shown  by  its  frac- 
tured condition.  Large  clear  crystals  are  found  at  the  Hot  Springs, 
Arkansas ;  these  have  been  formed  in  several  stages  of  growth  as  is 
indicated  by  the  internal  crystalline  outlines  known  as  "  phantoms," 


OXIDES  359 

caused  at  this  locality  by  fine  crystals  of  chlorite  being  deposited 
on  the  crystal  at  different  periods  of  its  growth.  More  compli- 
cated crystals  are  found  in  North  Carolina,  showing  rare  forms. 

Quartz  is  quarried  in  Connecticut,  Maryland,  New  York,  Wis- 
consin, and  North  Carolina,  for  various  purposes.  When  finely 
ground  it  is  used  as  a  filler  in  paints  and  scouring  soaps.  It  is 
used  in  pottery  and  glass,  and  recently  it  has  been  fused  and  blown 
as  glass,  in  chemical  ware,  such  as  evaporating  dishes,  flasks,  cru- 
cibles, and  ignition  tubes,  for  the  determination  of  carbon ;  here  it 
has  the  advantage,  due  to  its  low  coefficient  of  expansion,  of  not 
being  liable  to  crack  when  submitted  to  sudden  changes  of  tem- 
perature ;  even  when  at  a  bright  red  heat  it  may  be  plunged  into 
cold  water  without  the  least  danger  of  cracking.  After  fusion  it 
has  lost  its  crystalline  structure  and  is  amorphous  silica,  but  on 
repeatedly  heating  and  cooling,  the  molecules  will  rearrange  them- 
selves and  become  crystalline,  and  then  the  tubes  are  liable  to 
shatter  on  passing  the  inversion  temperature  of  ft-  to  a-quartz. 

Colored  quartz.  —  Owing  to  small  quantities  of  metallic  oxides 
or  organic  matter  as  impurities  quartz  may  appear  in  various 
colors.  Some  of  the  colored  varieties  have  received  special  names, 
as  citrine  or  yellow  quartz,  which  is  clear  and  transparent,  in 
appearance  very  much  like  the  topaz,  and  indeed  when  cut,  pol- 
ished, and  mounted  in  jewelry  is  sold  in  the  trade  as  topaz,  or  false 
topaz.  The  best  examples  of  this  variety  are  obtained  from  Brazil. 
Smoky  quartz  is  a  dark  colored  variety  of  crystalline  quartz  which 
owes  its  color  to  organic  matter  or  carbon  compounds.  The  evenly 
colored  transparent  specimens  are  polished  and  valued  as  a  semi- 
precious stone.  Disentis  and  St.  Gothard,  Switzerland,  are  noted 
localities.  These  crystals  generally  have  the  faces  x  and  s  well 
developed.  In  the  United  States,  it  occurs  at  Pike's  Peak,  Colo- 
rado; in  Richmond  County,  New  York;  and  very  large  crystals 
have  been  obtained  at  Paradise  River,  Nova  Scotia. 

Amethyst  is  a  purple  or  bluish  violet  quartz  which  is  colored  with 
small  amounts  of  manganese  or  possibly  by  organic  matter.  The 
color  may  vary  greatly  ;  in  most  specimens  it  is  unevenly  distributed 
through  the  crystal,  and  is  usually  concentrated  at  the  apex.  Dark, 
evenly  colored  crystals  are  much  prized  as  a  semiprecious  stone. 
The  best  colored  specimens  are  from  Siberia,  India,  Uruguay,  and 
Brazil.  Pale  varieties  are  widely  distributed.  In  the  United 
States  amethysts  of  good  color  are  found  in  Lincoln  and  Macon 
counties,  North  Carolina;  Nelson  County,  Virginia;  Rabun 


360  MINERALOGY 

County,  Georgia;    Thunder  Bay,  Lake  Superior;    also  at  Digby 
Neck,  Nova  Scotia. 

Other  varieties  are  cryptocrystalline  or  aggregates  of  radiated, 
parallel,  or  matted  fine  fibers ;  in  all  these  the  chemical  properties 


FIG.  428.  —  Onyx  from  Brazil. 

are  the  same  as  crystalline  quartz,  but  they  are  softer  and  of  a  little 

lower  specific  gravity. 

Chalcedony  is  a  translucent,  concretionary  form  of  Si02;  usually 

light  in  color,  often  stalactitic  or  botryoidal,  occurring  as  crusts 

lining  cavities,  and  in  geodes, 
which  often  contain  large  cavi- 
ties partly  filled  with  water, 
which  may  be  seen  through 
the  translucent  wall  on  rolling 
the  specimens  back  and  forth. 
When  very  marked  in  color 
they  have  been  given  special 
names,  as  carnelian  for  the 
translucent  red  variety. 
Chrysoprase  is  light  green, 
colored  by  the  oxide  of  nickel. 

Bloodstone  is  chalcedony  with  small  inclusions  of  red  jasper.    The 

banded  varieties  are  agates,  and  when  the  bands  are  flat  and 


OXIDES  361 

regular  it  is  onyx.  At  times  the  impurities  are  dendritic  and  ap- 
pear like  pieces  of  moss  inclosed  within  the  specimen;  these  are 
moss  agates.  When  dark  in  color  and  associated  with  limestones 
in  nodules  it  forms  flint.  Lydian  or  touchstone  is  also  a  very 
dark,  almost  black,  variety,  used  by  the  goldsmiths  to  test  the 
purity  of  their  gold  alloys,  by  means  of  the  color  of  the  streak 
made  by  the  metal  in  drawing  it  across  the  stone. 

Jasper  is  an  opaque  variety  of  impure  SiO2  of  a  red,  brown  ocher, 
gray,  green,  or  black  color. 

In  addition  there  are  pseudomorphs  of  SiO2  after  shells,  wood 
(silicified  wood),  or  bones ;  and  various  minerals,  as  carbonates  and 
sulphates,  may  be  replaced  by  SiO2  from  solution. 

TRIDYMITE 

Tridymite.  —  Silicon  dioxide,  Si02 ;  Hexagonal,  Hexagonal 
above  130°  C.,  below  probably  Orthorhombic ;  6  =  1.653;  0001  A 
1011  =  62°  21';  Common  forms,  c  (0001),  m  (10lO)2  a  (1120), 
p(1011);  Twinning  plane  1016  and  3034 ;  Cleavage,  1010  distinct ; ; 
Brittle,  fracture  conchoidal ;  H.  =  7;  G.  =  2.28-2.33 ;  Colorless 
to  white;  Luster,  vitreous ;  Streak,  white ;  €  =  1.477. 

B.B.  —  Like  quartz,  but  soluble  in  boiling  alkaline  carbonates. 
Fuses  at  1625°  C. 

General  Description.  —  Crystals  are  small  hexagonal  tablets, 
combinations  of  the  base,  prism,  and  pyramid,  or  aggregations  of 
these  small  scaly  crystals.  At  ordinary  temperatures  tridymite  is 
pseudo-hexagonal ;  above  130°  it  becomes  truly  uniaxial,  and  below 
it  shows  low  double  refraction  (.0018),  is  optically  (+),  and  is  prob- 
ably orthorhombic  in  symmetry.  It  is  not  a  common  mineral,  but 
found  in  acid  volcanic  rocks  and  lavas,  as  in  the  lavas  of  Vesuvius 
and  Krakatau,  Obsidian  Cliff,  Yellowstone  Park,  and  in  some 
meteorites.  It  was  discovered  at  Pachuca,  Mexico,  where  it  oc- 
curs in  aggregates  of  twins ;  in  the  cavities  of  an  andesine  rock.  It 
has  since  been  discovered  in  many  localities  connected  especially 
with,  and  in  cavities  of,  the  more  recent  acid  volcanic  rocks. 

Tridymite  and  quartz  are  polymers  of  SiO2.  The  transition 
temperature  between  the  two  is  near  800°  C.,  quartz  being  the  more 
stable  phase  below,  and  tridymite  the  more  stable  phase  above  that 
temperature.  Artificially,  tridymite  has  been  formed  both  in 
solution  and  in  dry  fusion.  When  a  silicate  is  dissolved  in  the  salt 


362  MINERALOGY 

of  phosphorus  bead,  the  silica  left  behind  as  the  silica  skeleton  is  in 
the  form  of  tridymite. 

HYDRATED  OXIDES 
BRUCITE 

Brucite.  —  Hydroxide  of  magnesium,  Mg(OH)2 ;  MgO  = 
69,  H2O  =  31 ;  Hexagonal ;  Type,  Dihexagonal  Alternating ; 
c  =  1.208 ;  r  A  r'  =  97°  38' ;  0001  A  lOTl  =  60°  20' ;  Common  forms, 
c  (0001),  r  (lOll),  p  (2021) ;  Cleavage,  basal  perfect,  laminae  sectile ; 
H.  =  2.5;  G.  =  2.4;  Color,  white,  greenish  or  bluish;  Streak, 
white;  Luster,  waxy  to  pearly;  Translucent  to  opaque;  €  = 
1.579;  <o  =  1.559;  €-<o  =  .020;  Optically  (+). 

B.B. —  Whitens  and  becomes  opaque ;  when  it  contains  Mn 
or  Fe  may  change  to  gray  on  heating.  After  ignition  reacts  alka- 
line with  turmeric  paper;  with  cobalt  solution  becomes  flesh- 
colored  (Mg).  Infusible;  in  the  closed  tube  yields  much  water. 
Soluble  in  acids. 

General  description.  —  Crystals  are  rare ;  they  occur  as  tabular 
combinations  of  the  base  and  rhombohedrons  at  Texas,  Pennsyl- 
vania. Brucite  is  usually  foliated  or  fibrous,  variety  nemalite. 

Brucite  is  a  secondary  mineral  formed  in  the  zone  of  hydra- 
tion  from  magnesium  silicates  and  is  nearly  always  associated 
with  serpentine,  both  being  derived  from  a  common  origin. 
By  the  action  of  carbon  dioxide  brucite  forms  hydromagnesite, 
3  MgCO2 .  Mg(OH)2 .  3H2O. 

It  occurs  at  the  Tilly  Foster  mine,  Brewster,  New  York ;  Rich- 
mond and  Westchester  Counties,  New  York.  At  Bergen  Hill, 
New  Jersey,  fibrous.  Artificial  crystals  are  produced  by  precipi- 
tating MgCl2  with  KOH  and  heating  to  200°  under  pressure; 
on  cooling,  brucite  separates.  It  has  also  been  noted  as  a  constit- 
uent of  boiler  scale. 

Pyrochroite,  Mn(OH)2,  is  isomorphous  with  brucite;  found  in 
simple  rhombohedrons  and  in  combinations  with  the  base  and  scale- 
nohedrons  at  Franklin  Furnace,  New  Jersey.  Crystals  are  nearly 
transparent  and  light  in  color  when  first  brought  to  the  surface, 
but  soon  darken  and  become  opaque  on  exposure.  Found  also  in 
several  localities  in  Sweden. 


OXIDES 


363 


IRON  HYDROXIDES 

Limonite.  —  Hydroxide  of  iron ;  Brown  hematite ;  Bog  iron 
ore ;  2  Fe2O3 .  3  H2O ;  Fe2O3  =  85.5,  H2O  =  14.5 ;  Amorphous ; 
Brittle;  H.  =  5-5.5;  G.  =  3.6-4;  Color,  dark  brown  to  ocher- 
yellow ;  Luster,  submetallic  to  dull ;  Streak,  ocher-yellow. 

B.B.  —  Like  hematite,  but  yields  water  in  the  closed  tube. 

General  description.  —  All  the  varieties  of  hydrated  oxides  of 
iron  are  colloids  with  no  constant  chemical  composition;  their 
water  content  varies  with 
their  state  of  dehydration, 
with  the  one  exception  of 
gothite,  FeO(OH),  which 
crystallizes  in  the  ortho- 
rhombic  system.  They  are 
all  secondary  minerals  pro- 
duced by  oxidation  and  hy- 
dration  in  the  belt  of  weath- 
ering ;  where  deposited  from 
solution  in  caves  and  cavi- 
ties they  form  stalactites, 
botryoidal  masses,  and 
crusts,  with  a  fibrous  or 
radiated  structure  which  at 
times  have  a  very  character- 
istic surface  as  if  varnished, 
or  as  if  having  been  fused, 
and  in  some  specimens 
beautifully  iridescent. 

In  the  oxidation  of  sulphides,  as  pyrite  or  marcasite,  ferrous 
sulphate  is  formed  and  carried  in  solution,  while  a  hydrated  oxide 
of  iron  remains  behind,  as  the  gossans,  capping  many  of  the  sulphide 
ore  deposits  of  the  West.  Many  crystals  of  pyrite  have  been  re- 
placed in  this  way  by  pseudomorphs  of  limonite  and  turgite 
(2Fe2O3 .  2  H2O) .  Large  specimens  of  thia  nature  are  obtained  from 
Australia,  and  small  crystals,  though  perfect  in  outline,  are  found 
near  Lancaster,  Pennsylvania.  Beautiful  pseudomorphs  of  limo- 
nite after  marcasite  occur  in  a  clay  in  Richland  County,  Wiscon- 
sin. Many  pyrite  crystals  in  Virginia  and  Maryland  have  a 
coat  of  varying  thickness  of  limonite  enclosing  the  still  unoxidized 
sulphide. 


FIG.  430.  — Limonite  Stalactite.     Ore  Hill, 
Connecticut. 


364  MINERALOGY 

The  waters  of  swamps  and  some  springs  carry  ferrous  salts  in 
solution,  either  connected  with  organic  acids  or  as  carbonate  or 
sulphate.  When  exposed  to  the  air  these  salts  oxidize  and  the  iron 


FIG.  431.  —  Limonite  Pseudomorph  after  Marcasite  from  Richland  County, 

Wisconsin. 

is  separated  as  ferric  hydrate,  which  may  be  noted  many  times 
as  an  iridescent  oil-like  film  on  the  surface  of  stagnant  water  of 
swamps.  When  the  surface  is  agitated,  the  film  of  heavy  oxides 
sinks,  and  constantly  accumulating'on  the  bottom  they  form  the  lake 
and  swamp  deposits  of  "  bog  iron  ore."  It  is  not  necessary  to  add  that 
such  ore  is  usually  impure,  from  the  nature  of  its  formation;  it 
is  mixed  with  sand  and  organic  matter  and  often  contains  consid- 
erable phosphorus  and  sulphur.  Numerous  iron  springs  are  so 
charged  with  iron  in  solution  that  their  banks  and  beds  become 
coated  with  a  bright  yellow  gelatinous  deposit  of  ferric  hydrate. 
Limonite  is  found  in  clays  and  as  a  residual  deposit  of  ferruginous 
limestones,  the  calcium  carbonate  having  been  removed  in  solu- 
tion, while  the  ferrous  carbonate  was  oxidized  and  remained  in 
place  and  concentrated  as  the  chemical  reactions  continued.  Such 
limonites  associated  with  limestones  are  found  along  the  Blue 
Ridge  in  western  Virginia  and  in  Tennessee,  where  they  are  mined 


OXIDES  365 

as  an  iron  ore.  The  hydrated  ores  of  iron  mined  in  the  United 
States  are  eight  or  ten  per  cent,  of  the  total  product.  They  are  mined 
in  Alabama,  Virginia,  Tennessee,  Georgia,  and  Connecticut;  at 
Salisbury  in  the  latter  state  beautiful  stalactitic  specimens  are 
obtained.  In  the  West  it  is  used  as  a  flux  in  the  smelters.  Large 
quantities  of  limonite  are  used  as  a  cheap  metallic  paint  and  as  a 
pigment  in  the  coloring  of  mortars,  as  ocher,  umber,  and  sienna ; 
after  the  water  is  driven  out  by  heat  they  form  burnt  umber  or 
burnt  sienna. 

HYDRATED  OXIDES  OF  ALUMINIUM 
DIASPORE 

Diaspore.  —  AIO(OH) ;  A1203  =  85.0,  H2O  =  15.0  ;  Ortho- 
rhombic  ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c  =  .9372  :  1 :  .6038 ; 
100  A  110  =  43°  9';  001  A  101  =  32°  48';  001 A  Oil  =  31°  7' 
35";  Common  forms,  c  (001) ;  b  (010),  m  (110),  h  (210) ;  Cleav- 
age, brachypinacoidal  eminent,  h  less  so;  Brittle,  fracture  con- 
choidal ;  H.  =  6.5-7 ;  G.  =  3.3-3.5 ;  Color,  white,  yellowish,  gray 
to  brown ;  Streak,  white ;  Luster,  brilliant  to  pearly  on  cleavage 
faces;  a  =  .702;  -y  =  1.750;  a--y  =  .048;  Optically  (+) ;  Axial 
plane  =  010 ;  Bxa  =  a. 

B.B.  —  Most  specimens  decrepitate  and  yield  water  in  the 
closed  tube;  after  ignition  with  cobalt  solution  becomes  blue. 
Infusible ;  after  ignition,  soluble  in  H2SO4,  otherwise  not  attacked 
by  acids. 

General  Description.  —  When  crystalline  it  is  tabular  or  in 
scales  and  flakes;  also  acicular  with  striations  parallel  to  the  c 
axis.  More  often  massive,  encrusted  or  stalactitic.  Generally 
associated  with  corundum  as  in  Macon  County,  North  Carolina, 
and  especially  with  the  impure  form  emery,  as  at  Chester,  Massa- 
chusetts, and  in  Chester  County,  Pennsylvania.  It  is  also  found 
with  chloritic  schists  and  in  dolomites.  As  a  decomposition 
product  it  may  be  formed  by  the  weathering  of  corundum,  nephelite 
and  sodalite,  or  the  reverse  of  this  is  also  true,  and  diaspore  by 
dehydration  forms  corundum. 

GIBBSITE  ' 

Gibbsite.  — -  Hydrargillite ;  Hydroxide  of  aluminium,  A1(OH)3 ; 
A12O3  =  65.4,  H2O  =  34.6;  Monoclinic;  Type,  Equatorial; 


366  MINERALOGY 

a  :  b:c  =  1.7089:1:1.9184;  P  =  85°  29'  =  100A110;  001 A 101 
=  50°  50';  001  A  Oil  =  62°  24' ;  Common  forms,  c  (001),  a  (100), 
m  (110) ;  Cleavage,  basal,  micaceous  laminae  tough  and  wills  how 
percussion  figures  as  in  mica;  H.  =  2.5-3.5;  G.  =  2.28-2.42; 
Color,  white,  green,  gray,  or  reddish;  Streak,  white;  a  =  1.5347 
Y  =  1.5577;  -y-a  =  .023;  Optically  (+) ;  fix*  in  the  angle  p, 
20°  57'  from  c ;  Axial  plane  =  010. 

B.B.  —  Like  diaspore,  except  it  is  soluble  in  H2SO4  and  in  potassa 
solution. 

General  description.  —  Hydrargillite,  the  crystalline  form,  is 
rare;  it  occurs  in  tabular  habit,  parallel  to  the  base  and  with  a 
hexagonal  outline ;  more  often  botryoidal,  stalactitic,  or  in  crusts, 
as  in  Dutchess  and  Orange  counties,  New  York,  and  at  Richmond, 
Massachusetts. 

Gibbsite  is  not  a  common  mineral  and  occurs  under  the  same  con- 
ditions and  associations  as  diaspore. 

Artificial  scaly  crystals  may  be  prepared  by  passing  CO2  through 
a  hot  alkaline  solution  of  aluminium  hydroxide. 

BAUXITE 

Bauxite.  —  A12O3.  2H20  ;  A12O3.  =  73.9,  H2O  =  26.1,  but  vari- 
able ;  Amorphous ;  H.  =  1-3 ;  G.  =  2.4-2.55 ;  Color,  white,  gray, 
when  iron  is  present  yellowish  or  red ;  Streak,  white ;  Luster,  dull 
to  earthy ;  Opaque. 

B.B  —  Infusible,  yields  water  in  the  closed  tube,  and  is  little 
attacked  by  acids;  otherwise  like  diaspore. 

General  description.  —  In  large  deposits  it  is  claylike,  or 
oolitic  and  porous ;  usually  brown  or  red  from  iron.  It  was  orig- 
inally described  from  Baux,  France,  where  it  was  found  in  dis- 
seminated grains  in  a  compact  limestone.  Its  origin  has  been 
attributed  to  the  direct  weathering  of  basalt-like  rocks ;  this  seems 
to  be  certainly  so  in  some  German  localities;  from  rhyolites; 
or  again,  as  in  case  of  the  Arkansas  bauxites,  connected  with  the 
decomposition  of  syenites.  Other  bauxites  have  been  formed  as  a 
chemical  precipitate,  when  hot  ascending  waters  have  brought 
solutions  of  alumina  to  the  surface,  where  they  have  been  precip- 
itated by  calcium  or  other  carbonates. 

The  bauxite  deposits  of  the  United  States  are  found  in  Ala- 
bama, Georgia,  and  Arkansas,  where  it  is  mined  in  large  quantities. 


OXIDES 


367 


Commercially  it  is  the  source  of  all  the  metallic  aluminium,  which 
is  reduced,  after  purification,  electrolytically  from  its  solution  in 
fused  cryolite.  While  bauxite  has  been  described  as  a  separate 
species,  it  may  be  only  a  mixture  of  the  other  two  hydroxides  of 
aluminium,  diaspore  and  gibbsite,  as  its  composition  is  variable. 
By  metamorphism  bauxite  forms  corundum;  in  fact  corundum 
wheels  are  now  made  from  a  product  obtained  by  heating  bauxite 
to  a  high  temperature. 

MANGANESE  HYDROXIDES 

MANGANITE' 

Manganite.  —  MnO(OH) ;  Mn203  =  89.7,  H2O  =  10.3 ;  Ortho- 
rhombic  ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c  =  .844  : 1 :  .545 ; 
100  A 110  =  40°  10' ;  001  A  101  =  32°  50' ;  Common  forms,  c  (001), 


FIG.  432.  —  Manganite  from  the  Harz,  Germany. 

m  (110),  1  (120),  d  (210) ;  Twinning  plane  e  (Oil)  common;  Cleav- 
age, brachypinacoidal  perfect,  m  imperfect;  Brittle,  fracture 
uneven ;  H.  =  4 ;  G.  =  4.2  —  4.4 ;  Luster,  submetallic  though 
bright ;  Color,  dark  steel  gray  ;  Streak,  brown  to  black ;  Opaque. 

B.B.  —  Infusible.  In  the  closed  tube  yields  much  water. 
Dissolves  in  HC1,  liberating  chlorine;  with  the  fluxes  reacts  for 
manganese. 

General  description.  —  Nearly  always  crystalline ;  in  habit 
elongated  parallel  to  the  vertical  axis  with  deep  longitudinal  stria- 


368  MINERALOGY 

tions  giving  the  crystals  the  appearance  of  parallel  growths  or  com- 
plex aggregates ;  also  acicular,  radiated,  fan-shaped  or  in  the  form 
of  dendrites. 

Beautiful  specimens  are  obtained  in  the  Harz,  Germany,  where 
it  is  associated  with  calcite.  Manganite  is  a  secondary  mineral 
formed  much  in  the  same  way  as  limonite.  Manganese  is  dissolved 
out  of  the  igneous  or  sedimentary  rocks  by  percolating  waters  and 
is  carried  in  solution  to  be  precipitated  either  as  carbonate  or  oxide. 
It  is  therefore  a  vein  mineral,  where  it  is  associated  with  calcite, 
quartz,  and  iron  oxides,  or  it  is  found  in  nodules  in  clays  and  sedi- 
mentary deposits.  By  dehydration  manganite  passes  into  pyro- 
lusite,  which  often  retains  the  form  of  manganite.  Small  equidi- 
mensional  crystals  of  manganite  are  found  at  Salisbury,  Connect- 
icut, associated  with  the  limonite.  It  also  occurs  in  the  Lake 
Superior  region;  in  Douglas  County,  Colorado.  For  use  see 
pyrolusite. 

Pyrochroite,  Mn  (OH)2,  is  isomorphous  with  brucit'e,  see  page  362. 

PSILOMELANE 

Psilomelane  is  an  amorphous  mixture  of  various  hydrated  oxides 
of  manganese  and  therefore  varies  greatly  in  its  composition.  It 
generally  contains  barium  oxide.  One  variety,  asbolite,  contains 
cobalt,  and  another,  lampadite,  contains  copper. 

Amorphous,  botryoidal,  stalactitic,  reniform,  or  in  crusts; 
Color,  black  to  steel-gray ;  Luster,  submetallic  to  dull  and  earthy ; 
Streak,  brown  to  black ;  Opaque ;  H.  =  5.6,  earthy  varieties  are 
soft,  soiling  the  fingers ;  G.  =  3.7-4.7. 

B.B.  —  Like  manganite,  but  usually  contains  barium  or  other 
elements  isomorphous  with  barium. 

General  description.  —  Psilomelane  is  associated  with  limonite 
and  they  are  both,  formed  by  the  same  agencies  and  under  the  same 
conditions,  so  much  so  that  all  limonites  will  yield  a  qualitative 
test  for  manganese,  and  in  some  cases  the  amount  of  manganese 
may  be  considerable. 

Wad  is  the  soft  earthy  form ;  it  is  also  known  as  bog  manganese 
ore,  as  it  is  deposited  from  solution  in  many  streams  and  marshes. 
The  pebbles  in  many  streams  are  covered  with  a  soft,  velvety  coat- 
ing of  amorphous  hydroxide  of  manganese.  These  oxides  also 
occur  as  nodules  on  the  floor  of  the  ocean,  where  the  oxides  have 
accumulated  around  some  object,  as  a  bone  or  shark  tooth,  as  a 


OXIDES 


369 


nucleus.     Nodular  manganese  oxides  occur  in  clays  and  other 
sedimentary  formations. 

Psilomelane  occurs  at  Brandon,  Vermont,  and  at  several  points 


FIG.  433.  —  Psilomelane  from  Langenberg,  Saxony. 

in  Arkansas.     Asbolite  is  found  at  Silver  Bluff,  South  Carolina. 

Lampadite,  as  well  as  wad,  occurs  in  the  Copper  Queen  mine, 
Arizona. 

OPAL 

Opal.  —  SiO2 .  n  (H2O) ;  Amorphous ;  The  amount  of  water 
varies  according  to  the  dehydration;  Brittle,  fracture  con- 
choidal;  H.  =  5.5-6.5;  G.  =  1.9-2.3;  Color,  all  colors;  Luster, 
vitreous  to  resinous ;  Streak,  white ;  Transparent  to  nearly  opaque  ; 
n  =  1.436-1.450.  In  thin  sections  isotropic  and  colorless,  fill- 
ing small  cracks  and  as  crusts  in  amygdaloidal  cavities. 

B.B.  —  Like  quartz,  but  yields  water  in  the  closed  tube; 
in  some  specimens  this  may  be  small  in  amount.  Soluble  in  boil- 
ing alkali  solution. 

General  description.  —  Opal  is  gelatinous  or  colloidal  silica, 
deposited  from  solutions.  Percolating  waters  decompose  many 
silicates,  especially  the  orthosilicates,  the  silica  of  which  may  be 
again  deposited  either  as  quartz  or  opal.  If  there  are  alkalies  pres- 
ent in  the  solution,  quartz  is  usually  the  mineral  formed ;  if  alka- 
lies are  absent,  opal  is  the  form  in  which  the  silica  is  deposited. 
Thus  opal  is  connected  as  a  secondary  component  of  lavas  and  sedi- 

2B 


370  MINERALOGY 

mentary  rocks,  especially  in  the  cavities  of  argillites  and  limestones ; 
also  as  a  deposit  around  hot  springs  and  geysers.  In  the  filling  of 
cavities  from  which  other  minerals  or  fossils  have  been  removed 
by  solution,  opal  appears  as  pseudomorphs,  especially  after  wood, 
as  wood  opal,  where  even  the  structure  of  the  wood  fiber  has  been 
replaced,  step  by  step,  and  so  faithfully  that  in  some  cases  the 
species  of  wood  may  be  microscopically  identified. 

Hyalite  is  a  clear,  colorless,  botryoidal  opal,  having  the  appear- 
ance of  frogs'  eggs,  from  which  it  takes  its  name;  it  is  a  very  dense 
opal,  showing  double  refraction  from  internal  stress.  The  best 
specimens  are  obtained  at  Waltsch,  Bohemia. 

Precious  opal  is  a  transparent  to  translucent  variety  yielding  a 
play  of  colors  or  flashes  of  light,  as  of  fire,  which  have  been  attrib- 
uted to  minute  fractures  formed  as  the  silica  lost  water.  The 
thin  films  of  air  in  the  cracks  disperse  the  light,  yielding  the  beau- 


FIG.  434.  —  Opal  Var.  Hyalite  from  Waltsch,  Bohemia. 

tiful  play  of  colors  which  is  so  much  admired  in  the  opal  as  a  gem. 
The  best  gem  opals  were  originally  obtained  in  Hungary,  where 
they  were  found  filling  cracks  and  small  cavities  in  an  andesite. 
More  recently  opals  with  a  good  play  of  color  have  been  found  in 
Queensland,  Australia ;  and  a  fire  opal  of  a  honey-yellow  color  is 
found  at  Queretaro,  Mexico,  in  a  rhyolite;  these  opals  are  jelly- 
like  and  lack  the  fire  of  the  Austrian  or  Australian  specimens. 
Geyserite  is  an  opaque  white,  but  very  porous  opal,  deposited 


OXIDES  371 

by  the  waters  of  hot  springs  and  geysers.  Large  quantities  of 
geyserite  are  being  deposited  by  the  geysers  and  hot  springs  of 
the  Yellowstone  Park,  and  in  New  Zealand  and  Iceland. 

Tripolite,  or  infusorial  earth,  is  an  opal-like  silica  secreted  by 
organisms,  as  the  shells  of  diatoms.  They  may  form  beds  of  con- 
siderable extent ;  such  deposits  are  found  in  Maryland,  Virginia, 
Alabama,  Missouri,  and  California. 

Tripolite  is  used  commercially  as  a  wood  filler ;  as  an  abrasive ; 
as  a  filter  stone ;  in  scouring  soap ;  as  a  polishing  powder ;  as  a  non- 
conductor of  heat  for  packing  steam  pipes  and  boilers ;  in  the  manu- 
facture of  dynamite,  as  a  holder  for  the  nitroglycerine ;  and  in 
California  as  a  building  stone. 

Artificially  opal  is  formed  by  the  drying  of  gelatinous  silica. 

SPINEL  GROUP 

The  spinels  form  a  small,  well-defined  group  of  isometric  minerals, 
crystallizing  in  the  ditesseral  central  type  with  an  octahedral  habit, 
or  in  combinations  of  the  octahedron,  rhombic  dodecahedron,  and 
the  cube,  while  the  cube  with  rare  exceptions  is  a  subordinate  form. 
Chemically  they  are  salts  of  the  general  acid  HR"'O2,  in  which  R'" 
may  be  Al,  Fe'",  Cr,  Mn'",  or  mixtures  of  these,  and  the  H  may  be 
replaced  by  Mg,  Fe",  Zn,  Mn",  or  Be ;  in  case  of  the  latter  in  com- 
bination with  Al  the  mineral  chrysoberyl  is  formed,  the  only 
mineral  of  the  series  which  is  not  isometric,  but  orthorhombic. 
They  are  aluminites,  ferrites,  manganites,  or  chromites,  and  have 
usually  been  classified  with  the  oxides.  They  may  be  primary  con- 
stituents of  the  igneous  rocks,  in  which,  when  they  occur,  they  are 
among  the  first  minerals  to  crystallize.  They  may  also  be  formed 
as  the  results  of  thermal  metamorphism  or  be  derived  from  the 
decomposition  of  other  minerals  as  precipitates. 

Spinel,  MgAl204.  Hercynite,  FeAl2O4. 

Gahnite,  ZnAl2O4.  Magnetite,  FeFe2O4. 

Manganoferrite,  MgFe204. 
Franklinite  (Fe .  Zn .  Mn)  (Fe,  Mn)2O4. 
Jacobsite,  (Mn .  Mg)  (Fe .  Mn)2O4. 
Chromite,  FeCr2O4. 

SPINEL 

Spinel.  —  Magnesium  metaluminite,  MgAl2O4 ;  MgO  =  28.2, 
A12O3  =  71.8;  Crystal  forms,  o(lll),  d  (110),  other  forms,  (001), 


372  MINERALOGY 

(311),  (322),  (221),  (332)  rare;  Twinning  plane,  111;  Cleavage, 
octahedral  imperfect ;  Brittle,  fracture  conchoidal ;  Streak,  white ; 
H.  =  8;  G.  =  3.5-4.1;  Luster,  vitreous;  Color,  gray,  brown  to 
black  in  opaque  specimens,  in  transparent,  red,  blue,  pink,  green 
yellow;  n  =  1.715. 

B.B.  —  The  light-colored  varieties  may  change  color ;  infusible  ; 
insoluble  in  acids ;  in  fine  powder  becomes  blue  with  cobalt  solu- 
tion. 

General  description.  —  Crystals  always  well  formed,  of  octahe- 
dral habit,  in  combination  with  the  rhombic  dodecahedron  and 
cube ;  other  forms  are  rare.  Pure  spinel  is  white  or  light  in  color, 
but  as  some  of  the  magnesium  is  always  replaced  with  other  metals 
of  the  isomorphous  group,  the  various  colors  are  due  to  these  and  to 
impurities.  In  sections  always  pale  in  color,  and  owing  to  its  high 
index  of  refraction  the  surface  is  rough,  showing  a  high  relief.  Crys- 
tals are  well  formed  with  good  outlines  or  in  rounded  grains,  but 
always  fresh  and  free  from  decomposition.  Some  crystals  have 
the  appearance  of  having  been  fused,  with  a  glazed  surface  and 
rounded  edges ;  this  is  a  characteristic  of  all  the  species  of  the  group. 

Spinel  may  occur  as  a  primary  accessory  mineral  in  igneous  rocks, 
especially  those  rocks  rich  in  magnesium  or  aluminium ;  often  as 
picotite,  a  variety  in  which  ferrous  iron  replaces  the  magnesium,  and 
chromium  some  of  the  aluminium.  Spinel  is  often  connected  with 
contact  metamorphism  and  occurs  in  a  granular  limestone,  from 
Sparta  in  northern  New  Jersey  to  Amity  in  New  York ;  these  are 
well-formed  gray  and  brown  crystals  associated  with  serpentine. 

In  North  Carolina  it  occurs  with  corundum,  from  which  it  may 
be  formed  as  a  decomposition  product.  It  may  also  result  from 
the  alteration  of  oli vine,  garnets,  or  other  minerals  rich  in  magne- 
sium or  aluminium. 

Ruby  spinel,  so  called  from  its  resemblance  in  color  to  the  true 
ruby,  and  is  nearly  as  hard,  is  used  as  a  gem.  The  gem  material1  is 
associated  with  corundums  in  the  gem-bearing  gravels  of  Ceylon  and 
Burma ;  of  the  various  colors  the  "  flame-red  "  and  blue  are  the 
most  sought  after.  Ruby  spinels  occur  also  at  Gold  Bluff,  Cali- 
fornia. 

Artificial  spinels  may  be  formed  by  fusing  the  oxides  or  fluorides 
of  magnesium  and  aluminium  with  boric  acid;  various  colors 
may  be  produced  by  the  addition  of  small  amounts  of  iron  and 
chromium  oxides. 


OXIDES  373 

MAGNETITE 

Magnetite.  —  A  ferrous  ferrite,  FeFe2O4 ;  Fe2O3  =  69.0,  FeO2 
=  31.0;  Isometric;  Type,  Ditesseral  Central;  Common  forms, 
o  (111),  d  (110),  other  rare  forms,  (001),  (311),  (531),  (310) ;  Twin- 
ning parallel  to  111;  Cleavage,  octahedral  imperfect;  Brittle, 
fracture  uneven;  H.  =  5.5-6.5;  G.  =  5.17-5.18;  Color,  iron- 
black  ;  Streak,  black ;  Luster,  metallic  splendent  or  dull ;  Natu- 
rally magnetic. 

B.B.  —  Fuses  with  difficulty  (1225°  C.),  and  in  a  strong  0.  F. 
changes  to  red  (Fe203)  and  loses  its  magnetism.  In  fine  powder  is 
soluble  in  hot  concentrated  HC1;  but  if  it  contains  titanium  or 
magnesium,  it  may  dissolve  with  difficulty.  Insoluble  in  HN03. 

General  description.  —  Crystals  are  octahedral  in  habit,  with 
the  edges  replaced  by  the  rhombic  dodecahedron ;  other  forms  are 
rare,  as  is  also  the  rhombic  dodecahedral  habit.     Some  simple 
rhombic     dodecahedrons 
are    found    at    the    Tilly 
Foster  mine,   New  York. 
In  twinning  magnetite  fol- 
lows  the    spinel    law,    in 
which  the  twinning  plane 
is  parallel  to  an  octahedral 
face,  and  when  this  is  re- 
peated   polysynthetically, 

*;   /   J  ,  «   1    j        FlG-  435.  —  Magnetite  Crystals  from  the  Tilly 

the    Crystals    are     Striated  Foster  Mine,  Brewster,  New  York. 

parallel  to  an  octahedral 

edge.  Large  deposits  of  magnetite  are  usually  granular,  coarse  or 
fine;  or  massive  with  a  parting  or  laminated  structure.  Its 
natural  magnetism  serves  to  distinguish  it  from  all  other  black 
minerals  with  the  exception  of  franklinite,  some  specimens  of  which 
are  quite  as  magnetic  as  magnetite. 

In  rock  sections  when  well  crystallized  magnetite  appears  either 
square  or  with  six-sided  outlines,  and  is  always  opaque ;  also  as 
rounded  grains,  irregular  masses,  or  very  fine  disseminated  opaque 
specks. 

In  rock  magmas  when  there  is  an  excess  of  iron  over  silica  to  form 
metasilicates  this  excess  usually  separates  as  magnetite.  Magnetite 
is  one  of  the  first  minerals  to  separate  on  consolidation.  It,  how- 
ever, may  contain  as  inclusions  both  zircons  and  apatite,,  earlier 


374  MINERALOGY 

products  of  crystallization,  but  less  often  will  it  contain  silicates. 
It  may  be  found  as  inclusions  in  most  of  the  silicates  and  quartz. 
Large  deposits  of  magnetite  have  resulted  from  this  magmatic 
differentiation,  as  in  the  Kirunavaara  district  of  Sweden,  where 
one  ridge  of  magnetite  has  been  estimated  to  contain  740  millions 
of  tons.  The  value  of  magnetite  as  an  iron  ore  and  in  the  produc- 
tion of  steel  depends  largely  upon  the  impurities  in  the  form  of 
sulphur  and  phosphorus  which  it  may  contain,  even  though  these 
are  present  in  only  a  fraction  of  a  per  cent.  The  sulphur  may  be 
oxidized  in  the  Bessemer  process  and  carried  off  in  the  slag.  The 
phosphorus  is  not  so  easily  handled,  and  very  little  renders  the  ore 
less  fit  for  the  production  of  steel  by  this  process.  When  titanium 
is  present  to  the  amount  of  several  per  cent,  the  blast  furnace  is  in 
danger  of  freezing,  owing  to  the  high  fusing  point  of  the  slag. 

Magnetite  deposits  in  the  Eastern  states  extend  from  northern 
New  York  through  the  intervening  states  to  Alabama.  Those 
deposits  connected  with  the  basic  igneous  rocks  are  generally  ti- 


FIG.  436.  —  Magnetite  twinned  after  the  Spinel  Law,  from  Brewster,  New  York. 

taniferous,  while  those  connected  with  the  gneisses  and  limestones 
are  non-titaniferous  and  are  therefore  more  valuable  to  the  iron 
industry.  Magnetite  in  small  amounts  is  very  widely  distributed 
as  a  secondary  product  derived  from  the  alteration  and  oxidation  of 
such  minerals  as  pyrite,  siderite,  garnet,  augite,  olivine,  amphibole, 
or  biotite.  Owing  to  its  high  specific  gravity  and  proneness  to 
decomposition  or  solution,  it  remains  behind  as  a  component  of 
the  black  sands  arising  from  the  weathering  of  metamorphic  and 
igneous  rocks. 

Lodestone  is  a  variety  of  magnetite  which  is  a  natural  magnet, 
showing  polarity;  good  specimens  are  found  at  Magnet  Cove, 
Arkansas.  It  is  thought  that  the  unknown  people  of  the  Southwest, 


OXIDES 


375 


precursors  of  the  American  Indian,  must  have  used  this  natural 
magnet  to  orient  their  temples,  as  they  are  all  placed  parallel  to  the 
magnetic  meridian.  Magnetite  may  be  formed  artificially  by  the 
fusion  of  an  iron  silicate  with  lime.  A  large  number  of  basic  rocks 
fused  in  this  way  will  yield  magnetite.  The  black  scales  formed 
on  red-hot  iron  when  cooling  are  magnetite. 

FRANKLINITE 

Franklinite.  —  (Fe  .  Mn .  Zn)  (Fe  .  Mn)204 ;  Isometric ;  Type, 
Ditesseral  Central;  Common  forms,  o  (111),  d  (110),  other  forms 
rare;  Cleavage,  octahedral  parting;  Brittle,  fracture  conchoidal 
to  uneven ;  H.  =  5.5-6.5  ;  G.  =  5.07-5.22 ; 
brownish  black ;  Streak,  brown  to  black 
Opaque. 


•Color,  iron-black  to 
Luster,    metallic ; 


FIG.  437.  — Crystals  of  Franklinite  in  Calcite.     Franklin,  New  Jersey. 

B.B.  —  Infusible ;  fused  with  soda  in  the  O.  F.  shows  green  so- 
dium manganate.  Fused  with  borax  and  soda  in  the  R.  F.  on  coal 
yields  a  zinc  coat.  Slowly  soluble  in  hot  HC1.  Some  specimens 
are  quite  magnetic. 

General  description.  —  Octahedral  in  habit  with  the  edges 
rounded  as  if  fused,  or  truncated  with  the  rhombic  dodecahedron ; 


376  MINERALOGY 

other  forms  as  the  cube,  tetragonal  and  trigonal  trisoctahedrons, 
occur  but  are  very  rare ;  also  granular  or  massive. 

The  only  deposit  of  franklinite  of  any  importance  is  at  Franklin 
and  Stirling  Hill,  New  Jersey,  where  it  occurs  in  lens-shaped 
masses,  in  a  crystalline  limestone,  and  associated  with  willemite  and 
zincite.  This  deposit  has  been  worked  for  years.  After  the  separa- 
tion of  the  willemite  and  zincite  mechanically,  the  zinc  is  volatilized 
and  collected  either  as  oxide  or  spelter  as  the  case  may  be ;  the  resid- 
ual, iron  and  manga*nese,  is  reduced  to  •"  spiegeleisen,"  an  alloy 
used  in  the  Bessemer  steel  process. 

CHROMITE 

Chromite.  —  Ferrous  chromite,  FeCr204  ;  FeO  =  32.0,  Cr203 
=  68,0;  Isometric;  Type,  Ditesseral  Central;  Common  forms, 
o(lll),  d(110);  Cleavage,  none;  Brittle,  fracture  uneven; 
H.  =  5.5;  G.  =  4.32-4.57;  Color,  brownish  black  to  gray  or 
yellowish ;  Luster,  metallic  to  submetallic,  dull  or  greasy ;  Streak, 
brown  to  grayish  brown ;  Opaque. 

B.B.  —  Infusible  ;  with  borax  in  O.  F.  yields  an  emerald  green 
bead,  which  treated  with  KNO3  in  O.  F.  and  dissolved  in  water 
and  several  drops  of  acetic  acid  yields  a  yellow  precipitate  with 
lead  acetate  (PbCr04).  Insoluble  in  acids,  but  easily  decomposed 
when  fused  with  sodium  peroxide. 

General  description.  —  Crystals  are  octahedral  in  habit ;  other 
forms  even  in  combination  are  rare.  Chromite  is  more  often  mas- 
sive, granular,  or  disseminated  in  rounded  grains. 

In  rock  sections  it  appears  usually  in  scattered  rounded  grains ; 
in  very  thin  sections  it  may  be  brown  or  reddish  in  color,  but  opaque 
otherwise,  and  not  to  be  distinguished  from  magnetite  except  by 
means  of  chemical  tests. 

Chromite  is  associated  with  the  basic  magnesian  rocks,  as  perido- 
tites  and  their  alteration  products,  like  serpentine,  and  while 
widely  distributed  in  small  amounts  there  are  no  large  deposits  in  the 
United  States.  It  is  mined  in  small  amounts  in  California,  but 
most  of  the  chrome  ore  used  is  imported  from  New  Caledonia, 
Greece,  Canada,  Newfoundland,  and  Portuguese  Africa. 

Crystals  occur  at  Texas,  Lancaster  County ,  and  massive  in  various 
localities  in  Chester  County,  Pennsylvania;  at  Baltimore,  Mary- 
land; also  at  the  Reed  Mine,  Hartford,  Maryland;  in  North 
Carolina  and  Wyoming. 


OXIDES  377 

Chromite  is  used  in  the  making  of  ferrochrome,  in  the  steel  in- 
dustry. It  is  the  source  of  all  chromium  salts,  which  are  used 
extensively  as  pigments  and  oxidizers.  The  powdered  mineral  is 
made  into  bricks  with  tar  as  a  binder,  which  are  used  in  furnaces 
where  a  refractory  lining  is  required  to  resist  the  corrosive  action  of 
the  fused  charge. 

Chromite,  like  magnetite,  resists  decomposition  by  weathering 
and  is  therefore  a  component  of  many  residual  sands,  some  of 
which  are  of  commercial  value  for  their  chrome  content. 

Other  members  of  the  spinel  group  are  rare  in  occurrence  and  are 
unimportant  minerals.  Gahnite,  ZnAl2O4,  the  zinc  spinel,  is  found 
at  Franklin,  New  Jersey;  at  the  Canton  mine,  Georgia;  in 
Mitchell  County,  North  Carolina.  When  the  molecule  contains 
manganese  and  iron  as  well  as  zinc  it  is  known  as  dysluite, 
(Zn .  Mn  .  Fe)  (Al .  Fe)2O4,  a  variety  of  gahnite  also  found  at  Stir- 
ling Hill,  New  Jersey. 

CHRYSOBERYL 

Chrysoberyl.  —  Be(A10)2,  BeO  =  19.8 ;  Al2Og  =  80.2 ;  Or- 
thorombic ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c  =  .470  :  1 : 
0.580;  100  A  110  =  25°  10';  001 A  101  =  50°  59';  001  A  Oil  = 
30°  7' ;  Forms,  a  (100),  o  (111),  m  (110),  n  (121) ;  Twinning  plane, 
031;  Cleavage,  Oil  distinct  and  010  less  so;  Brittle,  fracture 
uneven;  H.  =  8.5;  G.  =  3.5-3.85;  Color,  various  shades  of  green, 
brown,  yellow,  and  sometimes  sed ;  Streak,  white ;  Luster,  vitre- 
ous; Transparent  to  translucent;  a  =.1.747;  P  =  1.748;  "y  = 
1.7565;  <y-a  =  .009;  2V  =  45°  20';  2E  =  84°  43';  Axial  plane  = 
010;  Bxa  =  b;  Optically  (+). 

B.B.  —  Infusible ;  powdered  and  ignited  with  cobalt  solution 
becomes  blue.  Dissolves  slowly  in  the  S.  Ph.  bead.  Insoluble  in 
acids. 

General  description.  —  Crystals  are  usually  tabular  parallel  to 
the  macropinacoid,  with  striations  parallel  to  the  c  axis.  Twins 
are  common,  both  contact  and  penetrating,  often  repeated,  yielding 
a  six-sided  pseudohexagonal  shape.  In  these  twins  the  face  a  (100) 
is  common  to  all  the  individuals  and  will  locate  the  direction  of  the 
c  axis  in  each  by  its  striations. 

Chrysoberyl  is  a  rare  mineral  found  in  granites  and  pegmatites, 
where  it  is  associated  with  such  minerals  as  tourmaline,  beryl, 


378  MINERALOGY 

garnet,  and  columbite,  as  at  Haddam,  Connecticut ;  and  found  also 
in  granite  at  Saratoga,  New  York;  at  Norway,  and  Stoneham, 
Maine. 

Alexandrite  is  a  transparent  variety  from  Takowayo  in  the 
Urals  in  Russia,  named  for  the  Czar  Alexander  II.  It  is  a  gem  of 
great  beauty,  being  a  fine  green  by  daylight,  and  in  artificial  light 
changes  to  a  raspberry-red.  The  cat's-eye  is  an  opalescent  variety 
of  chrysoberyl,  the  best  specimens  of  which  are  obtained  from 
Ceylon.  It  has  been  used  as  a  gem  in  the  East  from  the  earliest 
times. 


CHAPTER  IX 

CARBONATES 

CALCITE 

Calcite.  —  Calcium  carbonate,  CaCOs ;  CaO  =  56,  CO2  = 
44;  Hexagonal;  Type,  Dihexagonal  Alternating;  c  =  .8542; 
0001 A 1011  =  44°  36';  rAr'  =  74°55';  e_Ae'  =  45°3';  VAV^  = 
75°  22';  Common  forms,  r  (1011),  m  (10TO),  e  (OU2),  f  (0221), 
y  (3251)L  v  (2131),  a  (1120),  c  (0001) ;  Twinning  planes,  lOll, 
0001,  1012 ;  Cleavage,  rhombohedral  (r)  perfect ;  Brittle,  fracture 
conchoidal,  but  difficult;  H.  =  3 ;  G.  =  2.713;  Color,  white  and 
pale  shades  of  yellow,  brown,  red,  purple,  or  gray  according  to  the 
impurities ;  Streak,  white ;  Luster,  vitreous ;  Transparent  to 
opaque;  €  =  1.487;  <o  =  1.659;  <o-€  =  .172;  Optically  (-). 

B.B.  —  On  heating  whitens  and  becomes  opaque,  yielding  a  cal- 
cium flame;  after  ignition  reacts  alkaline  with  turmeric  paper. 
Dissolves  in  cold  dilute  HC1  with  effervescence,  yielding  CO2; 
when  pure  leaves  no  residue.'  Calcite  may  be  distinguished  from 
aragonite  by  treating  the  fine  powder  in  dilute  cobalt  solution  in 
the  cold ;  aragonite  becomes  lilac-colored,  which  remains  on  boil- 
ing; calcite  remains  white  in  the  cold  solution  and  becomes  blue 
on  boiling.  If  powdered  calcite  is  heated  in  a  solution  of  ferrous 
sulphate,  it  precipitates  a  yellow  ferric  hydroxide,  while  aragonite 
will  produce  a  green  precipitate  of  ferrous  hydroxide. 

General  description.  —  Calcite  crystals  are  very  rich  in  forms ; 
some  330  forms  have  been  described,  with  about  one  hundred  addi- 
tional forms  which  are  doubtful.  With  so  many  possible  forms 
it  can  be  imagined  that,  while  occurring  very  widely  distributed,  the 
crystals  of  many  localities  are  combinations  locally  peculiar. 

Four  general  habits  may  be  recognized:  1.  Rhombohedral; 
several  rhombohedrons  occur  as  simple  crystals  as  e,  f,  or  r.  The 
flat  rhombohedron  e  occur  as  a  simple  crystal  and  in  flat  combina- 
tions, in  West  Cumberland,  England,  where  they  are  known  as  nail- 
head  spar,  from  their  resemblance  to  the  old  hammer-made  wrought- 

379 


380  MINERALOGY 

iron  nails.  The  sharp  rhombohedron  f  occurs  as  a  simple  form  in 
the  Fontainebleau  sand  crystals,  while  the  cleavage  rhombohedron 
r  occurs  as  a  simple  form  at  Sterlingbush,  New  York ;  from  this 
locality  individual  crystals  weighing  nearly  a  thousand  pounds 
have  been  obtained,  mostly  rhombohedral  in  form  with  but  slight 
modifications.  In  many  localities  this  rhombohedron  is  the  pre- 
dominating form,  as  at  Rossie,  St.  Lawrence  County,  New  York, 
where  r  is  well  developed  but  with  the  equatorial  belt  highly  modi- 
fied with  scalenohedrons.  Since  the  cleavage  of  calcite  is  so  per- 
fect and  in  every  specimen  may  be  recognized  either  by  the  internal 


FIG.  438.  —  Prismatic  Habit  of  Calcite,  Wanlockhead,  Scotland.    The  Small 
Specimen  is  the  Rhombohedron  e,  Freiberg,  Saxony. 

fractures  or  by  the  brilliant  cleavage  surfaces,  other  forms  occurring 
may  usually  be  oriented  by  their  relation  to  this  cleavage  rhombo- 
hedron r  (1011)  or  the  commonly  occurring  prism  m  (lOlO). 

2.  Prismatic  crystals  owe  their  general  appearance  to  the  pre- 
dominating prism  m  which  may  be  terminated  by  the  simple 
rhombohedron  e,  as  at  Keokuk,  Iowa,   or  at  Freiberg,  Saxony. 
Again  the  termination  may  be  by  the  rhombohedron  and  base,  as 
at  Patterdale,  England;    or  complicated  by  the  combination  of 
several  scalenohedrons  and  rhombohedrons,  as  at  Patterdale  and 
Derbyshire,  England. 

3.  When  the  prism  is  very  short  and  terminated  by  the  base,  or 
small  modifications  of  a  scalenohedron,  or  rhombohedron  in  com- 
bination with  these  two  predominating  forms,  the  crystalline  habit 


CARBONATES 


381 


is  tabular,  as  at  Andreasberg  in  the  Harz,  also  in  West  Cumberland, 
England. 

4.  Calcite  crystals  of  the  scalenohedral  habit,  designated  "  dog- 
tooth spar,"  is  probably  the  most  common.  In  this  form  the 
scalenohedron  v  (2131)  is  the  predominating  form.  At  Tavetsch, 
Switzerland,  this  scalenohedron  occurs  uncombined.  Nearly 
simple  scalenohedrons  of  large  size  but  slightly  modified  on  the 


FIG.  439.  —  Calcite  Rhombohedron  r,  and  Epidote  from  Sulzbachthal,  Tyrol. 

apex  by  rhombohedrons  and  very  flat  scalenohedrons  occur  at 
Joplin,  Missouri,  also  at  Mineral  Point,  Wisconsin.  Very  acute 
scalenohedrons  occur  at  Thunder  Bay,  Lake  Superior.  Probably 
the  clearest  and  most  beautiful  specimens  of  this  habit  are  those  of 
West  Cumberland,  England. 

Twinning.  —  1.  Twins  in  which  the  vertical  axis  c  is  the  twinning 
axis  and  the  base  is  the  composition  face.  The  reentrant  angle 
appears  in  the  plane  of  the  lateral  axes  as  in  Fig.  295,  which  are 
twinned  scalenohedrons  from  Guanajuato,  Mexico. 

2.  When  the  twinning  axis  is  the  normal  to  e,  and  the  face  e  (0112) 
is  the  composition  plane.  In  these  twins  the  vertical  axes  of  the 
two  individuals  stand  at  an  angle  of  52°  30'  30".  Fig.  294  are 
scalenohedrons  from  Mexico,  twinned  after  this  law.  This  style 
of  twins  also  forms  the  polysynthetic  twins  of  rock-forming  calcite 
and  the  parting  noticed  in  some  calcites  as  that  from  Franklin, 


382  MINERALOGY 

New  Jersey.     Some  very  large  rhombohedrons  twinned  after  this 
law  were  taken  from  a  cave  in  the  Joplin  district  of  Missouri. 


FIG.  440.  —  Calcite  with  a  Scalenohedral  Habit.     Cumberland,  England. 


FIG.  441. -Calcite  twinned  on  the  Rhombohedral  Face  e  (Oil 2).    Joplin,  Missouri. 


CARBONATES 


383 


3.  Twins  in  which  the  twinning  axis  is  normal  to  the  face  of  the 
rhombohedron  r  (1011)  are  not  uncommon;  here  the  vertical  axes 
of  the  two  individuals  lie  at  an  angle  of  90°  46',  as  shown  in  Fig.  442, 
which  are  twins  from  Cumberland,  England. 

4.  Twins  in  which  the  twinning  axis  is  normal  to  (0221)  are  rare. 
In  this  type  the  angle  between  the  c  axes  is  53°  46 ' . 

In  rock  sections  calcite  is  colorless  between  crossed  nicols  and 
yields  a  gray  interference  color  of  high  order.  The  relief  is  high 
when  the  ray  is  vibrating  parallel  to  co,  and  the  marked  cleavage 
cracks  and  surface  scratches  are  all  distinct ;  by  a  rotation  of  the 
section  through  an  angle  of 
90°  the  marks  are  less  dis- 
tinct, as  the  ray  is  now  vi- 
brating parallel  to  €,  which 
is  the  fast  ray  having  the 
least  index  of  refraction. 
The  cleavage  parallel  to  r  is 
nearly  always  to  be  noted  in 
sections,  and  in  many  cases 
poly  synthetic  twinning  la- 
mellae parallel  to  the  rhom- 
bohedron e  appear  as  light 
and  dark  bands  when  be- 
tween crossed  nicols. 

Owing  to  the  high  double 
refraction,  even  thin  basal 
sections  of  calcite  yield  good 
interference  figures,  with  one  or  more  colored  rings. 

Occurrence.  —  Calcium  carbonate  is  very  widely  distributed ;  as 
massive  rock  it  forms  the  large  beds  of  limestone,  which  crystallizes 
to  marble  when  associated  with  metamorphism.  As  a  product 
derived  from  the  weathering  and  solution  of  many  silicates  con- 
taining calcium,  calcite  may  occur  filling  the  cracks,  veins,  and  cavi- 
ties in  almost  any  rock,  and  it  is  therefore  a  very  common  vein 
filler,  associated  with  quartz,  sulphates,  sulphides,  fluorite,  oxides, 
carbonates,  and  other  oxidized  vein  minerals.  Calcium  carbonate  is 
soluble  in  water  containing  CO2  as  calcium  bicarbonate,  Ca(HC03)2, 
and  is  the  cause  of  the  temporary  hardness  of  natural  waters ;  on 
boiling  such  hard  waters  the  excess  CO2  is  driven  off  and  the  water 
becomes  milky  by  the  precipitation  of  calcium  carbonate;  from 
cold  solutions  calcite  is  formed,  and  from  hot  solutions,  aragonite, 


FIG. 442. — Calcite  twinned  on  r  (1011).  Cum- 
berland, England. 


384  MINERALOGY 

the  orthorhombic  calcium  carbonate.  Some  stalactites  or  stalag- 
mites may  be  calcitie  and  others  aragonite  according  to  the  tempera- 
ture at  which  they  were  formed.  The  large  caves  so  common  in 
limestone  regions  owe  their  origin  to  the  solubility  of  the  rock  in  the 


FIG.  443.  —  A  Section  of  Calcite,  Crossed  Nicols,  showing  Twinning  Bands  parallel 
to  the  Rhombohedron  e  (0112). 

natural  ground  waters  containing  CO2,  and  it  is  carried  out  in 
solution  to  some  locality  where,  by  relief  of  the  pressure  and  loss 
of  C02,  the  calcium  carbonate  is  redeposited  either  as  calcite  or 
aragonite. 

Calcite  appears  as  pseudomorphs  after  a  long  list  of  minerals, 
either  formed  by  replacement,  double  decomposition,  or  as  casts 
filling  cavities  after  the  decomposed  crystals  of  other  species  have 
been  carried  away.  A  beautifully  clear  and  pure  calcite  occurs  at 
Eskifjord,  Iceland,  known  as  Iceland  spar.  This  form  of  calcite, 
like  quartz,  has  been  a  great  servant  of  science,  for  its  perfect  cleav- 
age pieces  and  strong  double  refraction  led  Bartholinus  and  Huy- 
gens  to  the  discovery  of  the  law  of  double  refraction,  and  later 
Malus  to  the  discovery  of  polarized  light.  Iceland  spar  is  used  in 
the  construction  of  the  nicol  prism,  in  the  dichroscope,  and  for 
other  parts  of  optical  apparatus.  Unfortunately  the  Iceland  de- 
posit has  been  exhausted  and  calcite  suitable  for  optical  instruments 
is  very  expensive,  and  hard  to  obtain  at  any  price.  Good  crystals 
of  calcite  are  to  be  obtained  at  numerous  localities  in  the  United 
States ;  noted  localities  are  Rossie  and  Sterlingbush,  New  York ; 
Joplin,  Missouri;  Thunder  Bay,  Lake  Superior;  Mineral  Point, 
Wisconsin.  Calcite  may  appear  massive,  oolitic,  pisolitic,  or  sta- 


CARBONATES  385 

lactitic,  as  from  the  caves  of  Kentucky.  Chalk  is  a  soft,  earthy 
variety  derived  from  the  shells  of  organisms. 

Uses.  —  Limestone  is  used  in  the  building  industries  not  only 
as  a  building  stone,  but  it  is  heated  to  drive  off  the  carbon  dioxide, 
when  quicklime  (CaO)  remains;  this  is  slaked  with  water  and 
mixed  with  sand  for  mortar ;  or  is  sintered  with  eight  or  nine  per 
cent,  of  clay  and  then  ground,  forming  Portland  cement,  of  which 
nearly  fifty  millions  of  barrels  are  used  annually  in  the  United 
States,  and  which  now  largely  replaces  the  older  quicklime  mortars. 
Quicklime  is  often  air-slaked  and  applied  to  the  soil  as  a  fertilizer 
to  correct  the  acidity  and  as  an  aid  in  liberating  combined  potas- 
sium, in  a  form  in  which  it  is  available  for  plant  growth. 

Granular  limestones  and  marbles  are  crystalline  limestones ;  their 
color  varies  with  the  impurities  they  contain,  and  some  have  a  very 
pleasing  effect  as  ornamental  and  building  stones.  When  very 
pure,  white,  and  of  an  even  texture,  as  Carrara  marble,  it  is  used 
by  sculptors  as  statuary  marble.  There  is  a  deposit  of  statuary 
marble  in  Colorado  said  to  equal,  if  not  to  surpass,  in  quality  that 
of  the  noted  quarries  of  Carrara,  Italy.  Limestone  is  also  used  as 
a  flux  in  blast  furnaces,  where  it  combines  with  the  silica  of  the  ore 
to  form  a  fusible  slag.  Limestones  are  quarried  in  many  states, 
but  the  center  of  the  industry  is  around  Rutland,  Vermont. 
Tennessee  produces  colored  marbles  in  large  quantities,  and  Cali- 
fornia the  onyx  variety. 

Artificial.  —  Calcite  forms  from  ordinary  solutions  of  calcium 
bicarbonate  at  low  temperatures;  if  heated  or  at  temperatures 
near  the  boiling  point,  aragonite  will  form.  Calcite  will  also  form 
in  a  fusion  with  alkaline  carbonates,  while  from  an  alkaline  solution 
aragonite  will  form. 

MAGNESITE 

Magnesite.  —  Carbonate  of  magnesium,  MgC03;  Mg  =  47.6, 
C02  =  52.4 ;  Hexagonal ;  Type,  Dihexagonal  Alternating ;  c  = 
.8112^  0001  A10U  =  43°  V  45";  rAr'  =  72°  36';  Forms  r  (1011), 
m(1010),  c(0001);  Cleavage,  rhombohedral  perfect;  Brittle, 
fracture  conchoidal;  H.  =  3.5-4.5;  G.  =  3-3.12;  Luster,  vitre- 
ous to  dull  and  earthy;  Transparent  to  opaque;  Color,  white, 
gray,  or  brown;  €  =  1.515,  CD  =  1.7171,  <o  —  €  =.202,  Optically  (  — ). 

B.B.  —  Reacts  like  calcite,  but   dissolves  in  dilute  HC1  more 
slowly.     Becomes  flesh-colored  when  heated  with  cobalt  solution, 
2c 


386  MINERALOGY 

and    the  concentrated  HC1  solution  yields  no  precipitate  with 
H2SO4.     Yields  no  flame  coloration  in  the  forceps. 

General  description.  —  Magnesite  is  rarely  crystalline.  The 
best  crystals,  which  are  simple  rhombohedrons,  occur  in  a  chloritic 
schist  at  Hall  in  the  Tyrol ;  they  are  of  a  light  brownish  color, 
contain  10  per  cent,  of  ferrous  carbonate,  and  are  known  as  breun- 
nerite. 

The  massive  magnesite  is  dull  white  or  gray  and  very  tough. 
It  is  formed  by  the  same  agencies  as  calcite,  but  naturally  from  the 
decomposition  of  minerals  or  rocks  rich  in  magnesium ;  it  is  there- 
fore associated  with  serpentine,  olivine,  and  garnets,  in  talcose 
and  chloritic  schists.  White  magnesite  occurs  in  Massachusetts, 
Pennsylvania,  and  Maryland.  All  magnesite  mined  in  the  United 
States  is  from  the  California  deposits,  especially  those  of  Tulare 
and  Santa  Clara  Counties.  The  imported  magnesite  is  obtained 
from  Greece  and  Austria. 

Uses.  —  In  addition  to  being  the  source  of  all  magnesium  salts, 
calcined  magnesite,  made  in  the  form  of  bricks,  is  used  as  a  refrac- 
tory lining  in  the  basic  open  hearth  furnaces,  in  cement  kilns,  or 
as  a  lining  to  resist  chemical  corrosion  at  high  temperatures,  as 
its  fusing  point  is  about  2250°. 

Artificial.  —  If  a  solution  of  magnesium  carbonate  in  water  is 
heated  to  300°  with  a  porous  stopper,  allowing  the  CO2  to  escape 
gradually,  small  rhombohedral  crystals  of  magnesite  will  form. 
They  are  also  formed  when  sodium  carbonate,  magnesium  chloride, 
and  water  are  heated  in  a  sealed  tube  to  a  temperature  of  160°  to 
200°. 

DOLOMITE 

Dolomite.  —  Pearl  spar ;  Calcium  magnesium  carbonate, 
CaMg(C03)2;  Ca  =  30.4,  Mg  =  21.7,  CO2  =  47.8 ;_  Hexagonal ; 
Type,  Hexagonal  Alternating;  c  =  .832;  0001  A  1011  =  43°  51' 
37";  rAr'  =  73°  45';  Forms,  c(0001),  m(10lO),  r(l6Il);  Twin- 
ning, like  calcite ;  Cleavage,  rhombohedral  perfect ;  Brittle,  frac- 
ture subconchoidal ;  H.  =  3.5-4;  G.  =  2.8-2.9;  Color,  various 
shades  of  gray,  light  yellow,  and  brown ;  Luster,  pearly  to  vitre- 
ous; Streak,  white;  Transparent  to  opaque;  <o  =  1.682;  €  = 
1.502;  o>-€  =  .180;  Optically  (-). 

B.B.  —  Infusible,  whitens  by  loss  of  C02  and  reacts  alkaline  with 


CARBONATES 


387 


turmeric  paper.  Insoluble  in  dilute  HC1,  but  effervesces  in  hot 
acid.  The  concentrated  HC1  solution  yields  a  white  precipitate 
with  H2S04(CaSO4). 


FIG.  444.  —  Dolomite.     Traversella,  Switzerland. 

General  description.  —  In  habit,  generally  in  simple  rhombo- 
hedrons  which  may  occur  in  complex  aggregates,  with  curved, 
warped,  or  saddle-shaped  surfaces ;  this  warped  appearance  of  the 
crystal  faces  may  occur  in  any  of 
the  species  of  the  rhombohedral 
group  of  carbonates,  but  is  es- 
pecially characteristic  of  dolo- 
mite and  siderite.  Dolomite, 
while  placed  in  the  calc  te  group, 
is  not  an  isomorphous  mixture 
of  the  two  carbonates,  but  a 
double  salt.  This  :s  not  only 
shown  by  the  difference  of  sym- 
metry, but  by  the  specific  gravity 
of  dolomite  (2.85)  being  higher 
than  would  be  required  by  the 

molecular  mixture  (2.843).     That  magnesium  carbonates  do  enter 
the   calcite  molecule   and   form   isometric  mixtures  is  shown  by 


f 


FIG.  445.  —  Dolomite    Var. 
Teruel,  Spain. 


Teruelite. 


388 


MINERALOGY 


almost  any  analysis  of  calcite;  most  of  the  magnesian  lime- 
stones are  of  this  nature.  Beds  of  dolomitic  limestone  are  formed 
by  the  replacement  of  calcium  carbonate  with  magnesium  carbonate 
in  coral  reefs.  This  can  take  place  in  solutions  of  MgCl2  and 
MgSO4  common  in  sea  water,  and  especially  in  the  presence  of 
C02  and  at  a  temperature  of  20°  to  25°  C. 

Mountain  ranges  in  the  West  are  formed  by  dolomite  in  which  the 
rivers  have  eroded  steep  walled  canons,  which,  with  the  cross  valleys 
and  tributary  streams,  cover  the  country  with  the  peculiar  butte 
formations ;  such  a  section  in  the  Tyrol  is  known  as  the  Dolomites. 

An  interesting  variety  of  nearly  black  dolomite,  teruelite,  occurs 
at  Teruel,  Spain,  in  a  matrix  of  gypsum.  The  crystals  are  combi- 
nations of  an  acute  rhombohedron  and  the  base  equally  developed, 
resembling  very  closely  the  octahedron,  but  the  basal  faces  are 
always  rough  and  quite  different  from  the  rhombohedron. 

Uses.  —  Massive  dolomites  are  very  desirable  as  building  stones. 


SIDERITE 

Siderite.  —  Chalybite ;  Spathic  iron  ore ;  Ferrous  carbonate, 
FeC03;  FeO  =  62.1  (Fe  =  48.2),  CO2  =  37.9;  Hexagonal;  Type, 
Dihexagonal  Alternating;  c  =  .8184;  0001 A 1011  =  43°  22' 


FIG.  446.  —  Fluorite  and  Siderite.    Harz  Mountains,  Germany. 


CARBONATES 


389 


51";  rAr'  =  73°0';  Common  forms,  r(1011),  c  (0001) ;  Twin- 
ning plane,  e  (0112) ;  Cleavage,  rhombohedral  perfect;  Brittle, 
fracture  uneven ;  H.  =  3.5-4 ;  G.  =  3.83-3.88 ;  Color,  various 
shades  of  brown  and  gray ;  Streak,  white  unless  oxidized,  when 
it  may  be  brown;  Luster,  vitreous;  Translucent;  o  =  1.873, 
c  ;=  1.633;  <•>-€  =  .240;  Optically  (-). 

B.B.  —  Loses  C02  and  blackens.  In  R.  F.  becomes  magnetic. 
Fuses  at  4.5  and  reacts  for  iron  with  the  fluxes.  Effervesces  in 
dilute  HC1,  especially  when  hot,  and  dissolves  completely  when 
pure. 

General  description.  —  Crystals  are  simple  rhombohedrons  or 
complex  crystals,  which  are  curved  rhombohedrons  as  in  dolomite. 
Simple  rhombohedral  crystals  of  half  a  pound  in  weight  occur 
embedded  in  the  cryolite  of  Ivigtut,  Greenland.  At  the  Buckler's 


FIG.  447.  —  Cryolite  and  Siderite  from  Ivigtut,  Greenland. 

Mine,  Cornwall,  England,  crystals,  combinations  of  the  rhombo- 
hedron,  base,  hexagonal  prism,  and  a  scalenohedron  occur,  but  forms 
other  than  the  rhombohedron  are  rare.  Siderite  is  also  massive, 
granular,  or  disseminated. 

The  various  colors  depend  upon  the  amount  of  oxide  of  iron,  or 
oxidation,  and  at  times  it  is  very  dark,  due  to  the  presence  of  hema- 
tite or  limonite,  to  which  it  is  easily  transformed  by  oxidation. 


390  MINERALOGY 

\Yhen  mixed  and  crystallized  with  MgCO3,  it  forms  mesitite,  as  at 
Traversella  in  Piedmont,  which  has  a  rhombohedral  habit  and  is 
theoretically  iron  and  magnesium  carbonates  in  molecular  propor- 
tions. Ankerite  is  a  mixture  of  calcium,  iron,  and  magnesium  car- 
bonates, occurring  with  iron  ores  and  siderite,  as  in  Styria,  Siegen, 
Nova  Scotia,  and  northern  New  York. 

Siderite  like  other  carbonates  is  deposited  from  the  bicarbonate 
solution,  but  in  this  case  it  must  be  laid  down  under  non-oxidizing 
conditions  or  in  the  presence  of  organic  matter,  otherwise  hematite 
or  limonite  will  form,  as  ferric  carbonate  is  not  known  as  a  dry 
salt.  It  is  also  formed  by  the  replacement  of  calcium  in  calcite 


FIG.  448.  —  Siderite,  showing  the  Hexagonal  Prism  and  Base.    Cornwall,  England. 

and  limestones  and  is  therefore  an  important  constituent  of 
many  sedimentary  rocks,  especially  those  of  the  coal  formations. 
Siderite  as  a  vein  filler  is  associated  with  many  ore  deposits. 

Uses.— It  is  mined  as  an  iron  ore  in  Cornwall,  England,  but  it  is 
of  little  importance  as  an  iron  ore  in  the  United  States.  For- 
merly it  was  mined  at  Roxbury,  Connecticut. 

Artificial.  —  Microscopic  crystals  may  be  obtained  by  precipitat- 
ing a  solution  of  ferrous  sulphate  with  sodium  bicarbonate  and 
heating  in  a  sealed  tube  to  150°  for  several  hours. 

RHODOCHROSITK 

Rhodochrosite.  —  Manganese  carbonate,  MnCO3;  Mn  =  61.7, 
C02  =  38.3  ;  Hexagonal ;  Type,  Dihexagonal  Alternating ;  c  = 
8184;  0001,1011=43°  22'  50";  rAr'  =  73°  0';  Forms, 


CARBONATES  391 

r  (lOll),  e  (1112);  Cleavage,  rhombohedral  perfect;  Brittle, 
fracture  uneven ;  H.  =  3.5-4.5  ;  G.  =  3.45-3.60  ;  Color,  various 
shades  of  red  or  pink,  yellowish  gray,  brown  when  oxidized ;  Streak, 
white;  Transparent  to  opaque;  €  =  1.597;  Optically  (  — ). 

B.B.  —  Blackens  and  decrepitates,  infusible ;  with  the  fluxes 
reacts  for  manganese.  Dissolves  in  hot  dilute  HC1  with  effer- 
vescence, yielding  carbon  dioxide. 

General  description.  —  Crystals  are  not  common,  but  in  habit 
are  simple  rhombohedrons ;  other  forms  are  rare.  Complex 
warped  rhombohedrons  of  a  beautiful  pink  color  occur  in  Saguache 
County,  also  dark  rose-colored  transparent  rhombohedrons  asso- 
ciated with  cubes  of  pyrite  at  Alicante,  Lake  County,  Colorado. 
At  Butte,  Montana,  the  simple  rhombohedrons  are  coated  with 
quartz.  Fine  specimens  are  obtained  from  Franklin  Furnace, 
New  Jersey,  where  large  masses  of  cleavable  calcite  occur,  colored 
pink  with  manganese  carbonate  which  has  crystallized  with  it  as  an 
isomorphous  mixture. 

These  mixtures  of  calcium  and  manganese  carbonates  are  given 
definite  names  when  the  amount  of  manganese  reaches  a  consider- 
able percentage,  as  manganocalcite  for  the  25  per  cent,  mixture, 
or  manganosiderite  when  much  iron  is  present. 

Rhodochrosite  is  formed  under  the  same  conditions  as  the  other 
rhombohedral  carbonates,  as  it  is  more  stable  than  the  ferrous  car- 
bonate; when  they  are  contained  in  the  same  solution,  the  iron  is 
deposited  first,  and  the  manganese  may  be  carried  in  solution  much 
farther  from  the  original  source.  Owing  to  this  difference  of 
stability  in  solution  the  two  are  almost  always  deposited  apart. 

Artificial.  —  Microscopic  rhombohedrons  may  be  formed  by 
heating  a  solution  of  manganese  chloride  and  sodium  carbonate  in 
'a  closed  tube  to  160°  C. 

SMITHSONITE 

Smithsonite.  —  Dry  bone,  Carbonate  of  zinc,  ZnCOs ;  ZnQ  = 
64.8,  CO  =  35.2;  Hexagonal;  Type,  Dihexagonal  Alternating; 
c  =  .8063;  0001A1011  =_42°57'20";  rAr'  =  72°  20';  Forms, 
r  (1011),  e  (0112),  v  (2131);  Cleavage,  rhombohedral  perfect; 
Brittle,  fracture,  uneven ;  H.  =  5 ;  G.  =  4.3-4.45 ;  Color,  white, 
yellow,  green,  brown ;  Streak,  white  or  pale ;  Luster,  vitreous  to 
dull;  Transparent  to  opaque;  €  =  1.618;  Optically  (— ). 


392  MINERALOGY 

B.B.  —  Infusible;  reduced  with  soda  and  borax  on  coal  yields  a 
white  coat  which  becomes  green  with  cobalt  solution  (Zn).  Dis- 
solves in  hot  dilute  HC1,  completely  when  pure,  with  effervescence 
yielding  carbon  dioxide.  Highly  colored  green  varieties  contain 
copper ;  the  brown,  iron  or  manganese. 

General  description.  —  Crystals  are  not  common,  but  occur  in 
small  simple  rhombohedrons,  or  scalenohedrons,  as  at  Friedensville, 
Pennsylvania.  Smithsonite  is  more  often  massive,  incrusted, 
botryoidal,  banded,  or  earthy,  than  crystalline.  The  pure  zinc 
carbonate  is  white,  and  the  mineral  owes  its  various  colors  to  the 
impurities  and  isomorphous  carbonates  which  crystallize  with  it. 
A  very  beautiful  green  variety  containing  considerable  copper  occurs 
at  Kelley,  New  Mexico ;  also  in  Greece.  The  bright  yellow  variety, 
known  in  Arkansas  and  southern  Missouri  as  "  turkey-fat  ore/' 
is  colored  with  the  sulphide  of  cadmium,  greenockite,  which  is 
associated  in  small  amounts  with  the  zinc  ores  of  this  region. 

Smithsonite  is  connected  with  most  zinc  ore  deposits  as  a  second- 
ary mineral,  a  surface  oxidation  product  derived  from  sphalerite, 
after  which  pseudomorphs  are  common.  Zinc  may  be  carried,  by 
percolating  waters,  in  solution,  either  as  the  sulphate  or  bicarbonate, 
to  be  precipitated  as  sulphide  or  by  replacement  of  carbonate  of 
lime  as  carbonate.  Zinc  ores  therefore  fill  veins,  pockets,  or  lenses 
in  limestones,  and  may  have  been  concentrated  from  the  lime- 
stone itself  or  carried  to  it  from  neighboring  regions.  As  a  vein 
mineral  Smithsonite  is  associated  with  calcite,  barite,  siderite, 
galena,  pyrite,  most  of  which  have  originated  through  the  same 
agencies. 

Smithsonite  as  a  zinc  ore  is  of  minor  importance  in  the  United 
States.  It  is  mined  in  Virginia,  Pennsylvania,  Arkansas,  and  Colo- 
rado. 

Its  synthesis  is  effected  by  the  same  means  as  the  other  car- 
bonates of  the  group. 

Sphserocobaltite,  a  carbonate  of  cobalt  isomorphous  with  the 
calcite  group,  is  found  in  small  rhombohedral  crystals  at  Schnee- 
berg  in  Saxony.  Some  rhodochrosites  contain  small  quantities 
of  cobalt. 

ARAGONITE 

Aragonite.  —  Calcium  carbonate,  CaC02 ;  CaO  =  56L  C02 
=  44 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  &  :  b  :  c  = 


CARBONATES 


393 


.6224  :  1 :  .7205  ;  100  A  110  =  31°  54' ;  001  A  101  =  49°  10' 
42";  001  A  Oil  =  35°  46'  30";  Common  forms,  c(011),  b  (010), 
m  (110),  o  (112),  other  forms  numerous  but  not  common ;  Twinning 
plane,  110,  often  repeated; 
Brittle,  fracture  subcon- 
choidal;H.  =3.54;  G.  =  2. 
93L2.95;  Color,  white, 
yellow,  gray,  sometimes 
green;  Streak,  white; 
Luster,  vitreous ;  Trans- 
parent to  opaque;  a  = 
1.529;  p  =  1.681;  y  =  1.685; 
•y  -  a  =  .156;  Optically 
(_);  2V  =  18°  11';  Axial 
plane  parallel  to  100;  Bxa 
parallel  to  c. 

B.B.  —  Like  calcite. 

General     description.  — 

Crystalline  habit  prismatic, 
combinations  of  the  unit 
prism  with  the  brachy- 
pinacoid;  as  the  prism  angle  is  116°  12',  the  crystals  have  a 
hexagonal  outline.  Such  combinations  terminated  by  the  base 
are  common  at  Herrengrund,  Hungary.  At  Cleator,  England, 
crystals  occur  illustrating  the  chisel-shaped  habit,  where  the  ter- 
mination is  formed  by  the  long  steep  pyramid  (991)  and  dome 
(091),  in  which  the  dome  predominates,  giving  the  crystal  the 
flattened  or  chisel  appearance.  Twinning  in  aragonite  adds  to 
the  hexagonal  appearance,  as  the  composition  plane  is  parallel  to 
the  unit  prism;  when  repeated  and  joined  in  parallel  position, 
yield  short  stout  hexagonal  prisms  terminated  by  a  flat  face,  the 
base.  The  complexity  of  such  specimens  is  easily  seen  by  the 
striations  on  the  base,  or  by  the  concave  or  offset  prism  faces. 

Aragonite,  as  the  unstable  form  of  calcium  carbonate  at  ordinary 
temperatures,  is  less  common  than  calcite.  It  is  formed  from  hot 
solutions,  around  thermal  springs,  in  crusts,  botryoidal  shapes,  and 
peculiar  coral-like  masses  known  as  "flos-ferri,"  orflowers  of  iron,from 
their  form  and  association  with  beds  of  iron  ore .  The  ' '  sprudelstein' ' 
of  the  hot  springs  of  Carlsbad,  Bohemia,  is  aragonite,  as  well  as  the 
calcic  skeleton  of  corals  and  the  pearly  lining  of  many  shells. 


FIG.  449.  —  Aragonite  from  Herrengrund,  Hun- 
gary. 


394 


MINERALOGY 


Heretofore   all   non-crystalline   deposits   of   calcium  carbonate 
were  termed  aragonite,  but  calcite  is  deposited  in  a  massive  state 

at  l°w  temperatures 

also,  and  the  iden- 
tity of  such  forma- 
tions should  be 
determined  by  ac- 
tual tests  (see 
calcite) .  Aragonite 
passes  over  to  cal- 
cite, with  which  it 
is  dimorphous,  by  a 
change  of  its  physi- 
cal properties,  form- 
ing pseudomorphs, 
the  chemical  com- 
position remaining 
unchanged. 

Good  aragonite 
crystals  are  not 
common;  those  of 
Herrengrund  are 
probably  the  best. 
Pseudohexagonal 
forms  occur  at 

Aragon,  Spain,  where  the  mineral  was  first  discovered.  At 
Girgenti,  Sicily,  it  occurs  associated  with  celestite,  sulphur,  and 
gypsum.  Often  associated  with  zeolites,  and  in  clays  with  gypsum. 
Artificial.  —  When  a  solution  of  calcium  bicarbonate  is  heated 
above  30°  C.,  aragonite  separates;  below  30°,  calcite  separates. 


FIG.  450.  —  Chisel-shaped  Crystals  of  Aragonite  from 
Cumberland,  England. 


WITHERITE 


Witherite.  —  Barium  carbonate,  BaC03 ;  Ba  =  77.7,  C02  = 
22.3  ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c  = 
.6032:  1 :  .7302;  100  A  110  =  31°  6';  001  A  101  =  50°  26' 
30";  110 A 001  =  36°  8'  15";  Common  forms,  c  (001),  m(110), 
b  (010) ;  Twinning  plane  parallel  to  110;  Cleavage,  b  distinct,  m 
imperfect ;  Brittle ;  Fracture,  uneven ;  H.  =  3-3.75 ;  G.  =  4.29- 
4.35 ;  Luster,  vitreous ;  Color,  white,  gray,  or  yellow ;  Streak,  white ; 


CARBONATES  395 

Transparent   to   translucent;     P  =  1.740;    Optically  (  —  );    Axial 
plane  parallel  to  010;  Bxa  =  c;     2E  =  26°  30'. 

B.B.  —  Fuses  easily  to  a  globule  and  yields  a  green  flame  in  the 
forceps.  After  ignition  reacts  alkaline  with  turmeric  paper.  Effer- 
vesces with  cold  dilute  HC1;  the  solution  greatly  diluted  yields  a 
white  precipitate  with  H2SO4. 

General  description.  —  Witherite  occurs,  as  a  rule,  not  in  well- 
formed  crystals,  but  in  masses  with  a  radiated,  columnar,  botryoi- 
dal,  or  granular  structure.  When  crystals  are  well  developed,  they 
are  pseudohexagonal  in  shape,  from  the  repeated  twinning  parallel 
to  110. 

Its  genesis  and  associations  are  like  aragonite,  but  it  occurs  more 
often  in  veins  with  barite  and  galena.  By  the  action  of  waters 
containing  sulphates  in  solution  witherite  will  be  transformed  to 
barite.  Witherite  is  not  a  common  mineral,  and  it  occurs  in  the 
United  States  in  but  few  localities,  as  at  Lexington,  Kentucky, 
and  near  Thunder  Bay,  Lake  Superior.  At  Fallowfield,  England, 
it  is  mined  commercially;  splendid  crystals  are  otained  at  this 
locality. 

It  is  isomorphous  with  aragonite  and  alstonite  or  bromlite; 
(Ba,  Ca)  CO3  is  such  a  mixture,  found  in  pseudohexagonal  crystals 
at  Alston  Moore,  England,  while  barytocalcite,  BaCa  (CO3)2,  is 
probably  a  double  salt  and  monoclinic  in  symmetry. 

Witherite  is  the  source  of  barium  salts ;  it  is  used  as  a  substitute 
in  paints,  and  in  powder  as  a  rat  poison. 

Artificially  it  is  formed  when  the  carbonate  is  fused  with  sodium 
chloride,  or  in  the  precipitation  of  a  hot  solution  of  barium  salts 
by  ammonium  carbonate. 

STRONTIANITE 

Strontianite.  —  Strontium  carbonate,  SrC03 ;  Sr  =  70.1,  C02 
=  29.9 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  a :  b  :  c 
=  .6090 : 1 :  .7238 ;  100  A  110  =  31°  20'  30" ;  001  A 101  =  49° 
55'  30";  001 A  Oil  =  35°  54';  Common  forms,  c  (001),  b  (010), 
m  (110),  i  (021) ;  Twinning  plane,  110;  Cleavage,  prismatic  good; 
Brittle;  Fracture,  uneven;  H.  =3.5-4;  G.  =  3.68-3.71 ;  Luster, 
vitreous;  Color,  white,  yellow,  gray,  pale  green;  Streak,  white; 
Transparent  to  translucent;  a=  1.515;  p  =  1.664;  -y  =  1.666; 


396  MINERALOGY 

Optically  (— );    Axial  plane    parallel  to   100;    Bxa  =  c;    2E  = 
10°  36'. 

B.B.  —  Fuses  in  the  forceps  with  difficulty  and  yields  a  deep  red 
flame ;  after  ignition  reacts  alkaline  with  turmeric  paper.  Effer- 
vesces in  cold  dilute  HC1. 

General  description.  —  Crystals  are  often  sharply  pointed  in 
habit,  caused  by  the  development  of  the  acute  pyramid  and  brachy- 
dome,  as  at  Hamm  in  Westphalia,  also  columnar,  fibrous,  or 
granular. 

It  is  isomorphous  with  barium  and  calcium  carbonates  and 
usually  contains  some  of  these  salts.  In  its  occurrence  it  is  asso- 
ciated with  galena  in  ore  deposits,  and  with  barite  and  celestite ; 
from  the  latter  it  may  have  been  derived  as  a  secondary  product,  as 
at  Mount  Bonnell  near  Austin,  Texas;  occurs  also  in  Jefferson 
County,  New  York,  and  in  Monroe  County,  Michigan. 

Strontianite  is  the  commercial  source  of  the  salts  of  strontium, 
which  are  used  to  produce  the  red  fires  in  pyrotechnics.  The  hy- 
drate is  used  in  the  beet  sugar  industry  to  precipitate  the  sugar 
from  the  molasses ;  for  this  purpose  barium  hydrate,  owing  to  its 
cheapness,  is  sometimes  used  as  a  substitute. 

Artificially  produced  like  witherite. 

CERUSSITE 

Cerussite.  —  Carbonate  of  lead,  PbC03 ;  PbO  =  83.5,  C02 
=  16.5 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  fi  :  b  :  c 
=  .6099  : 1 :  .7230 ;  100  A  110  =  31°  22'  55" ;  001  A  101  =  49° 
51';  001A011=35°  52';  Common  forms,  c  (001),  m(110), 
b  (010),  P  (111)  i  (021) ;  Twinning  plane,  110;  Cleavage,  m  and  i 
distinct ;  Brittle ;  Fracture,  conchoidal ;  H.  =  3-3.5 ;  G.  =  6.46- 
6.57;  Color,  white,  yellow,  or  gray;  Streak,  white;  Luster,  ada- 
mantine to  resinous ;  Transparent  to  opaque;  a  =  1.804;  p  = 
2.076;  v  =  2.078;  Optically  (-);  Axial  plane  parallel  to  010; 
Bxa  =  c;  2V  =  8°  14';  2E  =  17°  8'. 

B.B.  —  Darkens  in  the  closed  tube  and  decrepitates.  On  coal 
in  R.  F.  reduces  to  malleable  lead  and  yields  a  lead  coat.  Dis- 
solves in  dilute  nitric  acid  with  effervescence,  yielding  carbon 
dioxide. 


CARBONATES 


397 


General  description.  —  Crystals  are  tabular,  pyramidal,  or 
elongated  prismatic  in  habit.  When  pyramidal  in  habit,  like  other 
members  of  the  group, 
they  are  pseudo-hexagonal 
in  appearance,  from  the 
equal  development  of  the 
pyramid  ,and  brachydome, 
as  at  Mies  in  Bohemia. 
When  tabular,  it  is  very 
apt  to  form  stellate  groups 
by  repeated  twinning  after 
the  aragonite  method,  but 
here  the  space  between  in- 
dividuals is  not  completely 
filled  as  in  aragonite,  as  at 
Mies  and  Pribram,  Bo- 
hemia. When  elongated  in 
habit,  the  prism  zone  is 
deeply  striated  lengthwise,  FIQ  45L  _Cerussite,  Twing  MieSi  Bohemia 
as  at  the  old  Wheatley 

Mine,  Chester  County,  and  Phoenixville,  Pennsylvania.  It  occurs 
also  in  crusts,  stalactitic,  granular,  or  massive,  often  colored  green 
or  blue  with  copper  carbonates. 

Cerussite  is  associated  as  a  secondary  product  in  the  superficial 
areas  of  lead  deposits  where  it  is  formed  from  galena  by  oxidation 
and  the  action  of  carbonated  waters,  which  may  also  form  carbon- 
ates from  anglesite,  the  sulphate  of  lead,  after  which  many  pseudo- 
morphs  are  found. 

Next  to  galena  cerussite  is  the  most  important  ore  of  lead  and 
is  mined  at  Leadville,  Colorado;  Arizona;  and  Utah. 

Artificially  it  may  be  formed  as  aragonite. 


MALACHITE 

Malachite. — Basic  carbonate  of  copper,  (Cu  .  OH)2CO3 ;  CuO 
=  71.9,  _C02  =  19.9,  H20  =  8.2;  Monoclinic;  Type,  Equato- 
rial; a  :  b  :  c  =  .881 :  1 :  .401 ;  p  =  61°  50'  =  001A100;  100A110 
=  37°  50';  001  A  Oil  =  19°  29';  Common  forms,  c(001),  a  (100), 
b  (010),  m  (110);  Twinning  plane,  100;  Cleavage,  basal  perfect, 
b  less  so;  Brittle;  Fracture,  uneven;  H.  =3.5-4;  G.  =  3.9-4; 
Color,  bright  green;  Streak,  pale  green;  Luster,  vitreous,  silky 


398 


MINERALOGY 


to  dull  and  earthy;  Translucent  to  opaque;  0  =  1.87;  Optically 
(-) ;  Axial  plane  parallel  to  010;  BxaAc  =  23°  39'  in  front;  2E  = 

89°  18';  2V  =  44°  7'. 


B.B. — Blackens  and 
fuses.  In  R.  F.  on  coal 
yields  copper  and  a 
green  flame.  In  the 
closed  tube  yields 
water,  dissolves  in 
dilute  HC1  with  effer- 
vescence, yielding  car- 

j    bon  dioxide. 

*n 

General  description. 
o  —  Crystals  are  seldom 
'<%  distinct  individuals, 
but  grouped  in  tufts, 
divergent  or  elongated, 
ri  acicular,  and  radiated, 
^  as  at  Betzdorf,  in 
2  Westphalia;  also  bot- 
ryoidal,  stalactitic, 
nodular,  and  curvi- 
linear masses,  formed 
with  concentric  layers 
of  different  shades  of 
green,  are  common,  as 
at  Bisbee,  Arizona. 
Malachite  is  a  second- 
ary mineral  formed 
as  an  oxidation  prod- 
uct of  other  copper  ores,  as  cuprite,  chalcocite,  chalcopyrite,  or 
melaconite,  by  the  action  of  percolating  waters  charged  with 
carbon  dioxide,  and  is  characteristic  of  the  surface  workings  of 
all  copper  deposits.  Numerous  pseudomorphs  of  malachite, 
especially  after  cuprite,  occur,  as  at  Chessy  near  Lyons,  France, 
where  there  are  beautiful  examples  of  octahedrons  and  rhombic 
dodecahedrons,  some  of  which  are  only  coated  with  a  crust  of 
carbonate,  while  the  interior  is  still  unaltered  cuprite.  Octahedral 
pseudomorphs  are  also  found  at  Bisbee,  Arizona. 

Malachite  is  of  common  occurrence  in  many  copper  localities ; 


CARBONATES 


399 


the  most  beautiful  specimens  in  the  United  States  are  obtained 
at  Bisbee,  Arizona,  The  massive  banded  variety  occurs  in  the 
Urals  in  Russia,  where 
it  is  much  prized  as  an 
ornamental  stone,  used 
in  the  veneering  of 
vases,  table  tops,  and 
decorations  in  build- 
ings. The  large  in- 
terior columns  of  St. 
Isaac's  Cathedral  at 
St.  Petersburg  are  of 
malachite. 

Malachite  is  also  a 
valuable  ore  of  copper. 

Artificial  malachite 
is  formed  upon  heat- 
ing a  solution  of  cop- 
per bicarbonate. 


FIG.  453.  —  Malachite  with  Concentric  Bands  of 
Various  Shades  of  Green.    Bisbee,  Arizona. 


AZURITE 

Azurite.  —  Chessylite ;  Basic  copper  carbonate,  Cu(OH)2 .  2  (Cu- 
C03) ;  CuO  =  69.2,  C02  =  25.6,  H2O  =  5.2 ;  Monoclinic ; 
Type,  Equatorial ;  a  :  b  :  c  =  .850  :  1 :  1.880 ;  P  =  87°  36'  = 
001  A  100;  100  A  110  =  40°  21';  001 A  101  =44°  46';  001 A  Oil 
=  41°  21';  Common  forms,  c  (001),  a  (100),  m(110),  p  (021) ; 
Twinning  plane  v  (201) ;  Cleavage,  p  perfect,  a  less  so  ;  Brittle ; 
Fracture,  conchoidal ;  H.  =  3.5-4 ;  G.  =  3.77-3.83 ;  Color,  azure- 
blue  to  deep  blue ;  Streak,  smalt-blue ;  Luster,  vitreous  to  dull ; 
Translucent  to  opaque ;  Optically  (+) ;  Axial  plane  perpendicular 
to  010;  BxaAc  =  -  12°  36';  2E  =  151. 


B.B.  —  Like  malachite. 

General  description.  —  Well-developed  crystals  are  more  com- 
mon than  those  of  malachite.  They  are  varied  in  habit,  and 
highly  modified,  often  tabular  combinations  of  the  base  and  unit 
prism  with  a  pyramid  and  dome,  with  the  base  striated,  as  at  Bisbee, 
Arizona.  Peculiar  complex  aggregates,  two  inches  in  diameter, 
some  rhombohedral,  others  rounded,  occur  in  the  claylike  pockets 
in  the  Copper  Queen  mine,  Bisbee,  Arizona ;  also  at  Chessy,  France. 


400 


MINERALOGY 


Azurite  is  also  radiated,  massive,  stalactitic,  granular,  earthy,  or 
botryoidal,  with  banded  concentric  layers  like  malachite,  from  which 
such  specimens  may  form  by  a  loss  of  CO2  and  hydration. 


FIG.  454. — Azurite  Crystals.    Bisbee,  Arizona. 

Azurite  is  a  secondary  mineral  formed  from  other  copper 
minerals  in  which  the  chemical  reactions  have  gone  a  step  farther 
than  in  malachite,  and  some  of  the  C02  has  been  replaced  by 
hydroxyl. 

Azurite  is  widely  distributed  in  the  superficial  workings  of  almost 
all  copper  deposits. 

Commercially  it  is  an  important  ore  of  copper,  and,  like  malachite, 
the  well-banded  and  colored  specimens  are  used  in  the  manufacture 
of  trinkets,  ornaments,  and  the  less  expensive  jewelry. 


NATRON 

sodium  carbonate,  Na^COa .  10  H2O  ; 
15.4,  H2O  =  62.9 ;  Monoclinic ;  Type, 
c  =  1.482  :  1  :  1.400;  P  =  58°  52'  = 
51°  46';  001 A  Oil  =50°  10' ;  Common 

forms,  b  (010),  m  (110),  e  (Oil) ;  Cleavage,  basal  distinct ;  Brittle  ; 

Fracture,  conchoidal;  H.  =  1-1.5;  G.  =  1.42-1.46;     Color,  white 


Natron.  —  Hydrous 
Na^O  =  21.7,     CO2  = 
Equatorial ;       a  :  S  : 
001  A  100;    100  A  110 


CARBONATES  401 

to  gray  or  yellow ;  Streak,  white ;   Luster,  vitreous  to  dull ;   Trans- 
parent to  earthy. 

B.B.  —  Soluble  in  water,  yielding  an  alkaline  solution ;  effer- 
vesces with  dilute  acids.  In  the  forceps  a  sodium  flame.  A  strong 
alkaline  taste. 

TRONA 

Trona.  —  Hydrous  sodium  bicarbonate,  Na3H(C03)2 .  2  H20 ; 
Na2O  =  41.2,  CO2  =  38.9,  H2O  =  19.9;  Monoclinic;  Type, 
Equatorial ;  a  :  b  :  c  =  2.846  :  1 :  2.969 ;  p  =  77°  23'  =  001 A  100  ; 
Common  forms,  c  (001),  a  (100),  o  (111) ;  Cleavage,  a  perfect; 
H.  =  2.5-3;  G.  =  2.11-2.14;  Color,  white,  gray,  yellowish ;  Streak, 
white;  Luster,  vitreous;  Translucent. 


B.B.  —  Like  natron. 


GAYLUSSITE 


Gaylussite.  —  Hydrous  sodium  calcium  carbonate,  Na2Ca- 
(CO3)2 .  5H20 ;  CaC03  =  33.8,  Na2C03  =  35.8,  H2O  =  30.4 ; 
Monoclinic ;  Type,  Equatorial ;  a  :  b  :  c  =  1.489  :  1 :  JL.444 ;  P 
=  78°  27' =  001 A  100;  100  A  110  =  55°  35';  001  A  101  =  49° 
44' ;  001  A  Oil  =  54°  45' ;  Common  forms,  c  (001),  e  (Oil), 
m(110),  r(112);  Cleavage,  prismatic  perfect,  basal  difficult; 
Brittle;  Fracture,  conchoidal;  H.=  2-3;  G.  =  1.93-1.95;  Color, 
white,  gray,  yellowish;  Streak,  white;  Luster,  vitreous;  Trans- 
lucent. 

B.B.  —  Whitens  and  fuses  to  a  white  enamel,  yielding  an  in- 
tense yellow  flame  (Na) ;  the  residue  after  fusion  reacts  alkaline 
with  turmeric  paper.  Soluble  in  acids  with  effervescence;  the 
solution  made  alkaline  with  ammonia  yields  a  precipitate  with 
ammonium  oxalate  (Ca) . 

General  description.  —  Natural  soda  occurs  in  numerous 
localities  where  the  dryness  of  the  climate  has  permitted  the  con- 
centration or  complete  evaporation  of  large  bodies  of  saline  waters. 
In  the  West  such  deposits  occur  in  Wyoming,  Nevada,  Utah,  and 
at  Mona,  Borax,  and  Owens  lakes,  California.  Most  of  these 
localities  have  been  worked  commercially  for  the  sodium  carbon- 
ates they  contain.  It  has  been  estimated  by  Loew  that  Owens 

2D 


402  MINERALOGY 

Lake  contains  in  solution  22  million  tons  of  dry  sodium  carbonate. 
Near  Ragtown,  Nevada,  crusts  of  trona  exists  over  the  surface  of 
Soda  Lake,  of  sufficient  thickness  and  strength  to  bear  the  weight 
of  a  man. 

A  third  sodium  carbonate,  thermonatrite,  Na^COs .  H2O,  with 
one  molecule  of  water,  is  deposited  from  these  solutions  under  fa- 
vorable conditions.  Of  the  three  carbonates,  trona  is  the  first  to 
be  deposited ;  as  concentration  increases,  the  very  soluble  natron 
separates,  mixed  with  sulphates  and  chlorides. 

Gaylussite  is  deposited  in  these  lakes  near  the  entrance  of 
small  streams  or  springs ;  as  the  waters  of  these,  carrying  calcium 
in  solution,  are  mixed  with  the  strong  soda  solution,  the  calcium  salt 
is  deposited  as  gaylussite. 

All  these  large  bodies  of  alkaline  carbonates  have  been  attributed 
to  the  concentration  of  solutions  from  volcanic  rocks,  or  by  the 
interaction  of  calcium  bi carbonates  in  solution  with  alkali  sulphates 
and  chlorides. 


CHAPTER  X 

SILICATES,  TITANATES,  ETC. 

FELDSPARS  AND  LEUCITE 

THE  FELDSPARS 

THE  feldspars  constitute  a  most  important  group  of  rock-form- 
ing minerals,  so  important  that  not  only  is  nearly  60  per  cent, 
of  the  igneous  rocks  composed  of  feldspars,  but  their  classifica- 
tion depends  to  a  great  extent  upon  the  quantity  and  species  of 
feldspar  that  is  present.  They  are  isomorphous  in  the  fullest 
sense  of  the  term,  and  even  though  they  belong  to  both  the  mono- 
clinic  and  triclinic  systems  and  are  also  salts  of  two  silicic  acids, 
yet  they  mix  in  all  proportions,  forming  solid  solutions.  One 
species  grades  gradually  into  the  others.  The  various  mixtures 
which  seem  to  be  more  constant  in  nature  have  been  given  special 
names,  and  in  the  past  were  considered  as  distinct  species. 

From  a  chemical  standpoint  there  are  four  species,  two  of  which, 
orthoclase  and  albite,  are  salts  of  the  trisilicic  acid  (H4Si3O8)  while 
the  other  two,  anorthite  and  celsian,  are  salts  of  orthosilicic  acid 
(H4Si04). 

Since  they  form  a  compact  isomorphous  group,  they  are  very 
similar  in  all  their  physical  and  crystallographical  properties. 

The  following  table  will  show  very  clearly  these  relations  and 
will  also  serve  for  their  microscopic  indentification. 

ORTHOCLASE 

Orthoclase.  —  Potassium  aluminium  trisilicate ;  KAlSisOg ; 
K2O  =  16.9,  A12O3  =18.4,  SiO2  =64.7;  Monoclinic;  Type,  Digonal 
Equatorial ;  a  :  b  :  c  =  .658  j_l :  .555  ;  p  =  116°  3'  =  001 A  100  ; 
100  A 110  =  30°  36' ;  001 A 101  =  50°  17' ;  _001  A  Oil  =  26°  31'  ; 
Common  forms,  c(001),  b(010),  m(110),  x(101),  y  (201),  n(011), 
o  (HI) ;  Twinning  common,  composition  face  b  twinning  axis 
c  (Carlsbad  twins),  composition  face  n  with  twinning  axis  ±  n 

403 


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SILICATES,   TITANATES,   ETC.  405 

(Baveno  twins),  composition  face  c  with  twinning  axis  _L  c  =  (Mane- 
bach  twins) ;  Cleavage,  basal  perfect,  b  less  so  and  M  imperfect  ; 
Brittle;  Fracture,  uneven;  H.  =  6;  G.  =  2.55;  Color,  white  to 
light  yellow,  pink,  or  red  and  gray ;  Streak,  white ;  Luster,  vitreous ; 
Transparent  to  opaque ;  a  =  1.519 ;  p  =  1.524 ;  -y  =  1.526 ;  -y  -  a 
=  .007;  Optically  (  — ) ;  Axial  plane  perpendicular  to  010;  BxaAa 
=  5°-7°  in  the  obtuse  angle  P;  2V  =  69°  43';  2E  =  120°. 

B.B.  —  Fuses  at  5,  about  1150°  C.,  rounds  on  thin  edges.  With 
potassium  flux  and  the  blue  glass  shows  potassium  flame.  Insol- 
uble in  acids. 

General  description.  —  Crystals  when  well  formed  are  generally 
combinations  of  c,  m,  b,  y,  or  x,  elongated  parallel  to  the  clino- 


FIG.  455.  —  Crystals  of  Orthoclase.     Two  are  Carlsbad  Twins,  Brice,  New  Mexico. 

axis ;  or  less  often  parallel  to  the  vertical  axis.  At  times  the  forms 
o  and  n  appear ;  other  forms  are  rare.  Crystals  are  short  and  stout ; 
when  elongated  parallel  to  the  clinoaxis  they  have  a  square  cross 
section ;  again  they  may  be  tabular,  flattened  parallel  to  the  base 
or  clinopinacoid.  Orthoclase  also  occurs  massive,  granular,  flint- 
or  jasper-like.  Twinning  is  very  common,  after  three  laws  which 
are  important. 

1.  The  Carlsbad  law,  where  the  twinning  axis  is  the  vertical  axis, 
with  the  composition  plane  parallel  to  the  clinopinacoid.  They 
may  be  either  interpenetrating  or  contact  twins,  and  if  repeated 
polysynthetically  produce  the  banded  appearance  between  crossed 
nicols  so  common  to  the  feldspar  on  the  base  or  basal  cleavage 
faces  parallel  to  the  clinopinacoid.  Carlsbad  twins  are  the  most 


406 


MINERALOGY 


commonly  occurring  twins  in  orthoclase.  The  name  is  derived 
from  Carlsbad  in  Bohemia,  where  they  are  found  in  great  perfection. 
2.  Baveno  law,  twinning  axis  perpendicular  to  n  (021),  with  n 
as  the  composition  plane.  Since  the  angle  n  A  n'  =  89°  13'  or 
nearly  90°,  the  outward  form  of  the  square  section  perpendicular 
to  the  clinoaxis  is  not  changed.  As  the  composition  plane  passes 
diagonally  across  the  prism  face  M,  making  unequal  angles  with 
the  two  faces,  upon  a  revolution  of  180°  around  the  twinning  axis 
there  will  appear  in  the  twin  crystal  a  reentrant  angle  at  one 
extremity  much  larger  than  at  the  other.  This  habit  of  twinning 
may  be  repeated  with  the  other  diagonal  of  the  square  section  as 
the  twinning  plane,  when  the  crystal  will  be  composed  of  four 
individuals.  The  name  is  derived  from  Baveno  in  Piedmont,  where 

the  pink  granite  quarries  furnish 
many  fine  crystals  twinned  after 
this  law. 

3.  Manebach  law,  twinning  axis 
is  perpendicular  to  the  base  and  the 
composition  face  is  the  base.  Since 
the  base  and  clinopinacoid  are  at 
right  angles,  the  square  section  of 
the  crystal  is  not  changed,  but  the 
form  which  terminates  the  crystal 
will  show  a  reentrant  angle  at  one 
extremity  only.  When  developed 
polysynthetically,  the  twinning 
planes  will  appear  on  the  clinopin- 
acoidal  cleavage  faces  and  not  on 
the  basal  cleavage. 

In  sections,  orthoclase  when  fresh 
is  colorless  and  transparent,  but 
often  milky  or  cloudy  through  kao- 
linization.  In  many  cases  the  crys- 
tal sections  will  show  numerous  fine  inclusions  of  hematite,  which 
may  impart  a  slight  reddish  tinge  to  the  mineral.  Orthoclase 
seems  to  possess  the  peculiar  habit  of  collecting  these  oxide 
of  iron  inclusions  from  the  magma,  and  when  porphyritic  the 
individual  crystals  may  be  pink  to  red  in  color  from  the  numerous 
inclusions. 

In  most  rocks  orthoclase  is  one  of  the  last  minerals  to  crystallize 
and  therefore  may  include  such  minerals  as  magnetite,  zircons, 


FIG.  456.  —  Baveno  Twins  of  Ortho- 
clase, from  Baveno. 


SILICATES,   TITANATES,   ETC. 


407 


apatite,  micas,  pyroxene,  or  amphiboles ;  or  it  may  form  a  ground 
mass  filling  in  the  cavities  between  the  crystals  of  earlier  crystalliza- 
tion. 

In  many  instances  quartz  and  orthoclase  crystallize  simulta- 
neously and  in  eutectic  proportions,  forming  intergrowths  of  the 
two  species,  termed 
pegmatites,  which 
when  fine  in  struc- 
ture are  micropeg- 
matites.  Cleavage 
is  always  present, 
and  at  right  angles 
when  the  section  is 
perpendicular  to  001. 
Interference  colors 
are  grays  of  the  first 
order,  and  relief  is 
not  marked.  The 
acute  bisectrix  lies 
in  the  large  angle  (3, 


FIG.  457.  —  Section  of  Orthoclase,  showing  the  Low 
Relief  and  Well-developed  Basal  Cleavage  Cracks. 


with  extinction  5° 
from  the  good  basal  cleavage.  This  angle  increases  with  the  re- 
placement of  potassium  by  sodium,  and  in  some  specimens  may  be 
as  large  as  10°. 

Orthoclase  is  the  important  feldspar  of  granites,  where  it  crys- 
tallizes at  about  the  same  time  as  quartz.  In  the  coarse  crystal- 
line pegmatites  the  two  minerals  are  in  eutectic  proportion,  two 
molecules  of  orthoclase  to  one  of  quartz,  or,  in  composition,  about 
35  per  cent,  quartz. 

Orthoclase  is  also  formed  as  the  result  of  metamorphic  agencies 
in  both  the  schists  and  gneisses  of  thermal  or  aqueous  origin. 
Secondary  orthoclase  sometimes  fills  veins,  and  is  also  associated 
with  metalliferous  fissure  veins  as  a  gangue  mineral.  It  is  easily 
decomposed,  yielding  its  alkali  to  solutions  containing  carbon 
dioxide,  especially  under  pressure,  when,  with  the  separation  of  some 
silica  and  hydration,  orthoclase  forms  kaolin.  It  is  by  this  con- 
tinuous decomposition  of  orthoclase  that  the  available  supply  of 
potassium  for  plant  growth  is  principally  sustained  in  the  soil ;  and 
it  is  well  known  that  soils  yielded  by  the  decomposition  of  granites 
are  the  most  fertile.  Other  decomposition  products  of  orthoclase 
are  muscovite,  biotite,  and  gibbsite. 


408 


MINERALOGY 


Adularia  is  a  transparent  orthoclase ;  the  most  noted  locality  is 
at  St.  Gothard  in  Switzerland.  The  name  is  derived  from  Mount 
Adula  of  that  region.  It  occurs  in  crystals  of  an  apparent  rhom- 
bohedral  habit,  being  a  combination  of  the  prism  m  and  the  base, 


FIG.  458.  —  Adularia  from  St.  Gothard,  Switzerland. 

nearly  equally  developed.  The  base  may  be  distinguished  by  the 
horizontal  striations. 

Beautiful  specimens  coated  with  chlorite  and  associated  with 
epidote  occur  in  the  Sultzbachthal,  Tyrol.  Adularia  when  cut 
and  polished  en  cabochon  is  the  moonstone  of  the  jewelers. 

Feldsite  is  a  compact  flint  or  jasper-like  rock,  showing  no  cleav- 
age, of  which  the  varieties  rhyolite,  trachyte,  and  phonolite  have 
a  chemical  composition  of  an  alkalic  feldspar.  Perthite  denotes  a 
structure  made  up  of  interlaminated  potassium  and  sodium  feld- 
spars ;  when  the  laminae  are  exceedingly  thin  it  forms  microper- 
thite. 

Potassium  and  sodium  feldspars  intercrj^stallize,  forming  alkalic 
feldspars,  while  sodium  and  calcium  feldspars  intermix,  forming 
plagioclases,  or  calcic  feldspars ;  although  the  potassium  and  cal- 
cium spars  are  isomorphous,  in  nature  they  are  seldom  found  in- 
ter crystallized. 

Sanidine  is  a  clear,  glassy,  granular  orthoclase,  usually  associated 
with  lavas  and  the  later  eruptive  rocks.  It  is  found  in  the  lavas 
of  Monte  Somma,  Vesuvius,  in  clear,  colorless  grains. 


SILICATES,   TITANATES,   ETC.  409 

Uses.  —  Orthoclase  is  quarried  in  Maine,  Massachusetts,  Con- 
necticut, Pennsylvania,  and  Maryland  from  the  pegmatites,  where 
it  is  associated  with  quartz,  muscovite,  and  small  quantities  of  mag- 
netite, tourmaline,  and  garnets.  It  is  ground  and  mixed  with  kao- 
lin and  quartz  in  the  manufacture  of  chinaware.  It  is  also  a  com- 
ponent of  both  the  glaze  and  underglaze.  It  is  also  the  flux  used 
in  the  making  of  corundum  wheels.  It  is  used  as  a  wood  filler ; 
in  scouring  soaps ;  and  in  opalescent  glass.  Artificially  orthoclase 
has  not  been  formed  by  the  dry  fusion  of  its  components,  but 
other  chemical  reagents  which  act  as  liquefiers  must  be  added,  as  the 
alkali c  feldspars  are  so  viscous  at  their  fusing  points  as  to  prevent 
crystallization  in  any  moderate  length  of  time.  If  fluorides,  tung- 
states,  or  magnetite  are  added  to  the  melt,  or  if  heated  in  the  pres- 
ence of  water  under  pressure,  crystals  are  formed. 

MICROCLINE 


Microcline.  —  KAlSi308 ;    A   triclinic   form   of  potassium   alu- 
minium trisilicate  in  which  the  cleavage  angle  varies  by  30'  from  90°, 


FIG.  459.  —  Section  of  a  Complexly  Twinned  Feldspar  between  Crossed  Nicols. 

or  the  angle  010  A  001  =  89°  30'.  In  all  other  respects  it  is  very 
much  like  orthoclase.  Their  difference  may  be  seen  in  the  table, 
page  404. 


410 


MINERALOGY 


Microcline  is  always  twinned  with  striations  differing  from  those 
of  orthoclase.  It  has  been  advanced  as  a  theory  that  o^thoclase  is 
a  triclinic  feldspar  and  that  its  monoclinic  symmetry  is  a  pseudo 
one,  and  caused  by  thin  laminae,  so  thin  as  to  be  submicroscopic, 
and  when  considered  from  this  standpoint  it  would  not  differ  from 
microcline. 

In  addition  to  the  twins  found  in  orthoclase  there  are  two  other 
important  twinning  laws  represented  in  the  twins  of  microcline  : 

1.  The  albite  twins,  in  which  the  twinning  axis  is  the  normal  to 
the  pinacoid  010,  and  the  composition  plane  is  parallel  to  010 ; 
when  repeated  polysynthetically,  this  produces  striations  on  the 
base  and  bands  between  crossed  nicols  on  the  basal  cleavage  parallel 
to  the  intersection  of  the  basal  and  brachypinacoids. 

2.  Pericline   twins,  in  which   the   twinning   axis   is   the   crys- 
tallographical  axis  b  and  the  composition  plane  is  parallel  to  the 
axis  b  and  inclined  to  the  base,  at  an  angle  depending  upon  the 
composition  of  the  feldspar.     These  twins  are  also  repeated,  and 
produce   striations    and    bands    on   both    the    brachypinacoidal 
cleavage  and  on  the   basal   cleavage.      Those   on   the   base   are 
at  right  angles  to  the  striations  produced   by  the  Carlsbad  and 
albite  twins  and  the  two  sets  of  bands  between  crossed  nicols  from 


FIG.  460.  —  Section  of  Microline,  showing  the  Gridiron  Structure  and  inclosing 
a  Crystal  of  Plagioclase. 


SILICATES,   TITANATES,   ETC.  411 

the  well-known  gridiron  structure,  recognized  in  sections  and  charac- 
teristic of  microcline,  serving  as  an  easy  means  of  distinguishing 
microcline  from  orthoclase. 

Microcline  is  associated  with  orthoclase  in  the  older  igneous 
rocks,  as  granites  and  syenites,  but  it  is  not  associated  with  sanidine 
in  the  more  recent  felsites  and  lavas. 

Amazon  stone  is  the  name  given  to  a  green  variety  of  microcline, 
beautiful  specimens  of  which  are  obtained  near  Pikes  Peak,  Colo- 
rado ;  in  fact  most  microcline  is  very  apt  to  be  light  green  in  color, 
the  cause  of  which  is  not  known. 

ALBITE 

Albite.  —  Sodium  aluminium  trisilicate,  NaAlSi3O8 ;  Na2O  = 
11.8,  A12O3  =  19.5,  Si02  =  68.7;  Triclinic;  Type,  Centrosym- 
metric;  a  :  b  :  c  =  .633  :  1 :  558  ;  a  =  94°  3';  p  =  116°  29'; 
•y  =  88°  9';  100  A  010  =  90°  3';  100  A  001  =  63°  35';=010A001 
=  86°  24';  Common  forms,  c  (001),  b  (010),  m(110),  M  (110), 
x  (101) ;  Twinning,  albite,  Carlsbad,  Baveno,  and  Manebach  laws 
common,  also  pericline  twins  ;  Cleavage,  basal  perfect,  brachypina- 
coidal  less  so,  and  m  imperfect ;  Brittle ;  Fracture,  uneven ;  H.  = 
6-6.5;  G.  =2.6-2.65;  Color,  white,  gray  to  reddish  ;  Streak,  white ; 
Luster,  vitreous ;  Transparent  to  opaque  ;  a  =  1.525;  p  =  1.530; 
V  =  1.531 ;  a  -  v  =  .006 ;  Optically  (+) ;  2  V  =  77°  39'. 

B.B.  —  Fuses  a  little  easier  than  orthoclase,  insoluble  in  acids, 
and  will  not  yield  a  potassium  flame,  at  least  not  a  strong  one. 

General  description.  —  In  habit  albite  is  tabular  parallel  to  the 
brachypinacoid  rather  than  elongated,  though  both  habits  occur. 
Crystals  are  combinations  of  the  base,  brachypinacoid,  and  the  two 
unit  prisms ;  other  forms  are  not  common.  Well-formed  crystals 
occur  in  cavities  and  veins  in  granites,  syenites,  or  gneisses,  espe- 
cially the  more  acid  varieties,  where  they  are  associated  with 
such  minerals  as  topaz,  beryl,  tourmaline,  chrysoberyl,  and  other 
rare  species.  In  color  albite  is  more  apt  to  be  white  than  is  ortho- 
clase, and  the  two  good  cleavages  are  not  at  90°  but  at  86°.  Twin- 
ning striations  caused  by  twinning  after  the  albite  law  are  charac- 
teristic and  nearly  always  appear  on  the  basal  cleavage.  In 
sections  albite  is  colorless,  with  very  low  relief,  and  is  similar  in  many 
respects  to  orthoclase,  with  which  it  may  be  intimately  intergrown 
in  laminae  parallel  to  the  macropinacoid  (perthite),  or  in  concen- 


412 


MINERALOGY 


trie  layers.     Since  it  is  triclinic,  extinction  is  always  at  an  angle, 
and  when  measured  on  the  basal  cleavage  in  regard  to  the  brachy 


FIG.  461.  —  Albite  from  Pfitsch,  Tyrol. 


cleavage  cracks  it  is  5° ;  when  measured  on  the  brachy  cleavage  in 
reference  to  the  basal  cleavage  cracks  it  is  19°.  The  interference 
color  is  a  little  higher  than  orthoclase,  but  still  gray  of  the  first  order. 
The  axial  plane  is  nearly  perpendicular  to  010.  The  acute  bisec- 


FIG.  462.  —  Pplysynthetic  Twinning  in  Albite,  Crossed  Nicols. 


SILICATES,   TITANATES,   ETC.  413 

trix  is  nearly  perpendicular  to  100  or  the  b  cleavage,  the  cleavage 
piece  yielding  a  nearly  symmetrical  interference  figure.  Opti- 
cally (+). 

Albite  is  associated  with  orthoclase  and  microcline  in  the  gran- 
ites, syenites,  gneisses,  and  schists,  and  in  the  macrocrystalline 
ground  mass  of  porphyries  as  well  as  in  phenocrysts.  It  crystal- 
lizes with  orthoclase,  forming  the  series  of  alkali  feldspars,  and 
with  anorthite  to  form  the  soda-lime  feldspars. 

In  decomposing  it  resembles  orthoclase  in  forming  kaolin,  but 
not  quite  as  easily,  since  in  the  same  sections  the  albite  may  appear 
clear  and  fresh,  while  the  orthoclase  will  be  clouded  by  kaolinization. 
It  is  associated  with  hornblende  in  diorite,  and  with  pyroxene  in 
gabbro;  here  the  feldspars  often  alter  to  a  substance  known  as 
saussurite,  a  mixture  of  albite,  zoisite,  etc. 

Albite  may  also  occur  as  a  vein  mineral  in  phyllites  and  clay- 
slates.  Artificially  albite  is  formed  under  the  same  conditions  as 
orthoclase,  except  that  sodium  salts  are  substituted  for  potassium. 

ANORTHITE 

Anorthite.  —  Calcium  aluminium  orthosilicate,  CaAl2Si208 ; 
CaO  =  20.12,  A12O3  =  36.72,  SiO2  =  43.16;  Triclinic;  Type, 
Centrosymmetric ;  a  :  b  :  c  =  .635  :  1 :  .550 ;  a  =  93°  13' ;  p  = 
115°  55';  v  =  91°  12';  100A010=83°  54';  100*001=63°  57'; 
010*001=85°  50';  Common  forms,  c(001),  b  (010),  m(110), 
M(110),  x(101),  y(201);  Twinning,  albite  law  common,  other 
laws  less  so ;  Cleavage,  basal  perfect,  010  less  so ;  Brittle ;  Fracture, 
uneven ;  H.  =  6-6.5  ;  G.  =  2.74-2.76 ;  Color,  white,  gray  to  reddish  ; 
Streak,  white;  Transparent  to  opaque;  a  =  1.574;  p  =  1.579; 
V  =  1.586 ;</-*  =  -012;  Optically  (-);  2V  =  77°  18';  Bxa  is 
53°  14'  to  the  normal  to  001  and  58°  to  the  normal  to  010. 

B.B.  —  Fuses  with  difficulty  (1532°  C.)  Decomposed  with  HC1 
gelatinizing.  The  solution  freed  of  silica  yields  tests  for  calcium. 

General  description.  —  Anorthite  in  the  pure  state,  unmixed 
with  albite,  is  very  rare ;  it  has  been  described  as  occurring  in  the 
lavas  of  Monte  Somma,  in  rounded  grains  and  in  small  complex 
crystals,  where  it  is  associated  with  hornblende  and  biotite.  It  has 
also  been  identified  at  Raymond,  Maine,  in  a  metamorphic  limestone. 

Twinning  is  polysynthetic,  after  the  albite  law,  with  twinning 
striations  on  the  base ;  other  twins  are  not  so  common. 

In  sections  anorthite  is  colorless  and  much  like  orthoclase  in  re- 


414  MINERALOGY 

lief.  The  interference  color  is  first  order  yellow,  and  extinction  is 
inclined  as  indicated  in  the  table  on  page  404.  The  interference 
figure  is  yielded  by  a  basal  cleavage  fragment,  showing  the  emer- 
gency of  an  optic  axis  in  the  plus  octant,  behind,  and  near  the  edge 
of  the  field,  with  the  plane  of  the  optic  axis  running  down  to  the  right. 

Anorthite  is  a  constituent  of  the  more  basic  igneous  rocks,  espe- 
cially gabbros,  basalts,  and  porphyrites,  and  it  also  appears  in  crys- 
talline limestones.  It  is  easily  decomposed  but  less  often  alters  to 
kaolin,  forming  rather  zeolites  and  quartz. 

Artificial  crystals  are  easily  obtained  by  the  simple  fusion  of  its 
constituent  oxides  in  the  right  proportion,  in  an  open  crucible. 
Anorthite  standing  at  the  basic  end  of  the  series  easily  forms 
crystals  in  the  simple  fusion;  this  property  is  lost,  however,  as 
albite  or  orthoclase  is  approached  at  the  acid  end  of  the  series. 

Celsian,  BaAl2Si20s,  is  a  rare  barium  feldspar  found  at  Jakobs- 
berg  in  Sweden ;  and  hyalophane  is  a  potassium  barium  feldspar, 
K^BaAUSigC^,  found  in  a  crystalline  dolomite  at  Binnenthal, 
Switzerland. 

SODA-LIME  FELDSPARS 

The  two  feldspars  albite  (ab)  and  anorthite  (an)  are  found  in 
nature  mixed  in  crystals  in  all  proportions,  and  since  they  are  solid 
solutions  and  not  double  salts,  there  are  no  theoretical  compositions 
for  members  of  the  series.  However,  mixtures  within  indicated 
limits  have  received  distinctive  names  and  form  a  complex  series 
grading  from  pure  albite  at  one  end  to  pure  anorthite  at  the  other. 

In  crystalline  habit  they  are  all  much  alike,  having  two  good 
cleavages  at  approximately  86°.  They  all  form  twins  after  the 
ordinary  laws,  but  polysynthetic  twinning  after  the  albite  law  with 
striations  or  bands  on  the  basal  cleavage  parallel  to  the  edge  c/b  is 
particularly  characteristic.  The  variation  and  trend  of  the  optical 
properties  with  the  change  in  the  composition  is  indicated  and  may 
be  seen  by  an  inspection  of  the  table  on  page  404. 

The  most  important  members  of  the  series,  with  their  composition 
expressed  in  terms  of  albite  (ab)  and  anorthite  (an),  are  as  follows: 

Albite ab   +  ano  to  ab6  +  an 

Oligoclase ab6  +  ani  to  ab3  +  an 

Andesine ab3  +  an  to  ab   +  an 

Labradorite ab   +  an  to  ab  +  ans 

Bytownite ab   +  ana  to  ab  +  an6 

Anorthite ab   +  an<5  to  ab0  -f  an 


SILICATES,   TITANATES,   ETC. 


415 


OLIGOCLASE 

Crystals  of  oligoclase  are  not  common,  but  when  well  developed 
they  are  of  the  same  general  habit  as  albite.  It  is  usually  massive 
or  cleavable.  Twinning  after  the  albite  law  is  the  common  form, 
producing  striations  on  the  base.  Pericline  twins  are  also  developed 


-b 


FIG.  463.  —  Diagram  of  the  Extinc- 
tion Angles  of  the  Feldspars  on 
the  Base  in  Reference  to  the  Pin- 
acoidal  Cleavage  010. 


+100' or  -80 
FIG.  464.  —  Diagram  of   the   Pinacoidal 
Section  010,  showing  the   Inclination 
of  the  Pericline  Twinning  Plane  in  Ref- 
erence to  the  Basal  Cleavage  Cracks. 


polysynthetically,  producing  striations  on  the  brachypinacoid ;  the 
inclination  of  these  striations  to  the  basal  cleavage  cracks  or  to  the 
edge  c/b  varies  with  the  composition  of  the  specimen,  and  is  of  great 
assistance  in  locating  the  position  of  the  crystal  in  the  series  of 
plagioclases ;  that  is,  in  roughly  determining  its  composition. 
Fig.  463  is  a  diagram  showing  how  this  angle  is  measured  in  a  plus 
or  minus  direction  and  also  giving  its  value  for  each  member  of 
the  series. 

Oligoclase  being  a  member  of  the  acid  end  of  the  series  is  associated 
with  orthoclase  and  albite  in  the  granites,  syenites,  diorites,  and 
their  porphyritic  equivalents,  where  the  individual  crystals  usually 
contain  large  quantities  of  glass  as  inclusions. 


416 


MINERALOGY 


In  decomposition  through  weathering  it  usually  forms  kaolin, 
and  under  varied  conditions  forms  zeolites,  epidote,  and  quartz. 

Labradorite  is  a  rock-forming  soda-lime  feldspar,  dark  gray  to 
nearly  black  in  color,  often  showing  a  play  of  colors  or  iridescence, 
In  composition  it  is  a  member  of  the  basic  end  of  the  series  of 

plagioclases.  The  relation 
of  its  optical  properties  to 
other  members  of  the  series 
is  shown  in  the  table,  page 
404. 

In  occurrence  it  is  asso- 
ciated with  the  more  basic 
rocks,  particularly  the  erup- 
tive rocks.  It  is  rarely  as- 
sociated with  quartz  or 
orthoclase.  In  sections  it 
appears  in  characteristic 
lath-shaped  crystals,  espe- 
cially in  the  diabases.  Since 
the  more  basic  feldspars  are 

FIG.  465.  — Diagram  of  the  Pinacoidal  Sec-  the   first  to  Crystallize  from 

tion  010,  showing  the  Extinction  Angles  of  a  magma,    they   often  show 

the    Feldspar    in    Reference    to   the  Basal  zonal     structure     in     which 
Cleavage  Cracks. 

the  central  portion  is   lab- 

radorite,  each  zone  outwardly  becoming  less  basic  or  calcic  in 
character.  This  change  in  composition,  from  basic  to  acid,  may 
also  take  place  in  passing  from  the  exterior  portions  of  a  rock 
mass  to  the  interior  portions.  Labradorite  is  associated  with 
pyroxene,  amphibole,  and  magnetite  in  those  rocks  in  which  the 
ferromagnesian  content  is  considerable. 


LEUCITE 

Leucite.  —  Potassium  aluminium  metasilicate,  KAl(SiO3)2; 
K2O  =  21.5,  A1203  =  23.5,  SiO2  =  55;  Pseudoisometric.  Forms, 
n(211),  d(110),  c(100);  Twinning  plane  =  110;  Cleavage,  110, 
imperfect ;  Brittle ;  Fracture,  conchoidal ;  H.  =  5.5-6 ;  G.  =  2.45 
-2.5 ;  Color,  white,  gray ;  Streak,  white ;  Luster,  vitreous ;  Trans- 
parent to  nearly  opaque;  €  =  1.509;  to  =  1.508;  €  -  co  =  .001  • 
Optically  (+). 


SILICATES,   TITANATES,   ETC. 


417 


B.B.  —  Infusible  (about  1400°),  shows  potassium  with  the  flux 
and  blue  glass.  Decomposed  with  HC1  without  gelatinization 
but  with  the  separation  of  silica.  The  solution  freed  of  silica 
yields  a  precipitate  of  aluminium  hydroxide  with  ammonia.  The 
powdered  mineral  often  becomes  blue  when  treated  with  cobalt 
solution. 


General  description.  —  Crystals  are  usually  simple  tetragonal 
trisoctahedrons,  rarely  in  combinations  with  the  cube  or  rhom- 
bic dodecahedrons ;  also  in 
rounded  grains,  as  in  the 
lavas  of  Vesuvius;  seldom 
massive.  Leucite  is  dimor- 
phous ;  the  apparently  sim- 
ple isometric  crystals  at 
ordinary  temperatures  are 
complex  aggregates  of 
twinned  lamellae,  and  prob- 


FIG.  466.  —  Leucite.    Vesuvius,  Italy. 


ably  orthorhombic.     When 

heated    above    500°    these 

complex  individuals  become  simple  isometric  crystals,  losing  their 

double  refraction,  and  are  isotropic.     The  isometric  form,  being 


FIG.  467.  —  Section  of  a  Leucitite,  showing  Rounded  Leucite  Crystals 
with  Dark  Inclusions  arranged  Symmetrically. 

2E 


418 


MINERALOGY 


stable  above  500°,  and  the  orthorhombic  form  the  stable  form 
below  that  temperature. 

Leucite  is  a  constituent  of  igneous  rocks,  especially  those  ter- 
tiary or  recent  lavas  rich  in  potassium  and  aluminium  in  which  the 
silica  is  insufficient  to  produce  feldspars.  It  is  associated  with  augite, 
haiiynite,  nepheline,  olivine,  apatite,  and  magnetite. 

In  rock  sections  the  crystals  appear  in  polygonal  or  nearly  circular 
outlines,  colorless  and  with  a  very  low  relief.  Inclusions  are  very 
common  and  symmetrically  arranged  in  concentric  belts  or  radiated 
starlike.  These  inclusions  may  be  glass  or  fine  crystals  of  those 
minerals  which  have  preceded  the  leucite  in  crystallization,  as 
magnetite,  apatite,  or  augite.  Between  crossed  nicols  the  twinning 
lamella?  will  appear  in  large  crystals,  showing  an  interference  color 


FIG.  468.  —  Outline  of  a  Leucite  Crystal  in  a  Section  of  Porphyry. 

of  a  very  low  gray  of  the  first  order,  while  the  small  rounded  grains 
may  appear  as  isotropic. 

Leucite  is  easily  decomposed,  yielding  its  potassium  to  percolat- 
ing waters,  which  may  at  the  same  time  replace  the  potassium  with 
sodium;  this  replacement  and  hydration  yields  analcite,  which 
crystallizes  in  the  same  form.  The  leucites  of  the  older  rocks  have 


SILICATES,    TITANATES,    ETC.  419 

all  had  their  potassium  replaced  at  least  in  part  in  this  way.  In 
other  instances  leucite  has  produced  feldspars  or  micas  which  have 
retained  the  crystalline  form,  producing  pseudomorphs.  Ulti- 
mately leucite  decomposes,  forming  kaolin. 

Artificially  leucite  has  been  produced  by  simply  fusing  its  con- 
stituents in  an  open  crucible  and  cooling  the  melt  slowly ;  also  by 
fusing  mixtures  of  natural  minerals,  as  microcline  and  biotite,  or 
from  the  fusion  of  muscovite  alone.  When  leucite  is  heated  to 
195°  in  a  solution  of  sodium  chloride  or  carbonate,  analcite  is  formed. 

PYROXENES 

The  pyroxenes  are  the  most  important  ferromagnesian  rock-form- 
ing minerals.  They  are  essentially  metasilicates  of  the  general 
formula,  R"SiO3  in  which  R"  may  be  Mg,  Ca,  Fe,  Mn,  Zn,  Na2, 
Li2,  or  the  triad  elements  Al,  Fe"',  or  Mn'",  which  enter  with  a  sec- 
ond more  or  less  complex,  as  R"  R'"  SiOe.  Again  silica  may  be 
replaced  in  part  by  Zr,  Ti,  or  Cb. 

The  series  in  symmetry  runs  through  the  orthorhombic,  mono- 
clinic,  and  triclinic  systems.  Magnesium  metasilicate  is  ortho- 
rhombic,  calcium  metasilicate  is  monoclinic,  manganese  metasili- 
cate is  triclinic,  and  the  ferrous  metasilicate  has  not  been  found 
unmixed  with  other  silicates  as  a  pyroxene,  but  as  an  amphibole. 
The  members  of  the  series  are  isomorphous,  and  have  many  char- 
acteristics in  common,  which  may  be  seen  by  an  inspection  of  the 
table  which  follows.  They  have  two  cleavage  directions  at  an  angle 
near  87°,  which  serves  to  distinguish  them  from  the  closely  related 
forms  of  amphibole.  In  crystalline  habit  they  are^  usually  short, 
stout,  prismatic,  or  columnar. 

ORTHORHOMBIC  PYROXENES 

The  orthorhombic  pyroxenes  are  mixtures  of  the  metasilicates 
of  iron  and  magnesium;  some  calcium  may  enter  the  molecule, 
though  always  in  small  proportions.  The  two  species  enstatite 
and  hypersthene  in  composition  grade  into  each  other,  the  varie- 
ties depending  upon  the  percentage  of  ferrous  iron  present ;  when 
this  amounts  to  10  to  12  per  cent,  the  mineral  is  known  as  bronzite ; 
when  much  greater,  hypersthene ;  when  much  less,  enstatite. 


GRAVITY 

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COMPOSITION 

MgSiOs 
(Mg  •  Fe)  SiOa 

CaMg(Si03)2 

I 

(Ca  •  Mg)(Fe  •  Mn)(SiO3)2 
(Ca  •  Mg  •  Zn)(Fe  •  Mn)(SiO3)2 

CaMg(SiO3)2 
Mg(Al  •  Fe)2SiOa 

NaFe(SiO3)2 
NaAl(SiO3)2 
LiAl(SiOs)2 

Na(Mn  -"Ca  •  Fe)(ZrO.F)(SiO3)2 
12(Na2Ca)(Si-Zr)O3 
RNb2O6  with  F 

Na2Ca3((Si  •  Zr  •  Ti)O3)4 
H  NaCa2(SiOs)3 

CaSiOs 

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Schefferite 
Jeffersonite 

S        .-§     .- 

Spodumine 

Lavenite 
Wohlerite 

Rosenbuschite 
Pectolite 

Wollastonite 

Rhodonite 

Fowlerite  * 

Hiortdahlite  \ 

Babingtonite 

II 

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420 


SILICATES,   TITANATES,   ETC.  421 


ENSTATITE 

Enstatite.  —  Magnesium  metasilicate,  MgSi03 ;  MgO  =  40, 
Si'O2  =  60 ;  Orthorhombic  ;  Type,  Didigonal  Equatorial ;  a :  b  : 
c  =  1.0307:1:. 588;  100 A 110  =  44°  8' ;  001 A  101  =  30°  29'; 
001A011=29°  43';  110A110  =  88°  16'.  Crystal  forms  rare, 
c  (001),  a  (100),  b  (010),  m  (110) ;  Twinning  rare,  forming  lamellae ; 
Cleavage,  prismatic,  easy ;  Brittle ;  Fracture,  uneven ;  H.  =  5.5  ; 
G.  =  3.1-3.3;  Color,  gray,  yellowish,  olive  to  brown;  Streak, 
white  to  gray ;  Luster,  vitreous  to  silky,  metalloidal  and  bronze- 
like;  Transparent  to  opaque;  a  =  1.665;  P  =  1.669;  *y  = 
1.674;  Y-a=.009;  Optically  (+) ;  Axial  plane  =  010;  Bxa 
=  c ;  2  V  =  70°,  varies  with  Fe  present. 

B.B.  —  Fuses  with  difficulty  (1400°).     Insoluble  in  acids. 

HYPERSTHENE 

Hypersthene.  —  (Mg.  Fe)  SiOa ;  with  FeO  greater  than  10  per 
cent.  Orthorhombic  ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c  = 
.9713  :  1 :  .5704 ;  100  A  110  _=  44°  10' ;  001  A  101  =  30°  25'  ; 
001  A 01 1  =  29°  42';  1 10 A  110  =  88°  20';  Crystals,  rare;  Cleav- 
age, b  perfect,  m  and  a  less  so;  Brittle;  Fracture,  uneven; 
H.  =  5-6 ;  G.  =  3.4-3.5 ;  Luster,  pearly  to  metalloidal,  bronze-like  ; 
Color,  dark  brown,  greenish  to  black;  Translucent  to  opaque; 
a  =  1.692;  p  -  1.702;  <y  =  1.705;  y  -  a  =  .013;  Optically 
(-);  Axial  plane  =  010;  Bxa  =  a;  2V  =  50°  varies  with  Fe. 

B.B.  —  Fuses  with  difficulty  to  a-black  slag.  On  coal  in  R.  F. 
becomes  magnetic.  Partially  decomposed  with  HC1. 

General  description.  —  These  two  minerals  are  rarely  found 
showing  crystalline  faces.  Large  crystals  of  enstatite,  combina- 
tions of  m,  a,  and  b,  terminated  by  a  series  of  brachydomes  and  the 
pyramid  (223),  occur  at  Bamle,  Norway.  Usually  they  occur 
lamellar,  foliated  or  fibrous  in  structure,  cleavable  at  an  angle 
near  88°.  The  transition  form  between  enstatite  and  hypersthene 
is  known  as  bronzite  from  its  peculiar  metalloidal  bronze-like 
luster. 

In  sections  enstatite  is  either  colorless  or  pale  shades  of  yellow 
or  green,  always  much  lighter  than  hypersthene,  which  is  rather 
reddish ;  both  are  irregular  or  in  rough  crystalline  outline,  with 
relief  well  marked,  and  surface  appearing  rough;  cleavage  well 


422 


MINERALOGY 


FIG.  469.  —  Enstatite.    Bamle,  Norway. 

developed  at  an  angle  near  88°.  Pleochroism  varies  with  the  iron 
content ;  it  is  almost  absent  in  enstatite  and  well  marked  in  hyper- 
sthene,  changing  from  brown  to  green  when  parallel  to  the  axis  t. 
Interference  color,  upper  first  order.  The  acute  bisectrix  in  ensta- 
tite is  parallel  to  c,  and  when  iron  reaches  10  per  cent,  it  changes 
to  the  axis  &. 

Inclusions  are  very  characteristic,  consisting  of  microscopic  crys- 


FIG.  470.  —  Section  of  Enstatite  showing  the  Regularly  Arranged  Inclusions 
causing  the  "  Schiller." 

tals  in  plates,  rods,  or  grains,  arranged  parallel  to  the  brachy- 
pinacoid,  producing  the  "  schiller  "  so  characteristic  of  these  two 


SILICATES,  TITANATES,  ETC.  423 

minerals.  Minerals  crystallizing  earlier,  as  apatite,  zircon,  or 
magnetite,  may  occur  as  inclusions,  while  lamellar  intergrowths 
parallel  to  010  with  monoclinic  pyroxene  are  not  uncommon. 

Orthorhombic  pyroxenes  are  common  pyrogenetic  constituents 
of  igneous  rocks,  especially  the  more  basic  varieties,  as  peridotites, 
norites,  diorites,  and  some  syenites  and  their  corresponding  por- 
phyries and  lavas,  where  they  are  associated  with  monoclinic  py- 
roxenes, olivine,  plagioclases  and  micas,  but  never  associated  with 
nepheline  or  leucite. 

In  their  weathering  and  hydration  they  form  bastite,  a  fibrous 
substance,  in  composition  much  like  serpentine ;  other  alteration 
products  are  serpentine,  talc,  carbonates,  iron  oxides,  and  quartz. 

Artificial.  —  There  are  four  modifications  of  magnesium  meta- 
silicate,  two  of  which  are  pyroxenes,  the  other  two  amphiboles. 
Of  the  two  pyroxenes  one  is  monoclinic,  the  other  orthorhombic. 
The  transition  temperature  between  these  two  modifications  is 
about  1100°  C. 

When  magnesia,  iron  oxides,  and  silica  are  fused  in  an  open 
crucible  and  allowed  to  crystallize  near  their  fusing  point,  the 
monoclinic  form  appears.  If  crystallization  takes  place  below 
1100°,  the  orthorhombic  form,  enstatite,  will  appear. 

If  serpentine  is  fused,  it  breaks  down,  on  cooling,  into  a  mixture 
of  enstatite  and  olivine. 

MONOCLINIC  PYROXENES 

In  the  monoclinic  pyroxenes,  calcium,  iron,  and  aluminium  enter 
the  molecule  and  are  more  important  than  magnesium.  While 
the  varieties  thus  formed  agree  closely  in  their  crystallographic 
habit  and  physical  properties,  they  differ  widely  in  their  occur- 
rence and  association.  The  varieties  from  diopside  to  acmite  and 
jadeite  are  included  under  the  species  pyroxene  and  they  may 
vary  in  their  composition  and  pass  one  into  the  other  gradually. 
Their  normal  compositions  and  relations  are  given  in  the  table, 
page  420. 

DIOPSIDE 

Diopside.  —  Calcium    magnesium    metasilicate,    CaMg(SiOs)2 ; 
Monoclinic;    Type,  Digonal   Equatorial;    &  :b:c  =  1.0921:1: 
.5893;   P  =  74°  10'  =  100A001;   100 A  110  =  46°  25';  001 A 101  = 
24°   21';    001 A Oil  =29°   33';  Common  forms,  c(001),  b(010), 
a  (100),  m(110),  s(lll),  w(lll);    Twinning  plane  110,  contact 


424 


MINERALOGY 


twins  and  repeated,  parallel  to  001  producing  striations  and 
parting ;  Cleavage,  prismatic  at  87°  12'  perfect,  a  less  so ;  Brittle  ; 
Fracture,  conchoidal;  H.  =  5-6;  G  =  3.2-3.6;  Color,  white, 
green  to  brown ;  Streak,  white  to  gray ;  Luster,  vitreous  to  dull  ; 
Transparent  to  opaque;  a=  1.6707;  p  =  1.6776;  -y  =  1.6996; 
•y-a  =  .0289;  Optically  (+);  Axial  plane  =  010;  Bx»Ac  =  38° 
in  front ;  2  V  =  59°  70'. 

B.B.  —  Fusibility  below  five,  but  varies  with  the  composition. 
If  much  iron  is  present,  it  may  become  magnetic  in  R.  F.  on  coal. 
Generally  insoluble  in  acids. 

General  description  of  the  pyroxenes.  —  In  habit  the  crystals 
are  short,  stout  prisms,  rarely  much  elongated  except  in  acmite 
and  diallage.  The  pinacoids  are  well  developed  and  usually  ter- 
minated by  the  plus  and  minus  unit  pyramids.  In  some  speci- 
mens the  two  end  terminations  may  differ,  indicating  a  symmetry 
lower  than  that  of  digonal  equatorial.  Diopside  is  the  light-colored 
variety  and  contains  but  little  iron  and  aluminium,  while  the 
dark  opaque  varieties  are  augite.  Jeffersonite  is  a  variety  from 
Franklin,  New  Jersey,  containing  zinc ;  and  schefferite  is  a  man- 
ganese variety  from  Sweden.  Hedenbergite  is  a  dark  green  variety 
containing  iron,  which  replaces  most  of  the  calcium;  it  was  first 
described  by  Berzelius  as  from  Sweden.  Diallage  is  a  variety 

elongated   in   habit,    and 

in  most  instances  is  lam- 

§inated    or    even    fibrous 
4^H      fi      mi?  parallel   to  010;   it  may 

fe          be   intergrown    with   the 
^^^0  orthorhombic     pyroxenes 

and  exhibit  the  same 
' l  schiller ' '  character]  stic 
of  those  varieties. 

In  thin  sections  it  is 
either  in  polygonal  out- 
line, rounded,  or  irregu- 
lar. When  crystals  are 
FIG.  471.  —  Augite  Crystals.  Bilin,  Bohemia,  well  formed  they  are  eight- 
sided  in  section,  formed 

by  the  two  pinacoids  and  the  unit  prism.  The  color  varies  from 
colorless  in  diopside,  through  various  shades  of  green  and  brown, 
in  augite  or  acmite,  the  color  depending  upon  the  amount  of  iron 


SILICATES,   TITANATES,   ETC. 


425 


FIG.  472.  —  Augite  from  Egausville,  Ren- 
frew County,  Canada. 


present.     Relief   is   high   and   the   sections    appear  very  rough. 

Cleavage  is  always  well  developed,  and  in  sections  at  right  angles 

to  the  vertical  axis  will  show 

the   true   prismatic  angle   of 

87°  12'.     Pleochroism  is  not 

marked.     The  cleavage  angle 

and  lack  of  pleochroism  serve 

to  distinguish  the  pyroxenes 

from  the  amphiboles. 

Crystals   may   show   zonal 

structure    and     intergrowths 

with    the    orthorhombic    va- 
rieties.   Extinction  in  sections 

parallel   to    the    orthoaxis   is 

either  straight  or  bisects  the 

cleavage  angle ;    in   all  other 

sectionsthere  is  inclined  extinc- 
tion, increasing  to  a  maximum 

in   the    plane    of    symmetry, 

where  it  varies   from  38°   to 

54°,  according  to  the  composition  of  the  specimen.     The  acute 

bisectrix  is  in  the  large  angle  |3 ;  the  angle  between  it  and  the 

axis  c  varies  with  the  variety,  as 
is  shown  in  the  diagram,  Fig. 
473 .  Double  refraction  is  strong, 
from  .020  to  .030,  always  yield- 
ing interference  colors  of  at  least 
the  second  order.  Interference 
figure  distinct,  showing  an  optic 
axis  in  the  basal  section  near  the 
center  of  the  field ;  the  other  axis 
emerges  in  the  orthopinacoid. 
Optically  (+). 

The  pyroxenes  are  constituents 
of  the  igneous  rocks,  especially  the 
ferro-magnesian  varieties,  erup- 
tives  and  lavas.  They  are  one  of 
the  essential  minerals  of  gabbros, 

FIG.  473.  — Diagram  of  the  Plane  of     where  they  are  associated  with  the 

the  Optic  Axes  010,  showing  the  Ex-         j      iod  biotite     hornblende, 

tinction   of  the  Various  Species  of      K  <  ° 

Pyroxenes.  olivine,  and  less  often,  with  quartz. 


426  MINERALOGY 

Diallage  is  the  usual  pyroxene  of  gabbro;  in  the  diorites  and 
norites  it  is  associated  with  hornblende  and  hypersthene.  The  dark 
colored  varieties,  as  augite  and  segirite,  are  more  often  associated 
with  nepheline  in  the  elseolite-syenites  and  with  leucite  in  the 
recent  alkali  lavas.  Pyroxenes  occur  less  frequently  with  the 
schists  and  sparingly  as  a  metamorphic  product  associated  with 
limestone.  The  green  sodium  pyroxene,  jadeite,  is  never  in  indi- 
vidual crystals,  but  occurs  in  very  tough  and  compact  masses, 
breaking  with  a  splintery  fracture.  It  is  much  prized  by  the  Chinese, 
who  have  used  it  for  ages  as  a  material  out  of  which  they  carve 
ornaments.  It  is  one  of  the  minerals  commonly  known  as  jade. 
An  alteration  characteristic  of  pyroxene  is  uralitization,  in  which 
there  is  a  physical  change  from  pyroxene  to  a  fibrous  amphibole 
with  little  or  no  chemical  change ;  the  fibers  of  the  uralite  always 
lie  parallel  to  the  c  axis ;  the  formation  of  epidote  or  zoisite  may 
accompany  uralitization.  Those  varieties  containing  much  mag- 
nesium are  apt,  in  their  weathering,  to  form  talc,  serpentine,  and 
bastite,  while  the  calcium  will  form  calcite;  when  more  iron  is 
present,  biotite,  chlorite,  and  calcite  are  produced. 

Pyroxenes  are  easily  crystallized  from  the  fusion  of  their  con- 
stituents in  open  crucibles.  They  are  common  minerals  of  blast 
furnace  slags. 

SPODUMENE 

Spodumene.  —  Lithium  aluminium  metasilicate,  LiAl(Si03)2 ; 
Li2O  =  8.4,  A12O3  =  27.4,  Si02  =  64.5 ;  Monoclinic ;  Type, 
Digonal  Equatorial ;  a  :  b  :  c  =  1.1283  : 1 :  .6234 ;  0  =  69°  32'  = 
001 A 100 ;  100  A 110  =  46°  30' ;  001 A  101  =  33°  25' ;  001 A  01  r  = 
30°  47' ;  Common  forms,  c  (001),  a  (100),  b  (010),  m  (110),  p  (110°) ; 
Twinning  plane  parallel  to  100;  Cleavage  prismatic  87°  perfect; 
Brittle;  Fracture,  uneven;  H.  =  6.5-7;  G.  =  3.13-3.20;  Color, 
gray,  yellowish,  green,  or  purple ;  Streak,  white ;  Luster,  vitreous 
to  pearly ;  Transparent  to  opaque ;  for  optical  properties  see  table, 
page  420. 

B.B.  —  Fuses  easily  to  a  clear  glass,  yielding  a  lithium  flame 
especially  with  the  potassium  bisulphate  flux.  Insoluble  in  acids. 

General  description.  —  Crystals  are  elongated  in  habit,  parallel 
to  the  vertical  axis,  with  striations  on  the  prism  zone  lengthwise. 
At  times  they  are  of  enormous  size,  as  at  the  Etta  mine  in  South 
Dakota,  where  it  is  said  that  crystal  faces  forty  feet  in  length  have 


SILICATES,   TITANATES,   ETC. 


427 


been  uncovered ;  the  mineral  is  mined  there  for  the  manufacture  of 
lithium  compounds.  There  is  a  lamellar  structure  very  often 
developed  parallel  to  the  orthopinacoid  and  when  prominent  yields 
a  parting. 

Spodumene  occurs  in  the  pegmatites  of  New  England,  as  at 
Mount  Mica  and  Windham,  Maine ;  at  Goshen,  Massachusetts,  and 
Branchville,  Connecticut.  In  these  pegmatites  it  is  associated  with 


FIG.  474.  —  Spodumene.    Goshen,  Massachusetts. 

tourmaline,  amblygonite,  lepidolite,  and  beryl.  The  green  variety, 
hiddenite,  is  found  in  cavities  in  a  gneiss-like  rock  at  Stony  Point, 
Alexander  County,  North  Carolina,  where  it  is  associated  with  a 
clear  green  beryl. 

Kunzite,  a  lilac-colored  variety,  is  mined  at  Pala,  San  Diego 
County,  California.  Both  of  these  clear  colored  varieties  are  used 
as  gems ;  beautiful  specimens  have  lately  been  discovered  in  a  coarse 
pegmatite  in  the  island  of  Madagascar,  where  they  are  asso- 
ciated with  magnificent  tourmalines,  lepidolite,  and  the  pink  beryl 
(morganite). 

By  an  interchange  of  alkali,  spodumene  alters  to  albite,  micro- 
line,  or  niuscovite. 

PECTOLITE 


Pectolite.  —  An  acid   sodium  calcium  metasilicate, 
(Si03)3;  Na2O  =  9.3,  CaO  =  33.8,Si02=  54.2,  H2O  =  2.7;  Mono- 


428 


MINERALOGY 


clinic ;  Type,  Digonal  Equatorial ;  a  :  b 
P  =  84°  40'  =  001  A  100;  100  A  110  =  47° 
10';  001 A  Oil  =  44°  29';  Forms,  c  (001),  a 
w  (140) ;  Twinning  plane,  100 ;  Cleavage,  a 
Fracture,  uneven ;  H.  =  5 ;  G.  =  2.68-2.8J 
Streak,  white ;  Luster,  silky ;  Transparent 
•y  —  a  =  .038 ;  Axial  plane  perpendicular  to 
of  90°  nearly  with  the  axis  c  ;  Bxa  =  b  ;  2V 


:c  =1.1140:  lr.9864; 
57' ;  001 A  101  =  39° 
(100),  h  (540),  q(340), 
and  c  perfect ;  Brittle; 
\ ;  Color,  white,  gray ; 
to  opaque;  n  =  1.61; 
010,  making  an  angle 


=  60°;    Optically  (+). 

B.B.  —  Fuses  quietly,  coloring  the  flame  yellow  (Na).  Gelat- 
inizes with  HC1;  the  solution  freed  of  silicia  yields  little  or  no 
precipitate  with  ammonia,  but  a  heavy  white  precipitate  with 
ammonium  carbonate  (calcium). 

General  description.  —  Crystals  are  elongated  and  needle-like, 
in  radiated,  nodular  masses,  with  an  opaque  porcelain  appearance 
or  silky ;  sometimes  in  groups  of  long,  slender  and  parallel,  though 


FIG.  475.  —  Pectolite.     Franklin,  New  Jersey. 

individual,  crystals.  Elongation  is  parallel  to  the  orthoaxis,  with 
the  base  and  orthopinacoid  as  the  prominent  faces,  terminated  by 
h  and  w.  Tabular  crystals  are  rare,  but  they  have  been  found  at 
Bergen  Hill,  New  Jersey. 

Pectolite  is  found  as  a  secondary  mineral  in  the  cracks  and  cavities 
of  the  basic  eruptive  rocks,  where  it  has  been  deposited  by  percolat- 
ing waters  and  associated  with  zeolites,  calcite,  apophyllite,  and 
datolite.  It  is  a  common  mineral  in  the  traps  of  Bergen  Hill  and  Pat- 
erson,  New  Jersey ;  Isle  Royal,  Michigan ;  Magnet  Cove,  Arkan- 


SILICATES,   TITANATES,   ETC.  429 

sas.     A  massive  variety  has  been  described  from  Tehama  County, 
California,  and  Alaska. 

WOLLASTONITfc 

Wollastonite.  —  Calcium  metasilicate,  CaSiOa ;  CaO  =  48.3, 
SiO2  =  51.7;  Monoclinic ;  Type,  Digonal  Equatorial;  &:b:c  = 
1.0531  :  1  :  .9676;  P  =  84°  30'  =  100  A  001;  100  A 110  =  46° 
21';  001 A 101  =  45°  5';  001  A  Oil  =  43°  55';  Common  forms, 
c(001),  a  (100),  h(540),  m  (110),  v  (101) ;  Twinning  plane,  100; 
Cleavage,  a  perfect,  c  less  so  ;  Brittle  ;  Fracture,  uneven ;  H.  = 
4.5-5 ;  G.  =  2.8-2.9 ;  Color,  white,  gray,  yellow,  brown,  or  pink ; 
Streak,  white  ;  Luster,  vitreous  to  pearly ;  Translucent  to  opaque ; 
a  =  1.621;  p  =  1.633;  v  =  1.635;  <y  =  a  =  .014;  Optically 
(-);  Axial  plane  =  010;  BxaAc=  32°  12'  behind;  2E=70°. 

B.B.  —  Fuses  quietly  to  a  white,  somewhat  glassy  globule.  Gelat- 
inizes with  HC1;  the  solution  freed  of  silica  yields  little  or  no 
precipitate  with  ammonia,  but  a  heavy  white  precipitate  with 
ammonium  carbonate  (calcium). 

General  description.  —  In  habit  crystals  are  tabular,  with  the 
base  as  the  prominent  face,  or  elongated  parallel  to  the  orthoaxis ; 
also  fibrous  or  divergent,  but  usually  in  cleavable  masses.  The 
color  is  white  to  gray  in  pure  material,  but  as  iron  and  manganese 
replace  the  calcium  the  color  becomes  brown  or  pink. 

Wollastonite  is  a  characteristic  mineral  of  contact  metamorphic 
regions  where  there  is  an  abundance  of  calcium ;  it  is  therefore  a 
common  mineral  in  crystalline  limestones,  where  it  is  associated 
with  garnets,  epidote,  vesuvianite,  diopside,  etc.  It  may  occur  in 
lavas  and  basalts,  where  it  is  associated  with  nepheline. 
A  pink  variety  containing  considerable  manganese  occurs  at 
Franklin,  New  Jersey.  Wollastonite  also  occurs  at  various  places 
in  Lewis  County,  New  York,  in  well-formed  crystals ;  in  the  Lake 
Superior  region,  and  at  Grenville,  Quebec. 

There  are  two  forms  of  calcium  metasilicate ;  the  /3-CaSi03  is 
stable  below  1190°.  This  form  is  the  mineral  Wollastonite.  The 
other  form,  a-CaSi03,  is  pseudo-hexagonal,  is  the  stable  form  above 
1190°,  and  is  formed  when  the  constituents  are  fused  in  an  open 
crucible  and  cooled  quickly.  It  is  the  common  form  of  calcium 
metasilicates  found  in  slags.  Wollastonite  is  formed  when  a  glass 
of  the  composition  of  CaSi03  is  heated  to  900°  or  1000°  and  kept 


430  MINERALOGY 

at  that  temperature  for  some  time;  or  a-CASi03  may  be  in- 
verted to  £-CaSi03  by  the  addition  of  a  fluoride  or  boric  acid  and 
cooling  slowly. 

RHODONITE 

Rhodonite.  — Manganese  metasilicate,  MnSiO3;    MnO  =  54.1 
Si02  =  45.9 ;     Triclinic ;     Type,    Centro-symmetric  ;     a  :  b  :  c  = 
1.0729  : 1 :  .6213 ;     a  =  103°  18' ;     p  =  108°  44' ;     y  =  81°  39' 
100  A  010  =  94°   26';    100  A  001  =  72°    36';    010  A  001  =  78°    42' 
100  A  110  =  4&°   33';     Crystal   forms,     c  (001),    a  (100),    b  (010) 
m  (110),  M  (110) ;  Cleavage,  prismatic  (92°  28')  perfect,  c  less  so  : 
Brittle;    Fracture,  uneven;    H.  =  5.5-6.5;   G.  =  3.4-3.7;   Color, 
shades  of  red  and  pink ;  Streak,  white ;  Luster,  vitreous  to  pearly, 
Translucent  to  opaque;  Optically  (-) ;  2  V  =  76°  12'. 

B.B.  —  Fuses  at  three  to  a  black  slag.  With  borax  yields  a 
manganese  reaction.  Nearly  insoluble  in  acids ;  some  may  effer- 
vesce from  containing  carbonates,  and  others  (fowlerite)  may  yield 
a  zinc  coat  on  coal. 

General  description.  —  Crystals  are  usually  large  and  rough, 
combinations  of  the  three  pinacoids,  flattened  parallel  to  the  base  or 
elongated  parallel  to  the  vertical  axis.  The  crystal  faces  often 
appear  glassy,  as  if  fused;  also  massive,  granular,  or  in  rounded 
separate  grains.  On  exposure  the  surface  blackens  from  oxidation, 
and  impure  varieties  may  be  yellow  or  green  ;  this  variation  in  color 
is  due  to  the  replacement  of  manganese  by  iron,  calcium,  or  zinc. 
Fowlerite  from  Franklin,  New  Jersey,  is  a  light-colored  form  con- 
taining as  much  as  7  per  cent.  ZnO ;  while  bustamite  is  a  variety 
containing  at  times  as  much  as  20  per  cent.  CaO. 

Rhodonite  is  found  in  schists  and  metamorphic  limestones ;  it  is  a 
product  of  contact  metamorphism.  At  Franklin,  New  Jersey,  it  is 
associated  with  wollastonite,  spinels,  and  garnets.  At  Filipstad 
in  Sweden  it  is  associated  with  the  iron  ores.  A  massive  variety  is 
mined  near  Ekaterinberg,  Urals,  Russia,  and  used  as  an  ornamental 
stone. 

In  weathering  the  manganese  forms  carbonates.  The  silica 
forms  a  soluble  silicate  and  quartz ;  when  the  decomposition  is 
incomplete,  the  mineral  is  a  mixture  of  silicate  and  carbonate. 


SILICATES,   TITANATES,   ETC.  431 

AMPHIBOLES  AND  BERYL 
AMPHIBOLES 

The  amphiboles  are  metasilicates  of  the  formula,  R/'SiOa  or 
R'R'"(SiO3)2  in  which  R  may  be  Na,  K,  B«"  =  Ca,  Mg,  Fe,  Mn,  and 
R'"  =  Al  or  Fe,  or  they  may  be  mixed  silicates  in  which  there  is 
one  molecule  of  an  ortho-  and  one  of  a  trisilicate ;  this  would  yield 
the  same  proportions  of  silicon  and  oxygen.  Such  radicals  as 
(A12.OF2)",  (Fea.OFj),",  (A120(OH)2)",  and  (Fe20(OH)2)"  also 
enter  the  molecule  of  the  more  complex  varieties.  They  are  dis- 
tributed through  the  orthorhombic,  monoclinic,  and  triclinic 
systems  as  are  the  pyroxenes,  but  the  orthorhombic  varieties  are 
very  unimportant  and  not  rock-forming  minerals.  They  differ 
from  the  pyroxenes,  with  which  the  simpler  forms  are  dimorphous, 
in  that  magnesia  or  its  equivalent  enters  the  molecule  in  greater 
proportions,  —  in  amphibole,  in  amounts  of  three  or  four  to  one  of 
calcium,  while  in  pyroxene  they  are  in  the  proportion  of  one  to  one. 
They  all  cleave  at  an  angle  approximately  124°,  while  in  pyroxenes 
it  is  87°.  In  crystalline  habit  they  are  elongated  or  fibrous,  with 
the  exception  of  the  basaltic  hornblende,  which  occurs  usually  in 
equidimensional  crystals. 

The  following  table  will  show  the  crystalline  and  optical  rela- 
tions of  the  various  amphiboles. 

TREMOLITE 

Tremolite.  —  CaMg3(SiO3)4 ;"  CaO  =  13.45,  MgO  =_28.83, 
SiO2  =  57.72 ;  Monoclinic ;  Type,  Digonal  Equatorial ;  a  :  b  :  c  = 
.5415  :  1 :  .2986 ;  p  =  74°  48'  =  100  A  001 ;  100  A  110  =  27°  54'  ; 
mAm'  =  55°  49;  001  A  101  =  24°  4';  001 A Oil  =  15°  46'; 
Common  forms,  c  (001),  a  (100),  b  (010),  m  (110),  r  (Oil),  t  (101)  ; 
Twinning  plane  100,  contact  twins,  also  lamellar  parallel  to  the 
base;  Cleavage,  prismatic  (124°  30'),  a  and  b  at  times  distinct; 
Brittle ;  Fracture,  uneven ;  H.  =  5-6 ;  G.  =  2.9-3.4 ;  Color,  gray, 
white,  brown,  or  nearly  black,  according  to  composition ;  Streak, 
white  to  gray;  Luster,  vitreous  to  pearly;  Subtranslucent  to 
opaque.  For  the  optical  properties  see  table,  page  432. 

B.B.  —  Like  the  pyroxenes  in  every  respect,  from  which  they 
must  be  distinguished  by  their  physical  and  optical  properties. 

General  Description.  —  Crystals  are  elongated  or  fibrous  paral- 
lel to  the  vertical  axis.  In  basaltic  hornblende  the  habit  is  short 


432 


MINERALOGY 


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COMPOSITION 

(MgFe)Si03 

CaMg3(SiO3)4 

Ca(MgiFe)3(SiO3)4 

(MgFe)SiOs 

d 

'-£ 

(•  Ca(MgFe)2SiO3 
|  Na,  Al(SiO3)2 
i  (Mg,Fe)(AlFe)2SiO6 

NaAl(SiO3)2(FeMg)S 

2  NaFe(SiO3)2FeSiO3 

Nas(CaMg)3(FeMn)i 
(Al,Fe)2Si2iO45 

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umrnington 

riinerite  . 

ornblende 

laucophane 

iebeckite 

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SILICATES,   TITANATES,   ETC. 


433 


and  stout  combinations  of  m,  b,  r,  and  t ;  the  pinacoid  a  is  never 
well  developed.  On  the  surface  these  crystals  appear  as  if  cor- 
roded, caused  probably  by  magmatic  resorption.  The  character- 


FIG.  476. — Actinolite  in  Talc,  from  the  Tyrol. 

istic  cleavage  of  approximately  124°  serves  to  distinguish  them  from 
the  pyroxenes.  Color  varies  greatly  with  the  composition,  being 
white  or  gray  in  tremolite,  where  there  is  little  or  no  iron ;  green  in 
actinolite;  black  or  dark  brown  in  the  basaltic  hornblende;  and 


. 


FIG.  477.  —  Fibrous  Amphibole  :  Var.  Asbestos  from  the  Tyrol. 

pink  in  hexagonite,  a  manganese  variety.  The  soft,  fibrous  variety 
is  asbestos  and  differs  from  the  serpentine  variety  of  asbestos,  as 
it  contains  no  water.  In  sections  it  is  irregular,  granular,  or 

2F 


434 


MINERALOGY 


rhombic  in  outline.  Tremolite  is  colorless ;  actinolite,  light  green ; 
hornblende,  olive  to  brown.  All  varieties  show  the  characteristic 
prismatic  cleavage  of  124°  30'  when  cut  perpendicular  to  the  ver- 
tical axis.  The  relief  is  distinct  and  the  surface  is  rough,  especially 
in  the  dark-colored  varieties.  Pleochroism  is  very  characteristic 
and  increases  with  the  depth  of  color,  absorption  being  the  stronger 
parallel  to  the  cleavage.  Interference  colors  are  higher,  second 
order,  or  very  high  in  case  of  some  of  the  basaltic  forms.  Extinc- 
tion varies  from  0°  to  20°  with  the  percentage  of  iron  present, 
though  not  regularly,  being  higher  in  the  darker  varieties.  The 
plane  of  the  optic  axes  is  in  the  plane  of  symmetry  with  the  acute 
bisectrix  in  the  small  angle  P,  Fig.  347,  page  199. 

Interference  figures  are  well  defined,  an  optic  axis  emerging 
in  the  basal  and  orthopinacoidal    sections.     Distinguished  from 


FIG.  478.  —  Section  of  Diorite,  showing  the  Relief  and  Cleavage  Angle,  124°,  of 
Hornblende  a,  and  also  the  Parallel  Cleavage  b,  when  cut  Parallel  to  the  Vertical 
Axis. 

pyroxenes  by  the  cleavage;  by  the  strong  pleochroism;  smaller 
angle  of  extinction ;  lower  interference  colors  and  somewhat  lower 
relief. 

The  amphiboles  are  important  rock-forming  minerals  widely  dis- 
tributed in  igneous  and  metamorphic  rocks.     Tremolite  and  ac- 


SILICATES,   TITANATES,   ETC. 


435 


tinolite  are  found  in  schists  and  granular  limestones  and  are  asso- 
ciated with  metamorphism  only,  while  hornblende  occurs  both  as  a 
primary  mineral  in  granites,  syenites,  diorites,  and  gabbros,  as  well 
as  in  the  gneisses  and  schists.  The  dark  basaltic  hornblendes 
are  igneous  only,  and  form  phenocrysts  in  ferruginous  lavas, 
basalts,  and  gabbros,  as  at  Aussig  in  Bohemia,  or  Monte  Somma, 
Vesuvius.  The  soda-iron  varieties,  barkevikite,  riebeckite,  arfved- 


FIG.  479.  —  Hornblende-diorite-porphyry,  showing  Section  of  a  Hornblende  Crystal. 

sonite  and  senigmatite,  occur  only  in  those  igneous  rocks  rich  in 
alkali,  as  the  nephelite-syenites  of  Greenland  and  Norway.  Arf- 
vedsonite  is  found  near  Magnet  Cove,  Arkansas.  Crocidolite  is  a 
fibrous  soda-iron  variety  of  varied  colors,  occurring  near  the 
Orange  River,  South  Africa ;  it  occurs  included  in  quartz,  making  a 
very  pleasing  ornamental  stone;  when  polished  and  mounted  in 
jewelry  it  is  the  semiprecious  "  tiger-eye/' 

The  decomposition  products  of  the  calcium  and  magnesium  va- 
rieties are  talc,  serpentine,  and  calcite,  while  those  containing 
iron  in  addition  form  hematite,  limonite,  or  siderite,  as  well  as 
chlorite,  epidote,  and  augite.  Uralite  is  a  fibrous  pseudomorph 
of  hornblende  after  augite,  having  the  crystalline  form  of  augite 
but  the  cleavage  of  hornblende;  under  suitable  conditions  this 
process  of  uralitization  may  be  reversed,  and  hornblende  will  pass 
over  to  augite. 


436  MINERALOGY 

In  the  formation  of  amphiboles  the  rate  of  cooling,  the  presence 
of  an  excess  of  magnesium  over  calcium,  as  well  as  the  presence  of 
water  and  pressure,  all  have  an  influence  in  determining  whether 
an  amphibole  or  pyroxene  will  form.  Amphibole  is  the  less  stable 
form  of  the  two,  as  when  fused  and  cooled  a  pyroxene  will  be  formed, 
it  being  impossible  to  form  amphiboles  by  a  simple  fusion  in  an  open 
crucible. 

The  amphiboles  artificially  are  produced  with  difficulty  and  only 
by  long-continued  heating  of  their  constituents,  with  water  and 
under  pressure.  The  magnesium  metasilicate  with  optical  proper- 
ties of  an  amphibole  has  been  formed  by  simple  fusion. 

BERYL 

Beryl.  —  Beryllium  aluminium  metasilicate,  Be3Al2(Si03)6 ; 
BeO  =  14.11,  A12O3  =  19.05,  SiO2=  66.84;  Hexagonal;  Type, 
Dihexagonal  Equatorial;  c  =  .4989;  0001  A  lOll  =  29°  57';  0001 
A  2021  =49°  2';  Common  forms,  c  (0001),  m  (1010),  r(l6ll), 
u  (2021) ;  Twinning  not  observed ;  Cleavage,  basal  imperfect ; 
Brittle;  Fracture,  conchoidal ;  H.  =  7.5-8;  G.  =  2.63-2.80;  Color, 
shades  of  green,  yellow,  white,  or  lilac;  Streak,  white;  Luster, 
vitreous;  Transparent  to  subtranslucent ;  o>  =  1.584;  €  =  1.578; 
<o-€  =  .006;  Optically  (-). 

B.B.  —  Whitens  and  fuses  with  difficulty,  only  on  the  thinnest 
edges.  When  containing  alkalies,  it  may  fuse  easier.  Insoluble 
in  acids. 

General  description.  —  Crystals  are  stout  prismatic  in  habit, 
usually  combinations  of  the  hexagonal  prism  and  pyramid  of  the 
first  order  and  the  base,  often  striated  or  furrowed  lengthwise. 
Various  other  pyramids  and  complex  combinations  have  been  de- 
scribed from  Alexander  County,  North  Carolina.  Hexagonal  prisms 
of  enormous  size,  weighing  hundreds  of  pounds,  occur  at  Grafton 
and  Acworth,  New  Hampshire.  It  is  also  common  in  the  pegma- 
tites of  Maine,  at  Albany,  Norway,  Bethel,  and  Paris,  usually 
associated  with  tourmaline,  spodumene,  and  lepidolite. 

The  clear  green  variety  is  the  gem  emerald,  but  owing  to  the 
prevalence  of  cavities  it  is  very  difficult  to  obtain  an  emerald  that 
is  flawless.  The  emeralds  of  commerce  are  in  large  part  mined  at 
Muzo,  United  States  of  Colombia,  where  they  are  found  in  a  crys- 
talline limestone  and  slates.  Fine  emeralds  are  also  obtained  from 


SILICATES,   TITANATES,   ETC. 


437 


Brazil ;  from  the  Ural  Mountains  in  Siberia,  where  they  are  asso- 
ciated with  topaz  and  quartz ;  from  the  Tyrol,  imbedded  in  a 
chloritic -schist;  from  North  Carolina,  where  they  are  associated 
with  the  green  spodumene,  hiddenite. 

Aquamarine,  the  clear  blue-green  variety,  is  more  common  and 
less  valuable  than  the  emerald.  A  pink  or  lilac-colored  beryl, 
morganite,  found  in  a  pegmatite  of  Madagascar  and  at  Pala, 


FIG.  480.  — Beryl. 


The  Simple  Crystal  is  from  Albany,   Maine,  the  Other  from 
Middletown,  Connecticut. 


California,  where  it  is  associated  with  spodumene  of  the  same 
color,  tourmaline,  and  lepidolite,  is  also  used  as  gem  material. 

The  various  colors  of  beryl  are  caused  by  the  replacement  of 
beryllium  in  part  by  the  alkalies  potassium,  caesium,  lithium,  and 
sodium ;  the  aluminium  may  at  the  same  time  be  replaced  by  chro- 
mium, which  yields  the  green  and  bluish  colors. 

Beryl  is  a  common  mineral  of  the  pegmatites,  where  it  is  asso- 
ciated with  tourmaline,  topaz,  spodumene,  corundum,  micas, 
feldspars,  and  garnets.  In  the  Black  Hills,  South  Dakota,  it  is 
associated  with  cassiterite.  As  a  product  of  metamorphosis  it 
occurs  in  the  crystalline  limestones,  chloritic  schists,  and  slates. 

In  weathering  the  beryllium  is  carried  away  in  solution  to  form 
secondary  minerals,  while  the  aluminium  and  silica  form  kaolin, 


438  MINERALOGY 

and  with  the  addition  of  alkali  and  water  form  micas.  Artificial 
beryl  has  been  formed  by  fusing  the  constituent  oxides  with  boric 
acid. 

ORTHOSILICATES 

SODALITE    GROUP 

COMPOSITION  GRAVITY  INDEX  OF  RE- 

FRACTION 

Sodalite       Na4  (AlCl)Al2(Si04)3  2.14-2.30         1.4827 

Haiiynite     (Na*.  Ca)2(NaSO4Al)Al2(SiO4)3    2.4-2.5  1.496 

Noselite      Na4(NaSO4Al)Al2(SiO4)3  2.25-2.4 

Lazurite      Na4(NaS3 .  Al)Al2(Si04)s  2.38-2.45 

The  four  species  included  in  this  group  are  all  alkali  aluminium 
orthosilicates  containing  either  chlorine  or  sulphur.  They  are  all 
isometric  in  symmetry,  have  a  low  specific  gravity  and  a  hardness  of 
5.5-6,  and  a  vitreous  or  greasy  luster ;  Crystal  forms  are  the  cube, 
rhombic  dodecahedron,  and  octahedron,  but  generally  massive  or 
granular;  Cleavage,  dodecahedral ;  Brittle;  Fracture,  conchoidal 
to  uneven.  Color,  white,  gray,  yellow,  blue,  green,  or  brown; 
Streak,  white  or  pale  blue.  Index  of  refraction  very  low.  All  iso- 
tropic,  haiiynite  may  in  cases  show  a  weak  double  refraction. 

B.B.  —  Fuse  at  3.5  to  4.5.  All  gelatinize  with  HC1 ;  the  solu- 
tion freed  of  silica  and  slowly  evaporated  will  yield  cubic  crystals 
of  salt  (NaCl).  Lazurite  when  dissolved  in  HC1  yields  hydrogen 
sulphide  and  usually  glows  with  a  blue  flame  when  first  heated 
in  the  O.  F. 

When  haiiynite  is  dissolved  in  HC1,  the  solution  freed  of  silica, 
and  the  Al  precipitated  with  ammonia  and  filtered  out,  the  filtrate 
will  yield  a  white  precipitate  with  ammonium  carbonate  (Ca) ; 
the  other  three  members  of  the  group  containing  no  calcium  will 
not  yield  a  precipitate.  Noselite  is  distinguished  from  sodalite 
by  yielding  a  sulphur  reaction  when  fused  with  soda  and  a  little 
coal  dust,  then  placed  on  silver. 

General  description.  —  These  four  minerals  are  associated  in 
nature  and  occur  as  constituents  of  those  lavas  low  in  silica  and 
high  in  their  alkali  content,  where  nepheline  and  leucite  are  their 
companions.  Haiiynite  and  noselite  are  found  in  the  lavas  of  recent 
geological  date  and  are  very  rare  in  the  coarsely  crystalline  ela3olite 
syenites,  here  sodalite  alone  of  the  group  occurs.  Lazurite  occurs 
in  crystalline  limestone  and  is  a  product  of  contact  metamorphism. 


SILICATES,   TITANATES,   ETC. 


439 


In  their  crystallization  from  the  magma  they  usually  follow  the 
pyroxenes,  forming  the  ground  mass  in  which  the  crystals  and  miner- 
als previously  crystallized  are  embedded,  as  is  often  the  case  with 
the  feldspars ;  from  this  relation  they  are  termed  the  feldspathoids. 
In  sections  when  well  formed  they  are  square  or  six-sided  in 
outline,  otherwise  rounded  and  irregular,  colorless,  greenish  to  blue. 


FIG.  481.  —  Section  of  Haiiynite,  showing  the  Dark,  Dust-like  Inclusions 
collected  near  the  Margin. 

The  relief  is  slightly  rough.  Inclusions  in  sodalite  are  rare,  but 
fine  particles  of  glass  and  cavities  filled  with  gas  are  characteristic 
of  haiiynite.  They  are  often  collected  at  the  center,  or  marginal 
or  in  concentric  bands  in  the  crystal  section.  The  feldspathoids 
are  easily  decomposed;  the  first  step  is  hydration,  forming  zeolites  ; 
numerous  pseudomorphs  of  natrolite  after  sodalite  occur.  When 
alteration  is  pushed  still  further  muscovite  and  kaolin  are  formed. 

Lazurite  is  used  as  an  ornamental  stone  and  in  inexpensive 
jewelry,  as  lapis  lazuli.  It  is  particularly  prized  in  Russia  for 
tables,  vases,  and  in  the  decoration  of  altars  in  the  churches.  It  is 
mined  at  Badakschan  in  Siberia.  A  fine  quality  also  comes  from 
Persia.  The  pigment  ultramarine  was  formerly  the  natural  mineral 


440  MINERALOGY 

ground  to  a  powder,  but  has  been  displaced  by  the  much  cheaper 
artificial  product,  formed  by  the  fusion  of  kaolin,  sodium  carbonate, 
and  sulphur  at  comparatively  a  low  temperature,  below  700°,  as 
these  minerals  decompose  or  are  unstable  above  that  temperature. 

IOLITE 

lolite.  — Cordierite;  Dichroite;  H2(Mg  .  Fe)4Al8SiioO37 ;  MgO  = 

10.2,  FeO  =  5.3,  A12O3  =  33.6,  SiO2  =49.4,    H2O  =  1.5;    Ortho- 
rhombic  ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c  =  .5871 :  1 :  .5585  ; 
100  A  110  =  30°   25';    001  A  101  =  43°    34';    001  A  Oil  =29°    11'; 
Common  forms,  c  (001),  a  (100),  b  (010),  m  (110),  d  (130),  s  (112)  ; 
Twinning  plane  m  forming  pseudo-hexagonal  shapes;    Cleavage, 
brachypinacoidal    distinct,  a  and  c  less   so;    Brittle;    Fracture, 
conchoidal ;   H.  =  7-7.5 ;   G.  =  2.6-2.66 ;  Color,  shades  of  blue  or 
smoky ;  Streak,  white ;  Transparent  to  translucent ;  Pleochroism 
very  strong,  a  bluish,  b  yellow,  c  blue;    a  =  1.532;    p  =  1.536; 
•y  =  1.539;  -y  -  a  =  .007;  Optically  (  -  );  Axial   plane  =  100; 
Bxa  =  c ;  2  V  =  39°  32'  but  variable. 

B.B.  —  Becomes  opaque,  and  fuses  with  difficulty  at  5.5.  The 
powdered  mineral  heated  very  hot  in  the  closed  tube  yields  water. 
Partially  decomposed  in  acids. 

General  description.  —  Crystals  are  short  prismatic  in  habit, 
pseudo-hexagonal  from  twinning ;  more  often  it  occurs  granular  or 
massive.  lolite  occurs  as  a  constituent  of  igneous  rocks,  but  is 
more  characteristic  of  schists,  gneisses,  and  contact  metamorphosis, 
where  it  is  associated  with  tourmaline,  biotite,  garnets,  sillimanite, 
quartz,  and  the  plagioclases.  It  is  widely  distributed  in  the  gneisses 
of  Connecticut,  Massachusetts,  and  New  Hampshire.  In  sections 
it  appears  very  much  like  quartz,  but  is  slightly  pleochroic  in  thin 
sections  and  is  biaxial.  The  variety  cerasite  from  Japan  has  a  large 
number  of  inclusions  symmetrically  arranged,  as  in  andalusite  or 
chiastolite.  It  decomposes  easily,  forming  by  hydration  a  series 
of  compounds,  as  fahlunite,  praseolite,  gigantolite,  and  with  the 
addition  of  iron  and  alkalies,  pinite  or  mica  is  formed. 

NEPHELITE 

Nephelite.  —  Nepheline ;  Elaeolite,  (Na  .  K)  AlSi04,  usually 
written  K^A^O-a ;  K2O  =  7.7,  Na«O  =  15.1,  A12O3  = 

33.3,  SiO2  =  44;     Hexagonal;     Type,    Hexagonal    Alternating; 
c  =  .8389 ;     0001  A  lOfl  =  44°     5' ;     Crystal     forms,     c  (0001), 


SILICATES,   TITANATES,   ETC. 


441 


m(1010),  p(1011),  a  (1120);  Cleavage,  prismatic  and  basal  dis- 
tinct ;  Brittle ;  Fracture,  subconchoidal ;  H.  =  5.5-6 ;  G.  =  2.55- 
2.65;  Color,  white,  yellow,  green,  blue,  brown,  or  red;  Streak, 
colorless  or  pale;  Luster,  vitreous  to  greasy;  Transparent  to 
opaque;  o>  =  1.5416;  €  =  1.5376;  o>-€  =  .004;  Optically  (-). 

B.B.  —  Fuses  quietly  at  3.5  (1100°),  yielding  a  yellow  flame 
(sodium).  Gelatinizes  with  HC1;  the  solution  freed  of  silica 
yields  a  precipitate  of  aluminium  hydroxide  with  ammonia. 

General  description.  —  Crystals  when  developed  are  short, 
stout,  hexagonal  prisms  terminated  by  the  base  or  tabular  parallel 
to  the  base.  Several  pyramids  of  both  the  first  and  second  orders 
have  been  described  on  small  crystals  from  the  lavas  of  Monte 
Somma,  Vesuvius. 

Elseolite  is  the  more  common  granular  or  massive  form  of 
nepheline,  of  the  older  syenites,  in  which  it  occurs  at  times  as  the 
dominant  mineral. 

Chemically  nepheline  is  probably  a  mixture  of  sodium  aluminium 
orthosilicate  (NaAlSi04),  which  has  been  produced  artificially, 


FIG.  482.  —  Nepheline  in  a  Section  of  Phonolite.    The  Small  Square  Crystals  are 
also  Nepheline.    The  Elongated  Crystals  are  Feldspar. 

but  is  not  known  as  a  mineral,  and  potassium  aluminium  orthosili- 
cate (KAlSi04),  which  is  the  mineral  kaliophilite,  with  possibly  a 
little  excess  SiO2  in  its  molecule.  Eucryptite  is  the  lithium  salt 
^),  while  cancrinite,  H6NaeCa(NaC03)2Al8(Si04)9,  is  a  min- 


442  MINERALOGY 

eral  very  similar  in  physical  and  crystalline  properties,  as  well  as 
associations,  but  contains  carbonates  in  its  composition. 

In  sections,  nepheline  when  crystalline  is  either  square  or  hex- 
agonal in  outline,  colorless,  with  no  relief ;  cleavage  is  not  marked 
except  when  decomposition  has  begun.  Inclusions  are  not  charac- 
teristic except  that  in  some  occurrences  it  may  be  filled  with  dust- 
like  inclusions  of  glass  and  gas  bubbles,  in  concentric  bands  or 
zonal.  Interference  color  very  low  grays  of  the  first  order,  and 
extinction  parallel  to  the  cleavage  or  symmetrical. 

Nepheline  is  a  primary  mineral  of  many  igneous  rocks,  especially 
those  rich  in  alkalies  and  low  in  silica.  It  seems  to  form  in  those 
magmas  in  which  the  sodium  is  in  excess  of  that  required  to  pro- 
duce feldspars,  separating  from  the  magma  in  most  cases  just 
before  the  feldspars  and  directly  after  the  sodalite  group,  with 
which  it  is  frequently  associated,  as  in  the  lavas  of  Vesuvius.  At 
Litchfield,  Connecticut,  it  is  associated  with  cancrinite  in  an  elseo- 
lite  syenite.  In  syenites  the  massive  elseolite  is  characteristic, 
lending  its  name  to  the  group,  elseolite  syenites. 

In  weathering,  zeolites,  especially  natrolite,  result ;  but  the  forma- 
tion of  sodalite  by  the  addition  of  chlorine  may  be  the  first  step 
in  this  reaction. 

Artificially  nephelite  has  been  produced  by  a  fusion  of  its  con- 
stituent oxides  at  a  low  temperature,  from  which  crystals  easily 
separate. 

GARNETS 

Garnets  are  orthosilicates  of  the  general  formula  R//3R///2(Si04)3, 
in  which  R"  may  be  Ca,  Mg,  Fe",  Mn,  and  R'"  =  Al,  Fe'",  Cr  ; 
Isometric;  Type,  Ditesseral  Central;  Common  forms,  d(110), 
n(211),  s(321);  Twins  rare;  Cleavage,  dodecahedral  sometimes 
distinct;  Brittle;  Fracture,  conchoidal ;  H.  =  6.5-7;  G.  =  3.15- 
4.3;  Color,  all  colors;  Streak,  white  or  pale;  Transparent  to 
opaqus;  Luster,  vitreous  to  resinous;  n  =  1.7-1.8. 

B-B.  —  Fuses  at  three,  except  uvarovite  which  fuses  at  six. 
After  fusion  gelatinizes  with  HC1,  otherwise  not  much  affected  by 
acids.  Those  containing  much  iron  become  magnetic  after  fusion 
in  0.  F. 

General  description.  —  Crystals  are  common  and  in  some 
instances  very  large,  up  to  a  foot  in  diameter;  in  habit  either 
rhombic  dodecahedrons  or  tetragonal  trisoctahedrons  or  combina- 


SILICATES,   TITANATES,   ETC. 


443 


tions  of  these  two  forms.  Other  forms  are  rare ;  the  cube  occurs  on 
crystals  from  Mill  Rock,  New  Haven,  Conn.,  and  the  octahedron 
on  crystals  from  the  isle  of  Elba. 

Striations  appear  on  the  dodecahedral  face  parallel  to  the  long 
diagonal,  and  on  the  trisoctahedral  face  parallel  to  its  intersection 


"    *• 

FIG.  483.  —  Garnet  Crystals  in  Schist.    Southbury,  Connecticut. 

with  the  rhombic  dodecahedrons ;    these  striations  appear  on  the 
weathered  or  water-worn  crystals  also. 

The  garnets  form  a  very  compact  isomorphous  group  in  which 
there  is  a  gradual  transition  from  one  variety  to  the  other.  They 
are  as  a  rule  grouped  under  six  heads,  with  varieties  under  each  of 
these. 

I.  Grossularite,    Calcium    aluminium     garnet,     Ca3Al2(Si04)3 ; 
CaO  =  37.30,  A1203  =  22.69,  Si02=  40.01;  H.  =  6.5;  G.  =  3.5; 
Color,  white,  amber  yellow  to  cinnamon  brown,  sometimes  green 
when  chromium  is  present.     Cinnamon  stone,  or  hessonite,  is  a 
brown  variety,  beautiful  specimens  of  which  are  obtained  from 
Ala,  Piedmont ;    also  Ceylon ;  and  Bethel  and  Rumford,  Maine  ; 
Amity,  New  York,  and  many  other  localities,  as  it  is  a  common 
variety  of  garnet. 

II.  Pyrope;     Magnesium    aluminium    garnet,    Mg3AJ2(Si04)3 ; 
MgO  =  29.82,    A1203  =  25.40,    SiO2  =  44.78;     H.  =  7.5;     G.  = 
3.7 ;   Color,  dark  red  to  nearly  black. 

This  is  the  precious  garnet  of  the  jewelers,  often  called  the 
"  Cape  Ruby."  Good  pyropes  are  associated  with  serpentine  at 


444 


MINERALOGY 


Bilin,  Bohemia;  pyrope  is  found  in  the  "Blue  Ground  "  with  the 
diamond  in  South  Africa ;  in  New  Mexico  on  the  Navaj  o  Reser- 
vation, where  it  is  associated  with  chrysolite. 

III.  Almandite;    Common  Garnet;    Iron  aluminium   garnet, 
Fe3Al2(Si04)3;    FeO  =  43.34,    A1203  =  20.34.    SiO2  =  36.15;    but 

composition  variable ;  Color, 
red  to  black;  Transparent  to 
opaque.  The  clear  red  speci- 
mens are  the  "  carbuncle," 
used  as  a  precious  garnet. 
Large  crystals  of  this  garnet 
are  of  common  occurrence,  as 
at  Salida,  Colorado;  Fort 
Wrangle,  Alaska,  where  sym- 
metrical combinations  of  the 
dodecahedron  and  the  tetrag- 
onal trisoctahedrons  are  ob- 
tained. Dodecahedrons  of  an 
inch  in  diameter  are  found  in 
a  schist  at  Southbury,  Con- 
necticut. Crystals  three  or 
four  inches  in  diameter  occur 

at  Arendal,  and  somewhat  smaller  specimens  at  Bodo,  Norway. 

All  the  well-developed  and  large  crystals  are  in  a  mica  schist. 

IV.  Spessartite;  Manganese  aluminium  garnet,  Mn3Al2(Si04)3 ; 
MnO  =  42.95,  A12O3  =  20.73,  SiO2  =  36.30 ;   H.  =  6.5 ;    G.  =  3.8  ; 
Color,   dark  hyacinth-red    to    brown-red.      Beautiful    specimens 
of  this  garnet  are  found  at  Amelia  Court  House,  Virginia ;  also  at 
Salem,  North  Carolina ;  Haddam,  Connecticut ;  and  Bethel,  Maine. 

V.  Andradite ;    Calcium   iron    garnet,    Ca3Fe2(SiO4)3 ;    CaO  = 
33.06,   Fe2O3  =  31.49,   SiO2  =  35.45;   in   the   black  melanite  and 
schorlomite   some  of  the  silica  may  be  replaced  by  titanium; 
H.  =  6.5 ;    G.  =  3.8 ;    Color,  various  shades  of  green   and  yellow 
to  black.     Demantoid  is  a  massive  green  variety  often  polished 
as  an  ornamental  stone.     Aplome  is  also  a  green  variety  found  at 
Schwarzenberg,  Saxony.     Topazolite  is  greenish  yellow  and  found 
at  Ala  in  Piedmont.     Polyadelphite  is  a  brown  variety  found  in 
large  crystals  and  massive  at  Franklin,  New  Jersey;   it  contains 
considerable  manganese.     Black  varieties  are  also  found  at  Frank- 
lin, while  the  titaniferous  garnets  occur  at  Magnet  Cove,  Arkan- 
sas; and  at  Henderson,  North  Carolina. 


FIG.  484.  —  Garnet. 


SILICATES,   TITANATES,   ETC. 


445 


VI.  Uvarovite  :  Calcium  chrome  garnet,  Ca3Cr2(Si04) ;  CaO  = 
29.27,  Cr2O3  =  32.50,  SiO2  =  38.23  ;  H.  =  7  ;  G.  =  3.4;  Color, 
bright  green.  Fuses  with  difficulty  and  will  not  gelatinize  after 
fusion.  This  garnet  is  usually  associated  with  chromite  and  with 
serpentine ;  at  Oxford,  Canada,  however,  it  is  found  in  the  cavities 
of  a  granular  limestone.  It  occurs  at  Wood's  chrome  mine,  Lan- 
caster, Pennsylvania ;  at  New  Idria,  California. 

In  rock  sections  garnet  appears  in  crystalline  outline,  granular 
or  irregular,  colorless  or  in  pale  colors,  with  a  high  relief  and  a 
rough  surface  and  parting  at  times  distinct.  A  zonal  structure 


FIG.  485.  —  Section  of  Garnet,  showing  the  High  Relief  and  Parting. 

is  often  noticed,  especially  in  the  titaniferous  varieties.  Isotropic, 
but  may  exhibit  anomalous  weak  double  refraction,  which  is  very 
often  a  characteristic  of  the  garnets  of  contact  zones. 

Occurrence.  —  Garnets  occur  as  accessory  minerals  in  rocks  of 
all  varieties,  the  kind  depending  upon  the  nature  of  the  magma. 
Andradite  and  almandite  are  found  in  granites;  pyrope  is  con- 
nected with  peridotites  and  serpentine;  spessartite  is  found  in 
quartzite  and  rhyolite;  grossularite  is  the  common  garnet  of  crys- 
talline limestone;  while  all  may  be  found  in  crystalline  schists  and 
gneisses,  as  well  as  in  metamorphic  and  contact  zones.  Eclogite 
is  a  rock  composed  almost  entirely  of  massive  garnet. 

Garnet  is  easily  decomposed  by  weathering,  and  forms  chlorite, 
iron  ores,  calcite,  kaolinite,  epidote,  and  a  large  number  of  second- 
ary minerals,  depending  upon  the  chemical  composition  of  the 
original  garnet.  Garnets  when  fused  break  down,  and  the  melt 
on  cooling  forms  other  silicates,  as  anorthite,  pyroxenes,  or  scapo- 
lite.  They  are  therefore  unstable  at  the  temperature  of  fusion. 
Some  garnets  have  been  formed,  as  spessartite,  by  a  simple  fusion 


446  MINERALOGY 

of  the  constituent  oxides ;  but  as  a  rule  some  flux,  as  calcium 
chloride,  must  be  added  to  lower  the  fusing  point  to  a  tempera- 
ture at  which  the  formation  of  garnets  is  possible. 

OLIVINE  GROUP 

The  olivine  group  is  composed  of  isomorphous  orthosilicates  of 
the  general  formula  R"2SiO4,  in  which  R"  is  Ca,  Mg,  Fe",  Mn,  Zn, 
or  mixtures  of  these  metals ;  they  are  of  orthorhombic  symmetry 
and  members  of  the  didigonal  equatorial  type,  crystallizing  usually 
in  combinations  of  the  three  pinacoids  with  the  unit  pyramid  and 
a  dome,  or  pyramidal  in  habit;  at  times  tabular,  parallel  to  b 
more  often  than  to  a.  They  have  two  well-developed  cleavages 
at  a  right  angle. 

The  following  table  will  serve  to  show  their  relations  both  opti- 
cally and  chemically. 

OLIVINE 

Olivine.  —  Chrysolite ;  Magnesium  iron  orthosilicate ;  MgFe- 
Si04 ;  MgO  =  49.19,  FeO  =  10.54,  SiO2  =  39.85 ;  a  :  b  :  c  =  .4656  : 
1:.5865;  1 10  A 100  =  24°  58';  001 A 101  =  51°  33';  001 A  011  = 
30°  24' ;  Common  forms,  a  (100),  b  (010) ;  c  (001),  m  (110).  s  (120), 
d(101),  e(lll),  k(021);  Twinning  plane,  Oil  rare;  Cleavage, 
b  distinct,  a  less  so ;  H.  =  6.5-7 ;  G.  =  3.27-3.37 ;  Color,  shades 
of  green  to  red  or  brown ;  Streak,  white  or  pale ;  Luster,  vitreous  ; 
Transparent  to  opaque;  a  =  1.653;  p  =  1.670;  -y  =  1.689; 
•y-a  =  .036 ;  Optically  (±) ;  Axial  plane  =  001 ;  Bxa  =  £  or  b  ; 
2  V  =  86°  89'. 

B.B.  —  Dark-colored  specimens  fuse  to  a  magnetic  slag,  while 
the  light-colored  specimens  whiten  and  fuse  with  difficulty.  Gelat- 
inizes with  HC1.  With  the  fluxes  reacts  for  iron. 

General  description.  —  Crystals  are  usually  small  and  nearly 
equidimensional  or  tabular,  parallel  to  a  or  b ;  more  often  granu- 
lar, friable  masses,  in  which  form  it  is  often  found  in  large  rock 
areas,  as  the  dunites  of  Georgia,  North  and  South  Carolinas,  where 
it  is  associated  with  corundum.  Chrysolite  and  peridote  are 
names  often  applied  to  olivine,  but  more  particularly  to  the  clear 
transparent  varieties,  which  are  used  as  gem  stones.  Peridote  is 
leaf-green  in  color,  and  for  a  long  time  was  gathered  along  the  shore 
of  the  Red  Sea,  where  the  water-worn  pebbles  were  thrown  up  by 


SILICATES,   TITANATES,   ETC. 


447 


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COMPOSITION 

CaMgSiO4 

6 

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CaMnSiO4 

(MgFe)2SiO 

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GO 

• 

Monticellite  .  . 

Forsterite  .  .  . 

Glaucochroite  .  . 
Olivine  .... 

Hortonolite.  .  . 

Fayalite  .... 

Knebelite  .  .  . 
Tephroite  . 
Roepperite  .  .  . 

448 


MINERALOGY 


the  waves  after  storms.     It  was  also  obtained  from  the  Arabs  of 

Egypt,  but  the  exact  locality  from  whence  it  was  gotten  is  unknown. 

Gem  material  is  found  near  Fort  Defiance,  Arizona. 

Chrysolite  is  a  yellow  variety  resembling  very  much  the  yellow 

topaz  when  cut  and  polished. 

Olivine  occurs  as  an  essential  constituent  of  many  igneous  rocks, 

which  are  low  in  silica  and  rich  in  the  alkali  earth  metals.     Its 

composition  is  variable, 
depending  upon  the 
proportion  of  mag- 
nesium, calcium,  iron, 
or  manganese,  present 
in  the  magma.  It  is 
found  in  such  rocks"  as 
peridotite,  gabbros, 
basalts,  nephelites,  and 
leucites,  and  many 
lavas,  while  dunite  is 
almost  entirely  olivine. 
It  appears  less  often  in 
andesites  and  trachytes. 


FIG.  486.  —  Olivine  Crystals  from  France.  In  rock  sections  it  is 

colorless  or  pale,  with 

crystalline  outline,  or  more  often  granular  or  irregular.  The  two 
cleavage  directions  are  well  marked,  that  parallel  to  b  more  so  than 
that  parallel  to  a.  Extinction  parallel.  The  index  of  refraction 
being  high,  the  relief  is  marked,  with  all  cracks  distinct.  Inter- 
ference colors  of  the  second  and  third  order.  The  plane  of  the 
optic  axes  is  parallel  to  001  and  the  optical  character  is  plus,  with 
the  acute  bisectrix  a,  when  the  ferrous  oxide  is  below  12  per 
cent. ;  when  above  12  per  cent.,  the  acute  bisectrix  is  b  and  the 
optical  character  is  minus.  Inclusions  are  not  characteristic,  but 
spinel,  chromite,  apatite,  and  hypersthene  appear ;  also  glass  and 
slag  in  the  lava  occurrences.  Pleochroism  is  marked  only  when 
the  iron  content  is  high. 

In  its  alteration  olivine  readily  forms  serpentine,  the  alteration 
following  the  fractures  or  cleavage  cracks  in  the  crystal,  with  the 
serpentine  fibers  lying  crosswise.  The  iron  at  the  same  time 
separates  as  oxide  and  is  deposited  along  the  cracks,  or  where  the 
specimen  is  rich  in  iron  as  layers  interlaminated  with  the  serpen- 
tine; carbonates,  as  magnesite  and  calcite,  and  also  opal,  quartz, 


SILICATES,   TITANATES,   ETC. 


449 


and  brucite  may  appear  as  alteration  products.  Chromite  is 
almost  always  associated  with  the  alteration  of  peridotite  or  with 
the  serpentine  resulting  from  it,  as  at  the  Maryland  locality. 


FIG.  487.  —  Section  of  Olivine,  showing  the  Cracks  filled  with  Secondary  Mag- 
netite and  a  Marginal  Band  of  Enstatite  and  Hornblende. 

Some  nickel  deposits,  as  the  garnierite  of  New  Caledonia,  are  asso- 
ciated with  olivine  or  its  alteration  products,  as  also  platinum  and 
the  diamonds  of  South  Africa. 

Artificially  the  olivine  group  is  easily  synthesized  by  a  direct 
fusion  of  their  constituents  in  the  right  proportion,  particularly 
if  a  little  boric  acid  is  added  to  lower  the  fusing  point.  They  are 
therefore  common  products  of  slags  and  are  also  found  in  meteors. 
When  olivine  is  fused  with  a  little  silica  in  excess,  enstatite  is 
formed;  if  the  silica  is  increased,  then  pyroxenes  are  formed. 

MONTICELLITE 

Monticellite.  —  Calcium  magnesium  orthosilicate,  CaMgSi04 ; 
CaO  =  35.9,  MgO  =  25.6,  Si02  =  38.5  ;  Orthorhombic  ;  Type, 
Didigonal  Equatorial ;  a  :  b  :  c  =  .4337  ;  1 :  .5758  ;  100  A 110  =  23° 
27';  001 A 101  =53*;  001 A011  =  29°  56';  Common  forms,  as 
in  olivine ;  Cleavage,  b  distinct ;  Brittle ;  Fracture,  conchoidal ; 
H.  =  5-5.5;  G.  =  3.03-3.25 ;  Color,  white,  gray;  Streak,  white ; 
Luster,  vitreous;  Transparent  to  translucent;  a  =  1.650;  p 
=  1.662;  -y  =  1.668  -y  -  a  =  .018;  Optically  (-);  Axial  plane 
001 ;  Bxa  =  b ;  2  V  =  37°. 

2G 


450  MINERALOGY 

B.B.  —  Fuses  on  the  thin  edges.  Gelatinizes  with  HC1 ;  this 
solution  freed  of  silica  yields  little  or  no  precipitate  with  ammonia, 
but  a  heavy  white  precipitate  with  ammonium  carbonate. 

General  description.  —  In  crystalline  habit  like  olivine ;  also 
granular  and  in  cleavable  masses.  It  is  a  product  of  metamorphism, 
and  as  such  occurs  in  ejected  blocks  of  limestone  at  Monte  Somma, 
Vesuvius.  Large  crystals  nearly  an  inch  in  length  occur  at  Mag- 
net Cove,  Arkansas.  In  all  other  respects  it  agrees  with  olivine. 
though  much  more  restricted  in  its  occurrence. 

FORSTERITE 

Forsterite.  —  Magnesium  orthosilicate ;  Mg2SiO4 ;  MgO  = 
57.00,  SiO2  =  42.9 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ; 
a:  b:c  =  .4666:1:  .5868;  100 A 110  =  24°  55';  001 A 101  =51° 
34';  001A011  =  30°  21';  Common  forms  as  in  olivine;  Cleav- 
age, b  distinct,  c  less  so ;  H.  =  6-7 ;  G.  =  3.21-3.33 ;  Color, 
white  and  light  shades  of  yellow  to  green ;  Streak,  white ;  Luster, 
vitreous;  Brittle;  Fracture,  conchoidal;  Transparent  to  trans- 
lucent ;  p  =  1.659 ;  y  -  a  high ;  Plane  of  the  optic  axes  =  001  ; 
Bxa  =  a;  2V  =  86°. 

B.B.  —  Infusible,  gelatinizes  with  HCl,  this  solution  freed  of 
silica,  an  excess  of  ammonia  added  yields  no  precipitate  with  am- 
monium carbonate  (calcium),  but  yields  a  white  precipitate  with 
ammonium  phosphate  (magnesium). 

General  description.  —  In  crystalline  habit  like  olivine,  but  is 
largely  a  product  of  metamorphism ;  as  such  it  occurs  in  the  ejected 
blocks  of  limestone  at  Monte  Somma,  Vesuvius.  A  variety,  Bol- 
tonite,  occurs  as  embedded  crystals  and  disseminated  grains  in  a 
limestone  at  Bolton,  Massachusetts. 

FAYALITE 

Fayalite.  —  Ferrous  orthosilicate ;  Fe2SiO4 ;  FeO  =  70.6,  Si02 
=  29.4 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c 
=  .4584  :  1 :  .5793  ;  100  A 110  =  24°  38' ;  001  A  101  =  51°  39' ; 
001 A Oil  =  30°  5';  Forms  as  in  olivine;  Cleavage,  b  distinct, 
a  less  so;  Brittle;  Fracture  uneven;  H.  =  6.5;  G.  =  4-4.14; 
Color,  yellow  to  nearly  black;  Streak,  pale  when  unoxidized; 
Transparent  to  opaque;  a  =  1.824;  p  =  1.864;  -y  =  1.874; 


SILICATES,   TITANATES,   ETC.  451 

Y  -  a  =  .050;    Optically     (-) ;     Axial     plane  =  001 ;     Bxa  =  b  ; 
2  V  =  49°  50'. 

B.B.  —  Fuses  easily  in  the  O.  F.  and  becomes  magnetic  on  coal 
in  R.  F.  Gelatinizes  with  HC1. 

General  description.  —  Fayalite  occurs  in  small  crystals,  flat- 
tened parallel  to  the  orthopinacoid,  in  the  rhyolites  and  ob- 
sidian of  the  Yellowstone  Park.  It  has  also  been  identified  in  a 
granite  at  Rockport,  Mass.  Originally  it  was  described  from 
Fayal  Island,  Azores. 

It  is  also  a  common  constituent  of  furnace  slags. 

It  is  easily  decomposed  by  weathering,  the  iron  being  oxidized 
to  ferric  oxide.  This  takes  place  along  the  cracks  and  on  the 
surface,  the  crystals  becoming  dark  brown  or  black  and  at  times 
iridescent. 

Knebelite  is  a  massive  mineral  containing  manganese,  or  may 
be  considered  as  a  mixture  of  fayalite  and  tephroite. 

Tephroite  is  the  manganese  orthosilicate,  Mn2SiO4  member  of 
the  olivine  group.  It  occurs  as  a  brown  or  red  massive  mineral 
at  Franklin,  New  Jersey,  where  it  is  associated  with  zincite,  wil- 
lemite,  and  franklinite. 

Roepperite  is  a  variety  from  the  same  locality  containing  some 
zinc. 

WILLEMITE 

Willemite.  —  Zinc  orthosilicate ;  Zn2Si04  ;  ZnO  =  73,  Si02 
=  27  ;  Hexagonal ;  Type,  Hexagonal  Alternating ;  c  =  .6775  ; 
0001  A  lOfl  =  38°  2' ;  l_0ll  AI011  =  64°  30';  Common  forms, 
c(0001),  a  (1120),  r(1011),  e  (0112) ;  Cleavage,  basal  and  pris- 
matic easy;  Brittle;  Fracture,  conchoidal;  H.  =  5.5;  G.  =  3.89 
-4.18;  Color,  shades  of  green,  yellow,  brown,  and  red;  Streak, 
white  or  pale;  Transparent  to  translucent;  to  =  1.6931;  €  = 
1.7118;  €-co  =  .0187;  Optically  (+). 

B.B.  —  Whitens  and  fuses  with  difficulty.  On  coal  in  R.F. 
with  soda  and  a  little  coal  dust  yields  a  zinc  coat.  Gelatinizes 
with  HC1. 

General  description.  —  Crystals  are  hexagonal,  prismatic  in 
habit,  terminated  by  rhombohedrons.  It  occurs  at  Franklin,  New 
Jersey,  in  stout  prisms  an  inch  across  and  several  inches  in  length ; 
these  crystals  contain  manganese,  and  are  red  or  ash  color  to  dark 


452  MINERALOGY 

brown,  known  as  troostite.  In  all  other  localities  the  crystals  are 
very  small,  as  at  Altenberg,  Saxony,  where  the  crystals  are  only  a 
few  millimeters  in  length. 

At  Franklin  it  occurs  in  sufficient  quantities  to  form  a  valuable 
ore  of  zinc,  but  mostly  in  cleavable  yellow  or  light  green  masses, 
associated  with  zincite,  franklinite,  and  tephroite.  At  times  it 
forms  long  needle-like  crystals,  apple-green  or  nearly  white  in 
color,  or  in  radiated  masses;  this  last  form  is  rare.  The  light 
colored  varieties  phosphoresce  under  the  influence  of  magne- 
sium light  or  the  radiations  of  radium,  especially  the  radiated 
variety. 

Willemite  has  often  been  observed  in  furnace  slags,  where  a  lead 
ore  containing  zinc  is  being  smelted. 

PHENACITE 

Phenacite.  —  Beryllium  orthosilicate ;  Be2Si04 ;  BeO  =  45.55, 
SiO2  =  54.45;  Hexagonal;  Type,  Hexagonal  Alternating;  c  = 
.6611j  0001  A  lOll  =  37°  21' ;  r  A  if  =  63°  24' ;  Common  forms, 
a  (1120),  m(1010),  r(1011),  x  (1232) ;  Cleavage,  a  indistinct, 
r  imperfect ;  Brittle ;  Fracture,  conchoidal ;  H.  =  5-8 ;  G.  =  2.97- 
3 ;  Color,  white,  pale  yellow,  brown,  or  red ;  Streak,  white ;  Luster, 
vitreous;  Transparent  to  translucent;  o>  =  1.6542;  €  =  1.6700; 
€-a=  .0158;  Optically  (+). 

B.B.  —  Infusible,  with  borax  in  fine  powder  fuses  to  a  clear 
glass.  With  cobalt  solution  yields  a  dull  blue.  Yields  reactions 
for  beryllium. 

General  description.  —  Crystals  are  rhombohedral  in  habit  or 
short  prismatic,  terminated  by  rhombohedrons,  at  times,  of  all 
three  orders.  In  its  occurrences  it  is  associated  with  beryl  and 
topaz. 

The  largest  crystals,  some  nearly  four  inches  across,  are  obtained 
near  Ekaterinberg  in  the  Urals,  Russia.  At  Pike's  Peak  it  is  found 
implanted  on  crystals  of  microcline. 

Phenacite  when  clear  and  free  of  flaws  is  polished  as  a  gem 
stone. 

DIOPTASE 

Dioptase.  —  An  acid  copper  orthosilicate,  H2CuSi04 ;  CuO  = 
50.4,  SiO2  =  38.2,  H2O  =  1 1 .4 ;  Hexagonal ;  Type,  Hexagonal 
Alternating;  c  =  .5341;  0001  A  1011  =  31°  40';  rAr'  =  54°  5'; 


SILICATES,   TITANATES,   ETC.  453 

Twinning  plane  1011  =  Common  forms,  a  (1120),  r  (HH1),  s  (0221). 
Cleavage,  rhombohedral  perfect;  Brittle;  Fracture,  conchoidal; 
H.  =  5;  G.  =  3.28-3.35;  Color,  emerald  green;  Streak,  pale 
green;  Luster,  vitreous ;  Transparent  to  translucent ;  <o  =  1.667; 
€  =  1.723;  €-<o  =  .056  Optically  (+). 

B.B. — Infusible,  but  blackens  and  decrepitates.  Reduced  on 
coal  with  soda  and  a  little  coal  dust  yields  metallic  copper. 
Gelatinizes  with  HC1.  Yields  water  in  the  closed  tube. 

General  description.  —  Crystals  are  prismatic  in  habit,  combina- 
tions of  the  hexagonal  prism  of  the  second  order,  terminated  by  the 
rhombohedron  s  or  several  rhombohedrons  of  the  same  series. 
The  rhombohedron  x  (1231)  also  appears  on  some  specimens,  fix- 
ing the  symmetry  as  hexagonal  alternating. 

Dioptase  is  a  rare  mineral,  found  only  in  a  few  localities  and  then 
in  very  small  amounts.  The  finest  specimens,  some  of  which 
are  nearly  an  inch  in  length  are  obtained  in  a  limestone  near 
Altyn-Tiibe,  Siberia.  Occurs  at  Copiapo,  Chili,  and  in  the  Clifton 
Mine  Graham  Co.,  Arizona. 

When  the  crystals  are  clear  and  flawless,  they  are  cut  and 
polished  as  a  gem. 

SCAPOLITE  GROUP 

The  chemical  composition  is  variable,  as  the  scapolite  group,  like 
the  plagioclase  feldspars,  is  formed  by  two  end  members,  meion- 
ite,  Ca4  A16  Si6  O25  (me)  and  marialite,  Na4  A13  Si9  024  Cl,  (ma)  ; 
these  two  species  form  isomorphous  mixtures  in  all  proportions, 
and  therefore  in  composition  the  series  passes  uninterruptedly 
from  one  extreme  to  the  other. 

C  0)  €  CO  — € 

A/T   -       -I  .onof1'594     L557       °-037 

Meionite      me   -f  mao  to  me3  -f  mai  0.4393  j  ^  ^     ^  ^     Q  Q2g 

Wernerite  me2  +  ma*  to  mei  +  mai  0.4384  1.570  1.547  0.023 
Mizzonite  mei  +  mai  to  mei  -f-  ma3  0.4424  1.567  1.550  0.017 

_  f  1.562     1.546    0.016 
Marialite     mei  +  ma3  to  me0  +  ma  0.4417  j  .  __  .  _  .  -  . .  -  ~  „.  ~ 

WERNERITE 

Wernerite.  —  Calcium  aluminium  orthosilicate,  Ca4Al6Si6O25 ; 
CaO  =  25.1,  A1203  =  34.4,  SiO2  =  40.5;  Tetragonal;  Type, 


454 


MINERALOGY 


Tetragonal  Equatorial;  c  =  .4384;  001A101  =  23°  40';  Com- 
mon forms,  a  (100),  c  (001),  m(110),  e  (101),  h  (210) ;  Cleavage, 
a  and  m  distinct;  Brittle;  Fracture,  uneven;  H.  =  5-6;  G.  = 
2.66-2.73 ;  Color,  white,  gray,  shades  of  yellow,  green,  red,  brown 
to  black;  Streak,  white  or  pale;  Luster,  vitreous  to  pearly; 
Transparent  to  opaque;  o>  =  1.562;  €  =  1.546;  CD  —  €=  .016; 
Optically  (-). 

B.B.  —  Fuses  easily  with  intumescence  to  a  blebby  glass.  The 
fused  mass  powdered  gelatinizes  with  HC1. 

General  description.  —  Crystals  coarse  prismatic,  combination 
of  the  two  prisms  a  and  m  terminated  by  the  unit  pyramid  or  the 
two  unit  pyramids.  The  third  order  prism  h  (210)  and  the  pyra- 
mid z  (311)  occur  on  crystals 
from  Grass  Lake,  New  York. 
Wernerite  is  often  granular  or 
massive. 

In  sections  the  scapolites  are 
colorless,  either  in  crystalline 
outline  or  rounded  grains ;  pris- 
matic cleavage  cracks  distinct; 
relief  is  very  low,  about  that  of 
quartz.  The  inclusions  are  not 
characteristic ;  double  refraction 
rather  strong,  yielding  interfer- 
ence colors  of  the  second  order 
and  increasing  with  the  amount 
of  calcium  present.  The  inter- 
ference figure  is  found  in  the 

sections  in  which  the  cleavage  cracks  are  at  right  angles,  yielding 
a  dark  cross  with  color  bands  in  thin  sections,  in  the  margin  of 
the  field  only.  Optically  negative.  The  scapolites  are  found  in 
igneous  rocks  as  secondary  minerals  only.  They  are  the  products 
especially  of  contact  metamorphism  and  commonly  occur  in 
granular  limestones,  where  they  are  associated  with  pryroxene, 
hornblende,  zircon,  spinel,  titanite,  and  garnets. 

Wernerite  occurs  in  fine  crystals  at  Pierrepont,  Gouverneur, 
Monroe,  and  Amity,  New  York,  and  at  numerous  points  in  New 
Jersey,  Pennsylvania  and  the  New  England  States. 

Meionite  and  mizzonite  occur  in  the  ejected  blocks  of  limestone 
on  Monte  Somma,  Vesuvius. 


FIG.  488.  —  Wernerite.    Laueinpaei, 
Finland. 


SILICATES,  TITANATES,  ETC.  455 

The  scapolites  are  easily  decomposed  by  weathering,  particularly 
those  containing  sodium,  the  ultimate  products  being  kaolin,  talc, 
or  micas,  with  calcite  and  quartz. 

The  artificial  production  of  the  scapolites  is  in  doubt. 

VESUVIANITE 

Vesuvianite.  —  An  orthosilicate  of  calcium  and  aluminium, 
Ca6(Al .  OH)Al2(Si04)5,  in  which  other  isomorphous  elements 
may  enter;  CaO  =  42.3,  A12  =  19.1,  SiO2  =  37.5,  H20  =  1.1; 
Tetragonal;  Type,  Didigonal  Equatorial;  c  =  .5372;  001A101 
=  28°  15';  001A111  =  37°  14';  Common  forms,  c  (001),  a  (100), 
m(110),  p  (111) ;  Cleavage,  prismatic  imperfect,  a  and  c  less  so; 
Brittle ;  Fracture,  uneven ;  H.  =  6.5 ;  G.  =  3.35-3.45 ;  Color, 
shades  of  brown  and  green ;  Streak,  white ;  Translucent ;  CD  = 
1.705;  €  =  1.701;  o>  -  €  =  .004;  Optically  (-),  sometimes  (+). 

B.B.  —  Fuses  with  intumescence  at  three,  to  a  brown  slag,  which 
when  powdered  gelatinizes  with  HC1.  Some  varieties  will  yield 
reactions  for  manganese  or  copper. 

General  description.  —  Crystals  are  well  developed,  stout  pris- 
matic combinations  of  the  prisms  of  the  first  and  second  orders 
terminated  by  the  base  and  the  unit  pyramid.  Striations  on  the 
prism  zone  lengthwise.  Beautiful  crystals,  combinations  of 
these  forms,  are  found  on  the  Vilui  River,  Siberia.  Crystals  from 
Wakefield,  Quebec,  have  the  pryamid  reduced  to  almost  a  line  and 
are  terminated  by  the  base.  Well-formed  crystals,  combinations  of 
all  seven  forms  of  the  type,  are  found  at  Poland,  Maine,  and  more 
complicated  combinations  are  found  on  the  crystals  from  Vesuvius, 
in  some  of  which  the  prism  zone  is  very  much  reduced,  yielding 
crystals  of  pyramidal  habit.  Vesuvianite  also  occurs  radiated, 
columnar,  irregular,  granular  or  massive.  A  compact,  green,  jade- 
like  variety  is  known  as  californite. 

Chemically  the  calcium  may  be  replaced  in  part  by  manganese, 
magnesium,  or  iron ;  also  fluorine  and  boron  may  be  present  in  small 
amounts.  In  sections  Vesuvianite  may  appear  in  crystalline  out- 
lines, rounded  or  irregular,  colorless  or  pale.  The  relief  is  well 
marked,  with  prismatic  cleavage  cracks  imperfectly  developed. 
Pleochroism  faint,  but  increasing  with  the  depth  of  color  of  the 
specimen.  Interference  colors  gray  of  the  first  order.  Basal 
sections  show  only  the  shadow  of  the  interference  figure  but  no 
colors.  Optically  negative,  rarely  positive. 


456 


MINERALOGY 


Zonal  structure  and  optical  anomalies  are  not  uncommon. 
Vesuvianite  is  a  mineral  produced  by  contact  metamorphism. 

It  appears  in  schists  but  more  often  in  granular  limestones,  where 

it  is  associated  with 
garnets,  epidote,wer- 
nerite,  wollastonite, 
and  diopside.  It  is 
a  common  mineral 
in  the  ejected  blocks 
of  limestone  on 
Monte  Somma,  Ve- 
suvius ;  in  small  bril- 
liant crystals  also  in 
the  Ala  thai,  Pied- 
mont. Clear  green 
brilliant  crystals  oc- 
cur at  Amity,  New 
York.  It  occurs  at 
Rumford  and  Po- 
land, Maine,  in  lime- 
stones associated 
with  garnets;  at 
Newton,  New  Jersey, 
with  corundum  and 
spinel. 
The  clear  crystals  are  sometimes  polished,  but  it  makes  an 

indifferent  gem  known  as  idocrase. 

It  is  seldom  found  altered,  though  pseudomorphs  after  vesuvian- 

ite  are  known. 

Vesuvianite  has  not  been  produced  artifically;    when  fused  it 

breaks  down  and  on  cooling  the  melt  produces  olivine,  anorthite, 

and  melilite. 


FIG.  489.  — Vesuvianite,  Poland,  Maine.    The  Upper 
Crystal  is  Viluite  from  Siberia. 


ZIRCON 

Zircon.  —  Zirconium  orthosilicate,  ZrSiO4 ;  ZrO  =  67.2 ;  SiO2 
=  32.8 ;  Tetragonal ;  Type,  Ditetragonal  Equatorial ;  c  =  .6493  ; 
001A101  =  32°  38';  11(^111=47°  50';  111A111=56°  40'; 
Common  forms,  p(lll),  m(110),  u(331),  x  (311) ;  Twinning 
plane  111,  geniculate  twins;  Cleavage,  m  distinct,  p  less  so; 
Brittle ;  Fracture,  conchoidal ;  H.  =  7.5 ;  G.  =  4.68-4.7 ;  Color, 


SILICATES,   TITANATES,   ETC. 


457 


pale  yellow,  brown,  red,  green,  white, 
black.  Streak,  white ;  Transparent  to 
€  =  1.968;  co  -  €  =  .045;  Optically  (+). 


and   at    times    nearly 
opaque;      co  =  1.923; 


B.B.  —  Whitens  but  infusible.  Only  very  slightly  affected  by 
acids.  Yields  a  zirconium  reaction  with  turmeric  paper. 

General  description.  —  Crystals  are  short  prismatic  combinations 
of  the  unit  prism  and  pyramid  of  the  first  order.  When  x  is  present 
the  crystals  are  acutely  pointed.  Microcrystals  are  more  apt  to 
be  pyramidal  in  habit.  Twins 
are  not  common,  but  genicu- 
late  twins  like  those  found 
in  rutile  occur  in  Renfrew 
County,  Quebec.  Zircon  is 
related  in  its  angles  and  axial 
ratio  to  cassiterite  and  rutile, 
and  these  with  thorite,  ThSiO4, 
constitute  an  isomorphous 
group.  In  rock  sections  zircon 
appears  either  in  crystalline 
outlines  or  rounded,  with  a 
very  high  relief,  and  white 
or  pale  yellow  or  brown  in 
color.  Cleavage  cracks  are 
not  marked. 

Interference  colors  are  high 

fourth  order,  and  the  interference  figure   shows  several  colored 
circles  in  addition  to  the  dark  cross.     Optically  positive. 

Zircon  is  very  widely  distributed,  occurring  as  one  of  the  most 
common  accessory  minerals  of  the  igneous  rocks,  as  the  granites, 
syenites,  and  diorites,  but  never  in  very  large  quantities ;  in  such 
magmas  it  is  the  first  silicate  to  separate.  It  is  also  of  common 
occurrence  in  pegmatites,  as  at  Green  River,  North  Carolina, 
where  it  is  separated  in  commercial  quantities;  near  Cash, Okla- 
homa, in  a  pegmatite.  At  Greenville,  Canada,  and  Amity,  New 
York,  it  occurs  in  a  crystalline  limestone.  At  several  points  in 
Essex  and  Orange  Counties,  New  York,  deep  brown  to  almost 
black  crystals  occur. 

It  is  decomposed  by  weathering  with  difficulty  and  is  found  in 
alluvial  deposits  and  gold-bearing  sands,  with  garnets,  cassiterite, 
magnetite,  and  other  heavy  minerals,  still  in  a  fresh  unaltered 


FIG.  490.  —  Zircon,  Buncombe  County, 
North  Carolina.  The  Small  Crystal  is 
from  Essex  County,  New  York. 


458  MINERALOGY 

condition.  At  times  by  hydration  it  becomes  dull  and  greasy  in 
appearance,  forming  several  varieties,  as  the  malacon  from  Hit- 
teroe,  Norway. 

Jargon,  jacinth,  and  hyacinth  are  clear  varieties  usually  obtained 
from  Ceylon,  which  are  cut  and  polished  as  gems. 

Artificial  zircon  may  be  produced  by  heating  gelatinous  silica 
and  zirconia  to  a  red  heat  under  pressure. 

TOPAZ 

Topaz.  —  Al .  Al(O,F2)Si04 ;  A12O3  =  55.44,  Si02  =  32.61, 
F  =  20.65 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  &  :  b  :  c 
=  .528;  1  :  .477;  110A110  =  55°  43';  120A120  =  86°  49';  001 
Alll  =  45°  35';  001,223  =  34°  14';  001A221  =  63°  54';  001 
A041  =  62°  20';  001A043  =  32°  27';  001A201  =  61°  30";  001 
A021  =  43°  39';  Common  forms,  a  (100),  b  (010),  c(001),  m(110), 
1(120),  d(201),  f(021),  y(041),  x(043),  o(221),  u(lll),  i  (223) ; 
Cleavage  basal  perfect ;  Brittle ;  Fracture,  subconchoidal ;  H.  =  8  ; 
G.  =  3.4-3.65 ;  Color,  white  and  pale  shades  of  yellow,  pink, 
blue,  green.;  Streak,  white ;  Transparent  to  translucent ;  a  =  1.615  ; 
P  =  1.618;  7  =  1.625;  V-a  =  -010;  2V  =  62°  33';  Axial 
plane  =  010;  Bxa  =  c;  Optically  (+). 

B.B.  —  Changes  color,  but  infusible.  Yields  a  fluorine  reaction 
in  the  closed  tube.  The  powdered  mineral  becomes  blue  with 
cobalt  solution.  Very  little  affected  by  acids. 

General  description.  —  Crystals  are  prismatic  in  habit,  combi- 
nations of  the  two  prism,  m  and  1,  both  of  which  are  striated  length- 
wise, but  1  more  than  m ;  terminated  by  one  or  two  domes  and 
pyramids,  rarely  doubly  terminated. 

In  chemical  composition  the  fluorine  is  variable,  as  it  may  be 
replaced  by  hydroxyl  (OH) ;  both  of  these  are  driven  off  at  a 
white  heat;  transforming  the  topaz  to  sillimanite. 

In  rock  sections  topaz  appears  with  crystalline  outlines  or  lath- 
shaped,  elongated  parallel  to  6 ;  the  basal  cleavage  is  well  marked 
by  cracks.  Relief  is  medium  and  the  interference  colors  are  first 
order  gray  or  yellow.  The  interference  figure  is  shown  in  the  basal 
section ;  the  angle  2  E  varies  with  the  fluorine  and  decreases  as  OH 
increases.  For  .93  per  cent.  H20,  2E  =  114°.  Cavities  elongated 
parallel  to  the  vertical  axis  and  containing  fluids  are  common. 

Topaz  is  an  accessory  mineral  in  many  granites,   and  is  also 


SILICATES,  TITANATES,  ETC.  459 

especially  characteristic  of  pegmatites  containing  cassiterite,  in 
which  it  is  also  associated  with  beryl,  tourmaline,  fluorite,  and  apa- 
tite. At  Schneckenstein,  Saxony,  it  is  associated  with  apatite 
chalcopyrite,  and  cassiterite. 

The  large  blue  crystals  from  Mursinka,  in  the  Urals,  are  asso- 
ciated with  smoky  quartz,  lepidolite,  and  feldspars.  At  Nathrop, 
Colorado,  and  in  the  Thomas  Range,  Utah,  well-developed  crystals, 
both  white  and  wine-colored,  are  associated  with  quartz,  in  cavi- 
,  ties  in  rhyolite. 

At  Stoneham,  Maine,  it  occurs  in  granite.  The  topazes  of  Minas 
Geraes,  Brazil,  occur  in  a  decomposed  schist ;  they  are  a  light  brown 
color  with  numerous  elongated  cavities,  containing  liquid  carbon 
dioxide. 

Topaz,  owing  to  its  hardness,  transparency,  and  delicate  coloring, 
has  for  a  long  time  been  used  as  a  precious  stone,  especially  those 
from  Siberia  and  Brazil.  The  Brazilian  topazes  may  be  improved 
in  color  and  the  yellow  and  brown  shades  changed  to  a  delicate 
pink  by  careful  heating ;  this  process  is  known  as  pinking. 

In  the  process  of  weathering,  topaz  takes  up  water  and  alkalies, 
forming  micas. 

The  synthesis  of  topaz  has  been  accomplished  by  heating  a  mix- 
ture of  silica  and  aluminium  fluoride  to  a  red  heat  and  then  igniting 
the  mixture  in  a  current  of  silicon  fluoride.  Both  the  synthesis  and 
the  associations  indicate  that  in  many  cases  topaz  has  been  the 
result  of  pneumatolytic  reactions  in  which  volatile  fluorides  were 
the  direct  agent. 

ANDALUSITE 

Andalusite.  —  Al(AlO)SiO4;  A12O3  =  63,  Si02  =  37;  Ortho- 
rhombic  ;  Type,  Didigonal  Equatorial ;  fi  :  b  :  c  =  .986  :  1 :  .702 ; 
100A110  =  44°  36';  001A011  =  35°  5';  Common  forms,  c  (001), 
m  (110),  s  (Oil) ;  Cleavage,  prismatic  distinct,  a  less  so;  Brittle; 
Fracture,  uneven ;  H.  =  7.5;  G.  =  3.16-3.20;  Color,  gray,  reddish, 
pink,  blue,  and  green;  Streak,  white;  Luster,  vitreous;  Trans- 
parent to  opaque;  a  =  1.632;  p  =  1.638;  \  =  1.643;  -y-a  = 
.011;  Optically  (-);  Bxa=  c;  2V  =  83°  37'. 

B.B.  —  Infusible,  not  attacked  by  acids.  The  fine  powder 
becbmes  blue  with  cobalt  solution. 

General  description.  —  Crystals  are  coarse,  prismatic,  simple 
combinations  of  the  nearly  square  unit  prism  and  the  base  or  dome ; 


460 


MINERALOGY 


FIG.  491.'' — Andalusite  from  Lancaster,  Massa- 
chusetts. 


such  combinations  are  found  at  Lisens  Alp,  Tyrol,  embedded  in  a 
chloritic  schist.     Transparent  crystals  which  display  very  strong 

dichroism  are  obtained 
in  Minas  Geraes,  Bra- 
zil, and  are  cut  as  gem 
stones  showing  green 
when  viewed  along 
one  direction  and  red 
when  viewed  in  the 
other. 

Many  specimens  of 
andalusite  contain  or- 
ganic inclusions  ar- 
ranged symmetrically, 
the  outline  or  cross 
section  of  which  varies 
with  the  position  in 
the  crystal.  Like  the 
symmetrical  inclu- 
sions in  leucite,  this 
arrangement  is  prob- 
ably due  to  a  skeletal  development  during  the  growth  of  the 
crystal ;  such  inclusions  are  especially  characteristic  of  the  variety 
known  as  chiastolite,  found  in  argillaceous  schists  and  clay  slates, 
the  crystals  of  which  are  slender,  prismatic,  almost  acicular. 

Andalusite  is  trimorphic  with  sillimanite  and  cyanite,  all  being  of 
the  same  percentage  composition  chemically,  but  differing  in  their 
physical  and  crystallographical  properties.  Of  the  three,  silli- 
manite is  the  most  stable  at  high  temperatures,  as  both  andalusite 
and  cyanite  when  heated  to  1400°  C.  pass  over  to  sillimanite  on 
cooling. 

In  rock  sections  andalusite  appears  in  almost  square  or  in  elon- 
gated outlines.  Relief  is  marked,  and  the  pleochroism  shows  only 
in  the  colored  varieties.  Inclusions  are  symmetrically  arranged. 
Interference  colors  are  yellows  of  the  first  order. 

Andalusite  is  the  result  of  metamorphism  and  is  developed  in 
some  gneisses  and  schists,  where  it  is  associated  with  sillimanite, 
cyanite,  iolite,  garnets,  corundum,  and  tourmalines.  Specimens 
with  typical  inclusions  are  found  at  Lancaster,  Massachusetts,  and 
Rochester,  New  Hampshire.  It  is  a  common  mineral  at  numerous 
points  in  New  England. 


SILICATES,   TITANATES,   ETC.  461 

The  name  andalusite  is  derived  from  the  noted  locality  of  Anda- 
lusia, in  Spain. 

Andalusite  in  weathering  is  decomposed  by  percolating  waters 
containing  alkalies,  forming  micas  and  kaolinite. 

SILLIMANITE 

Sillimanite.  —  Al(AlO)SiO4 ;  A103  =_63 ;  Si02  =  37 ;  Ortho- 
rhombic  ;  a :  b  :  c  =  .970  :  1 :  ?  110A110  =  88°  15' ;  230A230 
=  69°;  Common  forms,  a  (110),  b  (010),  m(110),  h  (230) ; 
Cleavage,  b  perfect;  Brittle;  Fracture,  uneven;  H.  =  6-7; 
G.  =  3.23-3.24;  Color,  shades  of  gray,  brown,  and  green ;  Streak, 
white;  Luster,  vitreous;  Transparent  to  opaque;  a  =  1.660; 
p  =  1.661 ;  -y  =  1.681 ;  y  -  a  =  .020;  Axial  plane  =  010; 
Bxa  =  c;  Optically  (+);  2V  =  31°  19'. 

B.B.  —  Like  andalusite. 

General  description.  —  Crystals  are  slender,  elongated,  parallel 
to  the  vertical  axis  ;  from  this  habit  the  mineral  is  sometimes  known 
as  fibrolite.  Terminations  have  never  been  observed.  Also  in 
compactly  massed  fibers  arranged  parallel  or  radiating.  In  sections 
sillimanite  may  be  distinguished  from  andalusite  by  the  fibrous 
structure,  by  the  higher  double  refraction  yielding  interference 
colors  of  the  second  order,  and  by  the  optical  character  being  posi- 
tive. 

In  occurrence  and  decomposition  by  weathering,  it  resembles 
andalusite. 

CYANITE 

Cyanite.  —  Disthene,  Al(AlO)SiO4;  _A12O3  =  63,  Si02  =  37  ; 
Triclinic ;  Type,  Centro-symmetric ;  a :  b  :  c  =  0.8994  :  1 :  .7089 ; 
a  =  90°  5' ;  (3  =  101°  2' ;  \  =  105°  44' ;  100A010  =  73°  56'  ; 
100  A  001  =  78°  30';  010A001  =  86°  45';  100A110  =  34°  17'; 
Common  forms,  a  (100),  b  (010),  c  (001),  m(110);  Twinning 
plane,  100 ;  Cleavage,  a  perfect,  b  less  so  and  parting  parallel  to  c ; 
Brittle;  Fracture,  fibrous;  H.  =  5-7.25;  G.  =  3.56-3.67;  Color, 
blue,  white,  reddish,  and  green;  Luster,  vitreous;  Transparent 
to  translucent;  a  =  1.717;  p  =  1.722;  <y  =  1.729;  -y  -  a  = 
.012 ;  Optically  (  -  ) ;  Bxa  nearly  normal  to  100 ;  2  V  =  82°. 

B.B.  —  Like  andalusite. 

General  description.  —  Crystals  are  long,  bladelike  in  habit, 
combinations  of  the  two  pinacoids  a  and  b,  with  the  unit  prism ; 


462  MINERALOGY 

the  a  pinacoid  is  often  striated  longitudinally.  Terminations  are 
very  rare.  The  two  pinacoidal  cleavages  are  at  an  angle  of  106° 
with  a  cross  parting  parallel  to  the  base.  The  hardness  is  remark- 
able for  its  variation  with  the  direction ;  when  tested  parallel  to 


FIG.  492.  —  Cyanite,  Pizzo  Eorno,  St.  Gothard,  Switzerland. 

the  length  of  the  crystal,  on  a,  the  hardness  is  nearly  7,  but  when 
tested  at  right  angles  to  this  direction  on  the  same  face  its  hardness 
is  only  4. 

Cyanite  is  a  mineral  produced  by  metamorphic  agents  and  is 
never  found  as  a  pyrogenetic  mineral  in  igneous  rocks,  but  is  asso- 
ciated with  staurolite  and  garnets  in  crystalline  schists. 

Typical  specimens  are  found  at  Litchfield  and  Newton,  Con- 
necticut ;  in  Yancey  County,  North  Carolina.  The  rare  white  and 
yellow  variety  rhaetizite  is  found  in  the  Zillerthal  and  Pfitschthal 
Tyrol.  Most  beautiful  specimens  occur  at  Pizzo  Forno,  St.  Goth- 
ard, Switzerland,  in  a  paragonite  schist,  associated  with  staurolite, 
with  which  it  is  often  in  parallel  position. 

In  its  alterations  cyanite  is  like  sillimanite   and  andalusite. 


SILICATES,  TITANATES,  ETC.  463 

When  the  constituent  oxides  in  the  required  proportion  are  fused, 
sillimanite  is  formed;  just  what  are  the  conditions  required  to 
produce  the  two  less  stable  forms,  cyanite  and  andalusite,  is  not  as 
yet  exactly  known. 

BASIC  ORTHOSILICATES 
DATOLITE 

Datolite.  —  A  borosilicate  of  calcium,  Ca(BOH)Si04;  CaO  = 
35.0,  B208  =  21.8,  SiO2  =  37.6,  H20  =  5.6;  Monoclinic; 
Type,  Digonal  Equatorial ;  a  :  b  :  c  =  .6344  : 1 :  1.2657 ;  p  =  89° 
51'  =  100A001;  100A110  =  32°24';  001A101  =  63°  16';  001A011 
=  51°  41';  Common  forms,  c  (001),  a  (100),  m  (101),  m,  (Oil), 
n  (111),  b  (010),  g  (012),  x  (102),  other  forms  also  numerous ;  Cleav- 
age, none;  Brittle;  Fracture,  uneven;  H.  =  5-5.5;  G.  =  2.9-3; 
Color,  white,  pale  green,  or  yellow ;  Streak,  white ;  Luster,  vitreous ; 
Transparent  to  translucent;  a  =  1.624;  p  =  1.652;  -y  =  1.669; 
Y  -  a  =  .045;  Optically  (  -  ) ;  Axial  plane  =  010;  BxaAc  =  1°- 
4°  in  the  acute  angle  p. 

B.B.  —  Fuses  easily  with  intumescence,  and  in  the  forceps  alone 
yields  a  green  flame  (boron).  Gelatinizes  with  HC1.  In  the  closed 
tube  yields  water. 

General  description.  —  Crystals  are  short,  prismatic,  or  equidi- 
mensional  combinations  of  a,  m,  c,  n,  x,  mx,  and  m,  all  of  which  are 
very  bright,  with  a  high  luster,  except  x,  which  is  dull.  A  massive 
form,  very  much  like  porcelain  on  fracture  surfaces,  occurs  in  the 
Lake  Superior  copper  regions. 

Datolite  is  a  secondary  mineral  deposited  from  solutions  in  the 
cavities,  cracks,  and  veins,  especially  of  basalts  and  gabbros,  where 
it  is  associated  with  calcite,  zeolites,  prehnite,  and  quartz.  It  is  a 
common  mineral  in  the  quarries  opened  in  the  traps  of  the  New 
England  and  Middle  states.  Fine  specimens  have  been  obtained 
at  Bergen  Hill  and  Patterson,  New  Jersey ;  at  Westfield,  Massa- 
chusetts, and  at  New  Haven,  Connecticut. 

EPIDOTE  GROUP 

The  epidote  group  is  made  up  of  basic  orthosilicates  of  the  general 
formula  R"2R"/2(AlOH)(Si04)3,  in  which  R"  is  Ca,  Mn,  or  Fe,  and 
R'"  is  Al,  Fe,  Ce,  or  Mn.  Some  epidotes  are  orthorhombic,  but 


464 


MINERALOGY 


**HJ     Cr   C2_      ^^     o   ^^ 

C?     r*""]                           Q^      r*H 

.S  ^  -2    .S  ^  ^    o 

XB 

II 

« 

1  •-  1  1  -s  I  T 

o        CP               o        ®       /\j 

« 

S?  g    M  7  1   N 

Ig 

PI 

o             S            o 
i—  i             i—  i            i—i 

0,                  0                   0 

> 

8 

3 

O                                   o 

oo              o 
oo              & 

1 

CO                     O5 
l>                    00 

°u 

+ 

1                -H              -H 

d 

CO 

O                    O                     O   oq 

1 

8 

q              q              S8 

b  % 
C  O 

cog  g 

CS|    (M   CO      C^    (N   CO        00 

§g 

CO   CO   !>• 

J 

3E 

II      II      II 

II      II       II         II       II       II           II 

IBS 

"w 

d  co.  ?- 

d    GO.    ?-       d    GO.    ?-       CO. 

O5 

CO                           CO                           rH 

CO                   (N                   Oi 

CO 

CO    rH                  CO    rH                  O    rH 

JD 

^ 

o                       o                       o 

C8 

8 

II    Is"             IIO              II    Ci 
ro    00             rn    C              fp    O 

q 

»o              co              >o 

rH                           rH                           rH 

co 

00                           rH                           iO 

CO                    *O                    CO 

CO 

CO                     CO                     CO 

O 

S         2  .?     ffi  §  § 

o 

fe 

s 

S             ^  ^        O  o3    ^ 

•>  —  ' 

0 

5 

M 

3 
w 

2         ^         ^  ^  ^ 
^          fe          ^fe   a 

1 

I 

o 

3 

1       1       *«5,* 

£ 

<£ 

o 

^              ^                          s 

CO                           CO                                              7"\ 

o         o                 S 

3 

• 

*                         D                            •                 OJ 

•t3                           .^ 

1 

S 
•a 

1    J     1   1 

N 

I      I      ^    1 

SILICATES,   TITANATES,   ETC.  465 

with  the  increase  of  iron  and  manganese  in  the  formula  they  become 
monoclinic. 

ZOISITE 

Zoisite.  —  A  basic  orthosilicate  of  calcium  and  aluminium ; 
Ca2Al2(A10H)  (SiO)3;  CaO  =  24.6,  A12O3  =  33.7,  SiO2  =  39.7, 
H2O  =  2.0 ;  Orthorhombic  ;  Type,  Didigonal  Equatorial ;  &  :  b:  c 
=  .6196:  1 :  .3429;  100A110  =  31°  47';  001A101  =  28°  58'; 
001A011  =  18°  56';  Common  forms,  a  (100),  b(010),  m(110), 
o(lll),  d(101),  f(011),  r(120);  Cleavage,  b  perfect;  Brittle; 
Fracture,  uneven;  H.  =  6-6.5;  G.  =  3.25-3.37;  Color,  gray, 
white,  or  pale  shades  of  green,  pink,  red,  or  yellow ;  Streak,  white  ; 
Luster,  vitreous  to  pearly;  a  =  1.696;  p  =  1.696;  7  =  1.702; 
Y  —  a  =  .006 ;  Optically  (  +  ) ;  Axial  plane  =  010  at  times  001 ; 
Bxa  =  a;  2V  =  0-60°. 

B.B.  —  Fuses  with  intumescence  at  three  to  a  white  blebby  slag, 
gelatinizes  after  fusion.  After  strong  ignition  in  the  closed  tube 
yields  water. 

General  description.  —  Crystals  are  prismatic  in  habit,  elon- 
gated parallel  to  the  vertical  axis,  while  epidote  is  elongated  paral- 
lel to  the  orthoaxis ;  in  the  comparison  of  these  two  minerals  the 
c  axis  of  zoisite  is  equivalent  to  the  b  axis  of  epidote.  Deep  stria- 
tions  on  the  prism  zone  parallel  to  the  vertical  axis  are  characteristic. 
The  crystals  are  rarely  terminated,  occurring  in  parallel  or  diver- 
gent groups ;  also  massive. 

Thulite  is  a  pink  variety  from  Norway,  and  Traversella,  in  Pied- 
mont ;  the  pink  color  is  due  to  manganese. 

Zoisite  contains  but  little  iron  and  is  essentially  an  aluminium 
epidote.  Clinozoisite  is  a  light-colored  variety  of  epidote  with 
small  amounts  of  iron ;  the  characteristic  pistachio-green  color  of 
epidote  deepens  with  the  increase  of  iron  in  the  molecule. 

In  rock  sections  zoisite  appears  in  elongated  crystals  or  granu- 
lar; colorless,  or  pale;  with  the  cleavage  cracks  parallel  to  the 
macropinacoid  distinctly  developed.  Relief  is  high,  but  the 
interference  color  is  a  low  first  order  gray.  Extinction  parallel. 
The  plane  of  the  optic  axis  is  usually  parallel  to  the  base,  but  at 
times  may  be  parallel  to  the  macropinacoid.  Optically  (  +  ). 

Zoisite  is  associated  with  the  crystalline  schists ;  rarely  is  it 
found  in  granites  or  igneous  rocks.  As  a  secondary  mineral  it  is 
2n 


466  MINERALOGY 

derived  from  the  alteration  of  the  plagioclases.     Saussurite  is  a 
mixture  of  plagioclase  and  zoisite  in  various  proportions. 

It  occurs  in  various  localities  in  Massachusetts,  Connecticut, 
and  North  Carolina ;  at  Ducktown,  Tennessee ;  and  in  the  Coast 
Range,  California.  The  synthesis  of  zoisite  is  uncertain,  as  the 
products  of  various  fusions  have  contained  no  water. 

EPIDOTE 

Epidote.  —  A  basic  orthosilicate  of  calcium  aluminium  and  iron, 
Ca2(AlFe)2(AlOH)(Si04)3;  CaO  =  23.73,  A1203  =  25.95,  Fe2O3 
=  10.18  (when  Fe  :  Al : :  4  : 1),  SiO2  =  35.20,  H2O  =  1.91 ;  Mono- 
clinic  ;  Type,  Digonal  Equatorial ;  a  :  b  :  c  =  1.5787  :  1 :  1.8036 ; 
p  =  64°  37'  =  100A001;  100A110  =  55°;  001A101  =  34°  43'; 
001A011  =  58°  28';  Common  forms,  c  (001),  a  (100),  m(110), 
e(101),  r(101),  o(011),  d(lll);  Twinning  plane,  100  contact 
twins,  also  001,  but  rare ;  Cleavage,  basal  perfect  and  a  imperfect ; 
Brittle ;  Fracture,  uneven ;  H.  =  6-7 ;  G.  =  3.25-3.5 ;  Color, 
shades  of  green,  also  yellow,  red,  or  gray ;  Streak,  white ;  Luster, 
vitreous;  Transparent  to  opaque;  a  =  1.724;  p  =  1.729;  y 
=  1.734;  -y  -  a  =  .010;  Optically  (  -  );  Axial  plane  =  010; 
BxaAc  2°-3°  in  the  acute  angle  p;  2V  =  73°-88°. 

B.B.  —  Fuses  at  three  with  intumescence  to  a  black  blebby  glass, 
which  when  powdered  is  generally  magnetic  and  gelatinizes  with 
HC1.  After  strong  ignition  in  the  closed  tube  yields  water. 


FIG.  493.  — Epidote,  Sulzbachthal,  Tyrol.    The  Central  Figure  is  from  Prince-of- 
Wales  Island,  Alaska. 

General   description.  —  Crystals  are  elongated  parallel   to  the 
orthoaxis,  with  terminations  generally  rich  in  faces ;  often  twinned 


SILICATES,   TITANATES,   ETC.  467 

as  is  indicated  by  the  reentrant  angle.  Particularly  fine  speci- 
mens of  this  habit  occur  in  a  chloritic  schist,  associated  with  adu- 
laria,  apatite,  titanite,  and  calcite,  in  the  Sulzbachthal,  Tyrol.  A 
tabular  habit,  though  not  as  common  as  the  elongated  habit, 
also  occurs,  good  specimens  of  which  are  obtained  on  the  Prince  of 
Wales  Island,  Alaska ;  these  are  also  twinned,  the  twinning  being 
revealed  by  the  striations  on  the  clinopinacoidal  face.  Massive 


FIG.  494.  —  Epidote.     Sulzbachthal,  Tyrol. 

and  granular  epidote  mixed  with  quartz  occurs  as  the  rock  epido- 
site;  it  is  derived  from  the  alteration  of  plagioclase  feldspars 
together  with  some  ferromagnesian  mineral,  as  pyroxene  or  amphi- 
bole. 

Piedmontite  is  a  brown  or  red  epidote  in  which  the  iron  is  re- 
placed by  manganese ;  it  occurs  in  Piedmont,  Italy,  and  also  in  a 
rhyolite  at  South  Mountain,  Pennsylvania. 

In  rock  sections  epidote  appears  colorless,  pale  yellow,  or  brown, 
depending  upon  the  percentage  of  iron;  in  tabular  or  elongated 
crystals,  at  times  intergrown  with  zoisite.  The  relief  is  high,  and 
the  basal  cleavage  cracks  are  distinct.  Pleochroism  is  strong  in  the 
colored  varieties,  and  much  less  in  the  colorless,  or  those  poor  in 
iron.  Interference  colors  are  high,  as  the  double  refraction  may 
vary  from  .03  to  .06.  The  extinction  is  inclined  and  varies  from  2 


468  MINERALOGY 

to  3°  with  the  imperfect  orthopinacoidal  cleavage  cracks.  Basal 
cleavage  fragments  show  one  optic  axis,  while  the  acute  bisectrix 
is  nearly  perpendicular  to  100.  Optically  (  —  ). 

Epidote  appears  as  a  secondary  mineral  in  igneous  rocks  and  in 
schists,  where  it  is  derived  from  the  alteration  of  feldspars  and 
pyroxene  or  amphibole,  usually  associated  with  chlorite.  It  is  also 
the  product  of  metamorphism,  and  is  found  in  contact  zones,  as 
well  as  in  granular  limestones  where  it  is  associated  with  vesu- 
vianite,  garnets,  hematite,  and  pyroxenes.  It  rarely  occurs  as  a 
primary  mineral  of  igneous  rocks.  In  the  United  States  epidote 
is  a  common  mineral  at  various  localities  along  the  Atlantic  slope 
in  the  New  England,  Middle,  and  Southern  states.  Its  synthesis, 
like  zoisite,  is  uncertain. 

ALLANITE 

Allanite.  —  Ca«(Al .  Ce  .  Fe)2(A10H)  (Si04)3 ;  Composition  vari- 
able ;  Monoclinic ;  Type,  Digonal  Equatorial ;  a  :  b  :  c  =  1.5509  : 
1:1.7691;  p  =  64°  39' =  100A001 ;  100A110  =  54°34';  001 A 101 
=  63°  24';  001A00:L  =  58°  3' ;  Common  forms,  c  (001),  a  (100)",  m 
(110),  d(lll),  n(lll);  Twinning  plane,  100;  Cleavage,  100  and 
001  in  traces ;  Brittle ;  Fracture,  uneven ;  H.  =  5.5-6 ;  G.  = 
3.5-4.2;  Color,  pitch  brown  to  black  or  yellowish;  Streak,  pale 
gray  or  greenish;  Luster,  pitchy  to  dull;  Opaque  to  subtranslu- 
cent. 

B.B.  —  Fuses  easily  with  intumescence.  Becomes  magnetic 
in  R.  F.  After  strong  ignition  in  the  closed  tube  yields  water. 
Gelatinizes  with  HC1 ;  the  solution  freed  of  silica  yields  reactions 
for  cerium,  page  571. 

General  description.  —  Either  tabular  in  habit  parallel  to  100, 
or  acicular  parallel  to  the  orthoaxis ;  also  granular  or  massive. 

The  elongated  variety  has  been  described  under  the  name  of 
orthite ;  it  contains  much  water,  even  as  high  as  17  per  cent.,  while 
the  true  allanite  contains  only  one  or  two  per  cent. 

Allanite  is  an  epidote  in  which  some  of  the  iron  is  replaced  by  the 
rare  elements,  cerium,  lanthanum,  didymium,  yttrium,  or  erbium  ; 
the  amount  of  each  varies  with  the  locality ;  the  total  of  them  all 
is  about  20  per  cent. 

Allanite  occurs  as  an  accessory  mineral  in  igneous  rocks,  more 
often  in  those  rich  in  silica,  as  granites  and  pegmatites;  also  in 
schists  and  crystalline  limestones. 


SILICATES,   TITANATES,   ETC.  469 

It  is  found  at  many  localities  along  the  Atlantic  slope,  as  South 
Mountain,  Pennsylvania ;  Edenville,  New  York ;  Haddam,  Con- 
necticut ;  Franklin,  New  Jersey ;  Amelia  Court  House,  Virginia ; 
Bethany  Church,  Iredell  County,  North  Carolina. 

AXINITE 

Axinite.  —  A  borosilicate  of  calcium  and  aluminium,  HCaa- 
Al2B(Si04)4;  Composition  variable;  Triclinic;  Type,  Cent'ro- 
symmetric;  a  :  b  :  c  =  .4921 :  1 :  .4797  ;  a  =  82°  54';  p  =  91°52'; 
V  =  131°  32' ;  010A100  =  48°  21/ ;  100A001  =  93°  49' ;  010A001  = 
97°  50';  110A100  =  15°  34';  110A100  =  28°  53';  Common  form, 
a  (100),  b  (010),  c  (001),  m  (110),  M  (FlO),  r  (111),  x  (111),  e  (111) ; 
Cleavage  010  distinct,  Brittle ;  Fracture,  conchoidal ;  H.  =  6.5- 
7;  G.  =  3.27  -  3.29;  Color,  brown,  blue, gray,  or  yellow;  Streak, 
white  or  pale ;  Transparent  to  translucent ;  a  =  1.685 ;  p  =  1.692 ; 
•y  =  1.695 ;  -y  —  a  =  .010 ;  Optically  (  —  ) ;  Bxa  nearly  perpendicu- 
lar to  111;  2V  =  71°  38'. 

B.B.  —  Fuses  easily  with  intumescence,  yielding  a  green  flame 
(boron).  Gelatinizes  with  HC1  after  fusion,  but  insoluble  before. 
May  yield  an  iron  or  manganese  reaction  with  the  fluxes. 


FIG.  495.  —  Axinite  from  Dauphine,  France. 

General  description.  —  Crystals  flattened,   with  the  forms    r, 
M,  and  m  prominent,  with  edges  sharp  like  an  axe,  hence  the  name 


470  MINERALOGY 

axinite.  Striations  on  the  prism  zone  parallel  to  the  vertical  axis 
are  characteristic.  The  color  is  usually  clove-brown,  but  varies 
with  the  replacement  of  calcium  with  iron  or  manganese. 

Like  most  other  borates,  axinite  is  formed  by  pneumatolytic 
action,  and  therefore  usually  appears  in  the  cracks  and  veins  of 
granites  and  diabases,  or  in  metamorphic  contact  zones. 

Fine  crystals  implanted  on  the  walls  of  veins  in  diabase  occur 
at  Bourg  d'Oisans,  Dauphine',  France;  at  St.  Gothard,  Switzer- 
land. In  the  United  States  it  occurs  a  Franklin,  New  Jersey,  in 
yellow  crystals  associated  with  garnets  and  rhodonite ;  at  Bethle- 
hem, Pennsylvania,  and  at  Phippsburg,  Maine. 

PREHNITE 

Prehnite.  —  An  orthosilicate  of  calcium  and  aluminium ;  H2Ca2- 
Al2(SiO4)3;  CaO  =  27.1,  A12O3  =  24.8,  SiO2  =  43.7,^  H2O  = 
4.4 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c  = 
.8401 :  1 :  .5549 ;  100A110  =  40°  2';  001 A 101  =  33°  27';  001A 
Oil  =  29°  2';  Common  forms,  c  (001),  a  (100),  b  (010),  m  (110), 
o  (061) ;  Cleavage,  basal  distinct ;  Brittle ;  Fracture,  uneven ; 
H.  =  6-6.5 ;  G.  =  2.8-2.95 ;  Color,  light  green,  oil-green,  yellow, 
white;  Streak,  white;  Luster,  vitreous;  Nearly  transparent  to 
translucent;  a  =  1.616;  p  =  1.626;  -y  =  1.649;  -y  -  a  =  .033; 
Optically  (  +  ) ;  Axial  plane  =  010;  Bxa  =  c ;  2  V  =  69°  22'. 

B.B.  —  Fuses  easily  with  intumescence  to  a  blebby  glass  and 
gelatinizes  after  fusion  with  HC1.  After  the  separation  of  silica 

, ,     and  aluminium  with 

ammonia,  yields  a 
heavy  white  pre- 
cipitate with  am- 
monium carbonate 
(calcium) .  Yields 
water  in  the  closed 
tube. 

General  descrip- 
tion.— Crystals  are 
small  and  rarely 
simple,  but  in  par- 

FIG.  496.  — Prehnite.    Bergen  Hill,  New  Jersey.  „.,  ... 

allel  position  or 
joined  in  ridged  groups  with  a  rough  surface  on  which  the  indi- 
vidual crystals  may  be  seen  to  be  joined  by  the  base,  with  the 


SILICATES,  TITANATES,  ETC.  471 

prism  angle  free  on  the  surface.  Often  globular  or  botryoidal 
with  very  small  crystal  faces.  The  nodules  when  broken  show  a 
radiated  structure.  The  color  is  nearly  always  light  green  or 
yellowish,  the  color  fading  on  exposure. 

Prehnite  is  a  secondary  mineral  formed  from  solution  in  the  cavi- 
ties and  veins  of  the  basic  igneous  rocks,  where  it  is  associated  with 
the  zeolites,  datolite,  calcite,  and  quartz.  It  is  a  common  mineral 
in  the  cracks  of  the  traps  of  Massachusetts,  Connecticut,  and  New 
Jersey ;  also  in  the  Lake  Superior  copper  regions. 

It  forms  pseudomorphs  after  analcite,  natrolite,  and  the  plagio- 
clases,  and  decomposes,  forming  chlorite.  •  ? 

When  fused  it  breaks  down  and  on  cooling  yields  wollastonite. 

CHONDRODITE 

Chondrodite.  —  [Mg^.  OH)]2Mg3(Si04)2 ;  Monoclinic ;  Type, 
Digonal  Equatorial ;  a  :  b  :  c  =  1.0863  :  1 :  3.1447 ;  p  =  90°  =  100A 
001;  100A110  =  47°  22';-  001A101  =  70°  57';  001,011  =  72°  22'; 
Common  forms,  c(001),  b  (010),  r2  (125),  r3  (123) ;  Twinning 
plane,  105;  Cleavage,  basal;  Brittle;  Fracture,  conchoidal; 
H.  =  6-6.5 ;  G.  =  3.1-3.2 ;  Color,  shades  of  yellow,  brown,  and  red ; 
Streak,  white  or  pale  yellow;  Luster,  vitreous;  Transparent  to 
translucent;  a  =  1.607;  p  =  1.619;  y  =  1.639;  <y  -  a  =  .032; 
Optically  (  +  )  axial  plane  perpendicular  to  010 ;  Bx0  A  c  =  25°  30; ; 
2V  =  79°  40'. 

B.B.  —  Infusible,  but  whitens.  Gelatinizes  with  HC1.  Yields 
a  fluorine  reaction  in  the  closed  tube.  Often  reacts  for  iron  with 
the  fluxes. 

General  description.  —  Crystals  are  equidimensional ;  at  times 
flattened  parallel  to  010,  with  striations  on  the  orthodome  parallel 
to  the  orthoaxis ;  crystals  are  complicated  and  very  rich  in  forms ; 
also  granular  and  massive. 

Chondrodite  is  a  member  of  the  humite  group;  the  other 
members  are  humite,  [Mg(F .  OH)]2Mg6(Si04)2 ;  clinohumite 
[Mg(F.OH)]2  Mg7(Si04)4. 

In  composition  they  differ  from  each  other  by  MgSiO4.  Humite 
is  orthorhombic,  while  the  others  are  monoclinic.  They  agree  very 
closely  in  their  occurrences,  associations,  and  physical  properties. 
They  occur  in  metamorphic  limestones  containing  considerable 
magnesia.  All  three  are  found  in  the  ejected  blocks  of  limestone 
on  Monte  Somma,  Vesuvius,  and  at  the  Tilly  Foster  mine,  Brew- 


472  MINERALOGY 

ster,  New  York,  associated  with  serpentine.  Chondrodite  occurs 
at  Sparta,  New  Jersey,  and  at  Amity  and  Warwick,  New  York, 
in  a  limestone  associated  with  spinel. 

By  hydration  they  alter  to  serpentine  and  sometimes  to  brucite. 

ILVAITE 

Ilvaite.  —  CaFe2(FeOH)  (SiO4)2 ;  CaO  =  13.7 ;  FeO  =  35.2 ; 
Fe203  =  19.6 ;  SiO2  =  29.3_;  H2O  =  2.2 ;  Orthorhombic ;  Type, 
Didigonal  Equatorial ;  a  :  b  :  c  =  .6665  :  1 :  .4427  :  100  A 110  =  33° 
41';  001A101  =33°  35';  001A011  =23°  53';  Common  forms, 
b  (010),  m  (110),  s  (120),  o  (111);  Cleavage,  b  and  c  distance  a 
and  m  less  so ;  Brittle ;  Fracture,  uneven ;  H.  =  5.5-6 ;  G.  =  3.99- 
4.05  ;  Color,  iron-black ;  Streak,  black  to  brownish ;  Luster,  sub- 
metallic;  Opaque. 

B.B. — Fuses  easily  with  some  intumescence  to  a  slag  which  ij 
magnetic ;  gelatinizes  with  HC1.  Manganese  may  replace  some 
of  the  iron,  when  it  will  yield  reactions  for  that  metal. 

General  description.  —  Crystals  are  prismatic  in  habit,  with 
striations  on  the  prism  zone  parallel  to  the  vertical  axis.  It  is 
not  a  common  mineral ;  it  occurs  on  the  isle  of  Elba  in  a  dolomite, 
•and  takes  its  name  from  the  Latin  term  for  the  island.  It  has  also 
been  reported  as  from  Somerville,  Massachusetts,  and  Cumberland, 
Rhode  Island. 

The  crystals  are  not  unusually  coated  with  a  yellow  or  brown 
oxide  of  iron,  which  is  the  result  of  oxidation. 

CALAMINE 

Calamine.  —  Hemimorphite ;  Hydrous  orthosilicate  of  zinc ; 
Zn2Si04.H20;  ZnO  =  67.5;  SiO2  =  25.0 ;  _H2O  =  7.5  ;  Ortho- 
rhombic  ;  Type,  Didigonal  Polar ;  a  :  b  :  c  =  .7834  : 1 :  .4778  ; 
100A110  =  38°  4';  001A101  =  31°  23';  001A011  =25°  32';  Com- 
mon forms,  b(010),  c  (001)*,  m(110),  i  (031),  v(121),  t  (301) i; 
Twinning  plane  001,  supplementary  twins;  Cleavage,  prismatic 
perfect ;  Brittle ;  Fracture,  uneven ;  H.  =  4.5-5  ;  G.  =  3.40-3.50 ; 
Color,  white,  brown,  or  yellow;  Streak,  white;  Luster,  vitreous 
to  adamantine;  Transparent  to  translucent;  Pyroelectric ; 
a  =  1.613;  p  =  1.617;  y  =  1.636;  y  -  a  =  .023 ;  Optically 
(  H-  ) ;  Axial  plane  100;  Bxa  =  c  ;  2E  =  78°  39';  2V  =  46°  9'. 


SILICATES,   TITANATES,   ETC. 


473 


B.B.  —  Fuses  with  difficulty.  With  soda  and  borax  in  R.  F.  on 
coal  yields  a  zinc  oxide  coat.  Gelatinizes  with  HC1.  In  the  closed 
tube  yields  water. 

General  description.  —  Crystals  are  tabular  in  habit  or  prismatic, 
flattened  parallel  to  010,  with  striations  lengthwise.  Polar  in  de- 
velopment, with  one  end  terminated  by  the  brachydome  zone, 
while  the  other  is  termi- 
nated by  pyramids.  They 
are  almost  always  im- 
planted on  the  pyramid 
termination,  with  the 
domes  free,  and  joined  by 
the  large  face  010  in  paral- 
lel positions,  forming  ridges 
and  crystalline  crusts,  with 
a  drusy  surface ;  or  in 
nodules  which  show  a  ra- 
diated structure  when 
broken.  Simple,  free  crys- 


FIG.  497.  —  Calamine  from  Stirling  Hill,  New 
Jersey. 


tals  are  rare ;  they  occur  at 
Altenberg,  in  Saxony. 

It  also  occurs  as  crusts,  stalactitic,  mamillary,  granular,  or 
earthy  and  amorphous ;  the  amorphous  variety  is  softer  than  the 
crystalline.  Calamine  is  a  secondary  mineral  deposited  from 
solution  at  low  temperatures.  The  percolating  ground  waters 
carry  zinc  silicate,  the  zinc  being  derived  from  the  oxidation  of 
sphalerite  which  unites  with  the  silica  in  solution,  forming  cala- 
mine. 

Calamine  is  characteristic  of  the  zone  of  oxidation  and  is  asso- 
ciated with  the  superficial  area  of  most  zinc  deposits.  Beautiful 
specimens  were  formerly  obtained  at  Stirling  Hill  and  Franklin, 
New  Jersey.  It  also  occurs  at  Friedensville,  Pennsylvania; 
Granby,  Missouri ;  Virginia ;  Colorado ;  and  Utah. 

TOURMALINE 

Tourmaline.  —  A  borosilicate  of  aluminium,  the  alkalies  and 
alkali  earth  metals,  of  the  general  formula  RgAlsCB  .  OH)2Si4Oi9, 
in  which  R  may  be  Li,  K,  Na,  H,  Ca,  Fe,  or  Mg  and  Al  may  be 
replaced  by  Fe  or  Cr,  and  OH  by  fluorine.  The  composition  is 
therefore  variable,  but  always  contains  B203  about  10  per  cent., 


474  MINERALOGY 

A1203  from  25  to  40,  and  Si02,  35  to  40  per  cent.;  Hexagonal ;  Type, 
Ditrigonal  Polarj  c  =  .4477;  0001  A  1011  =_27°  20';  101 1A  1011 
=  46°  52';  0112A01_12  =  25°  2' ;  0221^0221  =  7_7°  ;  Common 
forms,  c  (0001),  m  (1010),  a  (1120),  r  (1011),  o  (0221),  e  (0112) ; 
Other  forms  numerous;  Twinning  plane  0001,  supplementary 
twins,  other  twins  rare;  Cleavage,  a  and  r  difficult;  Brittle; 
Fracture,  uneven ;  H.  =  7-7.6 ;  G.  =  2.98-3.20 ;  Color,  commonly 
dark  brown  to  black,  but  all  shades  of  green,  blue,  red,  to  white ; 
Streak,  white  or  gray;  Transparent  to  opaque;  co  =  1.640; 
€  =  1.622;  co  -  €  =  .018;  Optically  (  -  ). 

B.B.  —  Generally  fuses  to  a  slag  or  glass,  with  a  change  in  color  ; 
but  some  varieties  fuse  with  difficulty,  or  are  infusible.  With 
Turner's  flux  yields  a  boric  acid  flame  (green).  Not  attacked  by 
acids. 

General  description.  —  Crystals  are  either  long  and  slender, 
parallel  to  the  vertical  axis,  or  short  and  stout,  with  striations  on  the 
prism  zone  lengthwise ;  combinations  of  the  hexagonal  prism  of  the 
second  order  and  the  trigonal  prism  of  the  first  order.  Oscilla- 


FIG.  498.  —  Tourmaline  from  Mesa  Grande,  California.    The  Smaller  Specimen  is 
from  Acworth,  New  Hampshire. 

tions  in  growth  between  these  two  forms  produce  the  striations 
so  characteristic.     They  may  be  deep  furrows  or  ridged,  in  which 
the  cross  section  becomes  only  approximately  trigonal ;  such  crys- 
tals are  well  represented  by  those  from  Mesa  Grande,  California. 
In  the  black  variety,  schorl,  the  polar  character  of  the  mineral  is 


SILICATES,  TITANATES,   ETC.  475 

well  illustrated  by  the  double  terminated  crystals  from  Pierrepont, 
New  York,  where  they  occur  in  a  limestone.  They  are  short,  stout 
prisms,  combinations  of  a  and  m,  and  less  commonly  the  ditrigonal 
prism  h  (4150)  in  the  prism  zone,  terminated  at  one  end  by  the 
pyramids  r  and  e,  which  are  flat.  At  the  other  end  they  are  termi- 
nated by  the  much  steeper  pyramid  o  and  a  small  basal  plane; 
the  two  ends  are  quite  different  in  appearance. 

Owing  to  the  great  variation  in  composition  possible,  tourmaline 
differs  widely  in  color.  The  transparent  pale  pink,  blue,  green, 
and  colorless  specimens  are  rich  in  alkalies.  Their  color  is  often 
unevenly  distributed,  as  different  parts  of  the  same  specimen  will 
differ  in  color ;  this  distribution  of  color  may  be  from  end  to  end, 
one  end  being  colorless,  green,  or  pink,  while  the  other  may  be 
blue;  or  the  distribution  may  be  concentric  around  the  vertical 
axis,  as  in  the  Brazilian  specimens,  in  many  of  which  the  central 
axis  is  pink,  then  a  colorless  area,  while  the  outside  is  green.  Beau- 
tifully colored  and  transparent  tourmalines  are  obtained  at 
Haddam,  Connecticut ;  Paris,  Maine ;  Mesa  Grande,  California ; 
Madagascar ;  and  Brazil.  When  transparent  and  flawless,  they 
are  cut  and  polished  as  gems.  The  pink  varieties  are  known  as 
rubellite,  the  green  as  Brazilian  emeralds,  the  yellow  as  Ceylon 
peridote,  the  blue  as  indicolite,  and  the  white  as  achroite. 

The  black  varieties  are  rich  in  iron ;  that  from  Pierrepont  con- 
tains 9.08  per  cent,  of  FeO.  The  brown  varieties  are  high  in  mag- 
nesium ;  that  of  Gouverneur,  New  York,  contains  14.9  per  cent,  of 
MgO. 

Tourmaline  is  very  strongly  pleochroic  ;  even  the  gems  cut  from 
the  transparent  specimens  are  often  of  different  color,  according  to 
the  direction  in  which  the  light  passes  through  the  crystal.  This 
absorption  is  more  strongly  marked  in  the  darker  varieties  and  is 
developed  to  such  an  extent  in  the  dark  brown  specimens  that  in  a 
section  parallel  to  the  vertical  axis  the  ordinary  ray  is  entirely 
absorbed.  Such  sections  are  used,  as  in  the  tourmaline  tongs,  to 
replace  the  nicols  in  viewing  interference  figures  of  mineral  sections 
placed  between  them. 

Tourmaline  is  also  the  best  example  of  the  pyroelectric  property 
in  minerals.  When  a  crystal  of  tourmaline  is  cooling  after  being 
heated,  one  end,  the  analogous  end  (the  sharp  end),  is  usually  nega- 
tively charged,  while  the  antilogous  end  (the  blunt  end)  is  positively 
charged.  The  reverse  of  this  is  true  when  the  temperature  is 
rising.  If  a  crystal  of  tourmaline  after  being  heated  is  dusted 


476 


MINERALOGY 


FIG.  499.  —  Tourmaline.  Jefferson 
County,  New  York. 


with  a  mixture  of  flowers  of  sulphur  and  powdered  red'  lead,  the 
positively  charged  end  of  the  crystal  becomes  yellow  as  it  attracts 

'the  sulphur,  and  the  negatively 
charged  end  becomes  red  as  it  at- 
tracts the  red  lead. 

In  rock  sections  it  appears  elon- 
gated, branched,  and  needle-like ; 
when  cut  perpendicular  to  the  verti- 
cal axis,  roughly  triangular.  In 
color,  white  and  pale  shades,  to 
brown  and  green,  especially  in  rock- 
forming  tourmalines.  Relief  is  well 
marked  and  absorption  very  strong, 
particularly  in  the  dark  varieties. 
Interference  colors,  low  second  or- 
der ;  the  basal  section  shows  a  dark 
cross  only,  in  thin  sections.  Opti- 
cally (  -  ). 

The  black  tourmaline  occurs  as  a  primary  component  in  some 
granites ;  other  tourmalines  are  mostly  formed  by  fumarole  action, 
caused  by  the  contact  of  igneous  intrusions  with  limestones  or 
sedimentary  rocks.  Boron  is  the  commonest  of  the  pneumatolytic 
agents.  Tourmaline  is  therefore  a  common  mineral  in  pegmatites, 
crystalline  schists,  and  granular  limestones,  and  is  usually  associated 
with  cassiterite,  topaz,  fluorite,  beryl,  lepidolite,  the  various  micas, 
and  quartz. 

In  the  United  States  the  noted  localities  for  varicolored  specimens 
are  Haddam,  Connecticut;  Paris,  Maine;  Mesa  Grande,  Cali- 
fornia, where  it  is  found  in  pegmatites  associated  with  lepidolite, 
spodumene,  and  beryl. 

The  brown  variety  occurs  at  Gouverneur,  New  York ;  Franklin, 
New  Jersey;  Unionville,  Pennsylvania,  in  limestones  associated 
with  scapolite,  spinels,  or  tremolite.  Also  in  many  of  the  Maine 
pegmatites;  near  San  Diego,  California,  in  large  black  crystals 
six  inches  across,  in  a  feldspar. 

Tourmaline  alters  to  chlorite,  muscovite,  or  biotite,  with  which 
minerals  it  is  generally  associated. 

Synthetically  tourmaline  has  never  been  produced  in  the  labora- 
tory. .  In  nature  it  must  be  formed  at  rather  a  low  temperature,  or 
under  pressure,  as  upon  fusion  it  breaks  down,  forming  minerals 
with  less  complex  molecules,  as  olivine  and  spinel. 


SILICATES,  TITANATES,   ETC.  477 

STAUROLITE 

Staurolite.  —  HFeAl5Si2Oi3 ;  Composition  variable ;  Ortho- 
rhombic  ;  Type,  Didigonal  Equatorial ;  a  :  b:  c  =  .4734  :  1 :  .6828  ; 
100  A 110  =  25°  20' ;  001  A  101  =  55°  16';  001  A  Oil  =  34°  19';  Com- 
mon forms,  c(001),  b  (010),  m(110),  r  (Oil) ;  Twinning  plane, 
x(032),  also  z  (232),  both  common;  y  (130) ;  rare;  Cleavage,  b 
distinct,  m  in  traces ;  Brittle ;  Fracture,  uneven ;  H.  =  7-7.5  ; 
G.  =  3.65-3.75 ;  Color,  shades  of  dark  brown  to  nearly  black  ; 
Streak,  white  to  yellowish ;  Luster,  vitreous  to  resinous ;  Trans- 
lucent to  opaque;  a  =  1.736;  p  =  1.741;  y  =  1.746;  Optically 
(  +  ) ;  -y  -  a  =  .010 ;  Axial  plane  =  100 ;  Bxa  =  c ;  2  V  =  88° 
46'. 

B.B.  —  Infusible ;  when  containing  much  manganese  fuses  at  4. 
With  borax  and  S.  Ph.  yields  an  iron  reaction.  Insoluble  in  acids. 

General  description.  —  In  crystalline  habit,  short  prismatic, 
combinations  of  the  unit  prism,  macrodome,  and  the  basal  and 
brachypinacoids.  The  surface  is  often  dull  from  alterations. 


FIG.  500.  —  Staurolite.    Windham,  Maine. 

Interpenetrating  twins  in  which  the  composition  face  is  032,  and 
the  vertical  axes  of  the  two  individuals  are  at  right  angles,  are 
characteristic,  and  it  is  from  this  twinning  that  the  name  staurolite, 
"  cross-shaped,"  is  derived,  Fig.  297.  Twins  in  which  the  vertical 


478  MINERALOGY 

axes  are  at  60°,  with  232  as  the  composition  face,  are  also 
common. 

In  rock  sections  staurolite  is  pale  yellow  to  reddish  brown,  usually 
in  crystalline  outline,  containing  numerous  inclusions,  often  of 
carbonaceous  matter,  at  times  symmetrically  arranged.  Relief 
is  high,  interference  colors  gray  to  yellow  of  the  first  order.  The 
interference  figure  is  in  the  basal  section.  The  optic  axes  lie  with- 
out the  field  of  view. 

It  is  a  product  of  contact  metamorphism,  occurring  in  argilla- 
ceous shales  and  schists,  associated  with  cyanite,  sillimanite,  and 
garnets. 

Beautiful  specimens  occur  at  Pizzo  Formo,  near  St.  Gothard, 
Switzerland,  where  it  is  found  in  a  paragonite  schist  associated  with 
cyanite,  often  in  parallel  growths. 

At  Windham,  Maine,  crystals  nearly  two  inches  in  length  are 
embedded  in  a  mica  slate  associated  with  cyanite  and  small  garnets. 
It  occurs  also  at  Franconia,  New  Hampshire ;  at  Sheffield,  Massa- 
chusetts, at  many  points  in  Connecticut,  Virginia,  North  Carolina, 
and  Georgia.  In  Lincoln  and  Fannin  counties,  Georgia,  many  sim- 
ple and  twinned  crystals  lie  loose  in  the  soil  or  in  the  decomposed 
schist.  It  is  not  easily  altered  by  weathering,  but  at  times  forms 
muscovite,  or  a  steatite-like  substance  mixed  with  quartz. 

The  synthesis  of  staurolite,  like  many  other  contact  and  pneu- 
matolytic  minerals,  has  not  as  yet  been  accomplished  with  cer- 
tainty. 

ZEOLITES 

The  zeolites  are  a  group  of  hydrated  silicates  of  aluminium  with 
either  calcium  and  the  alkalies,  or  both.  Calcium  may  be  replaced 
by  barium  or  strontium,  but  there  is  little  or  no  magnesium.  In 
their  occurrence,  they  are  not  found  as  primary  minerals,  but  they 
are  all  secondary  products  derived  by  the  hydration  of  such  primary 
minerals  as  the  feldspars,  nepheline,  or  leucite.  They  may  be 
found  in  the  mass  of  the  rock  containing  the  minerals  of  which 
they  are  hydration  products,  or  they  may  be  found  filling  the  pores, 
cracks,  and  other  cavities  in  which  they  have  been  precipitated  or 
crystallized  from  solution. 

They  are  common  minerals  associated  with  such  rocks  as  dia- 
base, diorite,  syenites,  and  those  containing  leucite,  nepheline,  or 
sodalite.  They  are  also  less  commonly  found  connected  with  the 
metamorphosed,  or  even  the  semimetamorphosed,  sedimentary 


GO 


SILICATES,  TITANATES,  ETC. 

_  O  o  O  _ 

lO  CO 


479 


O 


T-J  O     CO     lO  tO 

^n  id   co   cs  co 


CO  00 
<M  00  LO 

10  <M  co 


r-     0? 


00 


t^  -^        Oi        O 
O  00  C5  <N  T-H  <M  00 

10  co  T-H  oi  I-H  oi  10 

(M 


pq       ^       _<  So       ® 

|>I  CO  |>I  CO  i-H  T-H   O  TJH 


co  cq 
-^  06 


O  b-   iO   O 

OS  T-H  00  <N  00  O5 


OCOCOI^T-HOO5COCO(MCOCOT—  (CO 


COCOI^T-HO 
T-Hr-i(M(M(M 


iO 

CO 


O 
^  <M 


T^  TtH  lO 


tOT-H 
CO  lO 


<N  00 

cd  CO 

r^  CO  ^H  ^  CO  CO 


1 


.- 


480  MINERALOGY 

rocks.  Their  formation  takes  place  at  comparatively  low  tempera- 
tures and  some  even  at  temperatures  near  zero,  as  phillipsite  has 
been  found  in  the  volcanic  mud  at  the  bottom  of  the  Pacific  Ocean. 

Zeolites  are  formed  synthetically  by  heating  their  constituents  in 
the  correct  proportions,  in  each  case,  in  a  sealed  tube  with  water, 
to  temperatures  between  170°  and  250°.  In  a  similar  way  they  are 
often  separated  from  the  percolating  waters  of  many  hot  springs. 
Their  water  content  is  held  with  different  degrees  of  firmness; 
some  even  lose  their  water  to  dry  air  and  reabsorb  it  without  any 
material  physical  change.  This  water  is  therefore  said  to  take  no 
part  in  the  formation  of  the  crystalline  molecule.  Zeolites  which 
have  had  their  water  driven  out  at  a  temperature  below  redness 
will  not  only  reabsorb  the  water  from  moist  air,  but  will  take  up  in 
the  same  way  alcohol,  ammonia,  silicon  chloride,  and  coloring  matter. 
If  the  zeolite  is  fused,  it  loses  this  property,  as  the  molecule  is  changed 
in  nature,  for  from  fused  natrolite  on  cooling  nepheline  is  formed ; 
from  apophyllite,  hexagonal  calcium  metasilicate ;  from  chabazite, 
anorthite ;  from  heulandite  pyroxene  is  produced. 

The  zeolites  are  so  near  alike  in  their  occurrence  and  appearance 
that  unless  well  crystallized  it  is  difficult  to  distinguish  them,  and 
often  a  quantitative  determination  of  silica  and  water  is  necessary 
for  the  identification  of  species.  They  all  boil  when  heated  in  the 
blowpipe  flame,  from  which  the  group  takes  its  name. 

They  are  usually  associated  with  datolite,  prehnite,  pectolite, 
quartz,  calcite  and  pyrite,  or  chalcopyrite. 

APOPHYLLITE 

Apophyllite.  —  H14K2Ca8(SiO3)i6 .  9  H2O ;  a  hydrated  metasilicate 
of  calcium  and  potassium ;  Tetragonal ;  Type,  Ditetragonal  Equa- 
torial; c  =  1.X2515;  001 A 101  =  51°  22';  001,111=60°  32'; 
Common  forms,  a  (100),  c  (001),  m(110),  p(lll);  Twinning 
plane,  111  rare;  Cleavage,  basal  perfect,  m  less  so;  Brittle;  Frac- 
ture, uneven;  H.  =  4.5-5;  G.  =  2.3-2.4;  white,  gray,  or  pale 
shades  of  yellow,  green,  or  red ;  Streak,  white ;  Transparent  to 
translucent;  Luster,  vitreous,  on  the  base  pearly ;  <o  =  1.534;  €  = 
1.536;  <o  —  €  =  .002;  Optically  (  ±  )  often  showing  anomalies. 

B.B.  —  Exfoliates  and  fuses  to  a  white  blebby  enamel.  Colors 
the  flame  violet  (potassium)  or  shows  potassium  through  the  blue 
glass.  In  the  closed  tube  yields  water.  Decomposed  with  HC1 
without  gelatinization.  Often  shows  fluorine. 


SILICATES,   TITANATES,   ETC. 


481 


General  description.  —  Crystals  are  prismatic  or  tabular,  paral- 
lel to  the  base ;  less  often  are  they  pyramidal.     Usually  combina- 
tions of  the  base,  the  unit  pyramid  of  the  first  order,  and  the  unit 
prism  of  the  second. 
The  prism    face  is 
vertically    striated, 
and  the  base  is  often 
dull. 

Apophyllite  is  a 
common  mineral  in 
the  fissures  of  the 
traps  of  the  New 
England  and  Mid- 
dle states.  Beauti- 
ful specimens  of 
stout  prismatic 
habit  have  been 
taken  from  the  va- 
rious railroad  .cuts 
through  Bergen 
Hill,  New  Jersey, 


FIG.  501.  —  Apophyllite  with  Small  Crystals  of  Albite. 
Paterson,  New  Jersey. 

where  it  is  asso- 
ciated with  other  zeolites,  prehnite,  pectolite,  and  datolite.  It  is 
also  common  in  the  Lake  Superior  copper  region;  at  Peter's 
Point,  Nova  Scotia.  Light  pink  crystals  occur  at  Andreasberg  in 
the  Harz,  which  are  pyramidal  in  habit ;  also  at  Aussig  in  Bo- 
hemia, which  is  a  noted  European  locality  for  zeolites  in  general. 


HEULANDITE 

Heulandite.  —  H4CaAl2(SiO3)6 .  3H20;  Monoclinic;  Type,  Di- 
gonal  Equatorial ;  a  :  b  :  c  =  .4035 ;  1 :  .4293  ;  p  =  88°  34'  = 
001 A 100 ;  100  A 110  =  21°  58' ;  001 A  Oil  =  23°  13' ;  201 A  001  =  63° 
40' ;  001  A  201  =  66° ;  Twinning  plane,  100 ;  Common  forms, 
c(001),  b(010),  m(110),  t(201),  s  (201) ;  Cleavage,  b  perfect; 
Brittle;  Fracture,  subconchoidal ;  H.  =  3.5-4;  G.  =  2.18-2.22; 
Color,  white,  gray,  brown,  or  brick-red;  Streak,  white;  Luster, 
vitreous,  on  cleavage  face  pearly;  Transparent  to  translucent; 
a  =  1.498;  p  =  1.499;  -y  =  1.505;  <y  -  a  =  .007 ;  Optically 
(  +  ) ;  Axial  plane  perpendicular  to  010 ;  Bxa  normal  to  the  ortho- 
axis  ;  2  V  =  0°  to  90°. 
2i 


482 


MINERALOGY 


B.B.  —  Exfoliates  and  curls,  fusing  at  2-2.5  to  a  white  enamel. 
Yields  water  in  the  closed  tube.  Decomposed  with  HC1  without 
gelatinization,  the  solution  freed  from  silica  yields  a  precipitate 
with  ammonia  (Al),  after  filtering,  the  filtrate  yields  a  white  pre- 
cipitate with  ammonia  carbonate  (Ca). 

General  description.  —  Crystals  are  flattened  in  habit,  parallel 
to  b,  and  usually  joined  along  the  same  face  in  parallel  positions 
forming  ridges,  the  top  of  the  ridge  being  the  base  and  the  sides 
the  two  forms  s  and  t,  which  are  vitreous  in  luster,  while  the  strong 


FIG.  502.  —  Heulandite  from  the  Faroe  Islands. 

pearly  luster  on  b  is  very  characteristic.  The  face  b  is  often  con- 
cave, while  s  and  t  are  convex  and  striated  parallel  to  their  inter- 
section with  b.  Sometimes  it  occurs  in  globular  shapes  or  granular. 

Chemically  the  calcium  in  part  may  be  replaced  by  sodium,  po- 
tassium, barium,  or  strontium,  when  it  approaches  brewsterite, 
another  very  closely  related  zeolite. 

Heulandite  is  a  very  common  mineral  in  the  trap  of  Bergen  Hill, 
and  in  the  quarries  at  Paterson,  New  Jersey.  Red  specimens  are 
found  in  Nova  Scotia ;  Fassathal,  in  the  Tyrol.  It  also  occurs  in 
Iceland  and  the  Faroe  Islands. 


HARMOTOME 

Harmotome.— H2(K2.Ba)Al2(SiO3)5  5H2O;  Monoclinic;  Type, 
Digonal  Equatorial ;  a  :  b  :  c  =  .7032  :  1 :  1.2310 ;  p  =  55°  10' 
=  001 A 100;  100  A 110  =  29°  59';  001*101  =35°  42';  001,011  = 
45°  18';  Common  forms,  c(001),  b  (010),  a  (100),  m(110); 


SILICATES,  TITANATES,  ETC.  483 

Twinning  plane  the  base,  penetrating  twins  common;  Cleavage, 
b  easy,  c  less  so ;  Brittle;  Fracture,  uneven  ;  H.  =4.5;  G.  =  2.44- 
2.50 ;  Color,  white,  gray,  yellow,  or  brown  ;  Streak,  white ;  Luster, 
vitreous;  Translucent  to  transparent;  a  =  1.503;  y  =  1.508; 
-y  -  a  =  .005  ;  Optically  (  +  ) ;  Bxa  =  b ;  Bx0  A  a  =  60°  in 
front. 

B.B.  —  Whitens,  crumbles,  and  fuses  at  3.5  to  a  white,  transpar- 
ent glass.  Yields  water  in  the  closed  tube.  Very  little  affected 
with  HC1 ;  fused  with  soda  and  freed  of  silica  and  aluminium,  the 
filtrate  yields  a  white  precipitate  with  dilute  sulphuric  acid  (ba- 
rium). 

General  description.  —  Crystals  are  usually  twinned  in  cross- 
shaped,  interpenetrating,  or  contact  twins,  in  which  the  composition 
face  is  the  base,  yielding  a  crystal  orthorhombic  in  appearance,  in 
which  the  unit  prism  m  appears  as  if  a  pyramid  face,  and  the  faces 
b  and  c  as  if  prisms.  This  habit  of  twinning  is  also  characteristic 
of  other  zeolites,  as  phillipsite  and  stilbite. 

The  barium  and  strontium  zeolites  are  rare  and  local  in  their 
occurrence.  In  the  United  States  harmotome  occurs  associated 
with  stilbite  in  places,  in  the  gneiss  of  Manhattan  Island,  New 
York. 

STILBITE 

Stilbite.  —  (Na2 .  Ca) Al2Si6Oi5 .  6H20 ;  Monoclinic ;  Type,  Digonal 
Equatorial ;  a  :  b  :  c  =  .7622  :  1 :  1.1940 ;  p  =  50°  50'  =  001 A 100 ; 
100  A 110  =  30°  35' ;  001 A 101  =  89°  30' ;  001 A  011*=  42°  47' ;  Com- 
mon forms,  c  (001),  b  (010),  m  (110),  e  (Oil) ;  Twinning  plane,  the 
base  penetration  twins ;  Cleavage,  basal  perfect ;  Brittle ;  Fracture, 
uneven;  H.  =  3.5-4;  G.  =  2.1-2.2;  Color,  white,  gray,  yellow, 
brown,  or  red;  Streak,  white;  Luster,  vitreous,  b  pearly; 
Transparent  to  translucent;  a  =  1.494;  p  =  1.498;  -y  =  1.500; 
•y-a  =  .006;  Optically  (-);  Axial  plane,  J_  100;  Bx»Ac  = 
31°  10'  in  front;  2V  =  33°. 

B.B.  —  Exfoliates,  curls,  and  fuses  at  2-2.5  to  a  white  enamel. 
Yields  water  in  the  closed  tube.  Decomposes  with  HC1  without 
gelatinizing. 

General  description.  —  Crystals  flattened  parallel  to  010  and 
joined  by  this  face  in  complex  bundles  of  parallel  or  divergent  ag- 


484  MINERALOGY 

gregates,  often  restricted  in  the  middle  or  sheaf-like,  a  form  very 
characteristic  of  stilbite.  Twinning  also  as  in  harmotome.  When 
the  crystals  are  tabular  and  simple,  the  very  strong  pearly  luster  on 
the  orthopinacoid  has  the  appearance  of  fish  scales. 

Chemically  the  calcium  may  be  replaced  in  part  by  sodium,  and 
the  brown  color  of  some  specimens  is  due  to  the  presence  of  iron. 
Stilbite  is  a  common  mineral  in  the  cracks  of  the  basic  eruptive 


FIG.  503.  — Stilbite  from  Nova  Scotia. 

rocks.  Very  fine  white  specimens  have  been  taken  from  the  rail- 
road cuts  through  Bergen  Hill,  New  Jersey.  It  is  also  found  in  the 
trap  quarries  at  Moore's  Station,  New  Jersey,  where  it  is  associated 
with  natrolite,  prehnite,  calcite,  and  pectolite. 

Cabinet  specimens  are  obtained  at  Partridge  Island,  Nova  Sco- 
tia and  the  Lake  Superior  region. 

CHABAZITE 

Chabazite.  —  (Ca .  Na^)  Al2(Si03)4 .  6  H20  ;  Hexagonal ;  Type,  Di- 
hexagonal  Alternating ;  c  =  1.0860;  0001_A  1011=51°  26';  1011 A 
1011=85°  14';  Common  forms,  r(lOfl),  e(0112),  s (0221) ; 
Twinning  axis  c,  interpenetration  twins ;  Cleavage,  rhombohedral 
distinct;  Brittle;  Fracture,  uneven;  H.  =  4.5;  G.  =  2.08-2.16; 
Color,  white,  yellow,  gray,  flesh-red,  or  pink;  Streak,  white; 


SILICATES,   TITANATES,   ETC.  485 

Luster,  vitreous;    Transparent   to  translucent;    n  =  1.46;    co  —  € 
=  .002 ;  Optically  (±)  depending  upon  the  amount  of  water. 

B.B.  —  Intumesces  and  fuses  easily  to  a  blebby  glass.  Yields 
water  in  the  closed  tube.  Decomposes  with  HC1,  yielding  slimy 
silica. 

General  description.  —  Crystals  are  simple  rhombohedrons, 
nearly  cubical  in  appearance,  but  the  angle  is  85° ;  or  combinations 
of  r  and  e  with  striations  on  both  forms  parallel  to  their  intersec- 
tions. Interpenetrating  twins  in  which  the  vertical  axis  is  the 


FIG.  504.  —  Chabazite.    Swan's  Point,  Nova  Scotia. 

twinning  axis  are  common.  Upon  the  whole  chabazite  has  very 
much  the  appearance  of  fluorite,  also  granular  or  amorphous. 

Chemically  the  calcium  and  sodium  content  is  variable,  and  they 
replace  each  other,  and  at  times  barium,  strontium,  or  iron  may 
enter  the  molecule. 

Chabazite  in  its  occurrences  is  associated  with  the  other  zeolites 
and  under  the  same  conditions.  Good  specimens  are  obtained  at 
Bergen  Hill,  New  Jersey;  at  various  points  in  Nova  Scotia;  as 
well  as  at  the  noted  zeolite  localities  of  Europe,  as  the  Giant's 
Causeway,  Faroe  Islands,  Iceland,  and  Aussig. 

ANALCITE 

Analcite.— NaAl(Si03)2 .  H2O ;  Isometric ;  Type,  Ditesseral  Cen- 
tral ;  Common  forms,  n  (211)  other  forms,  as  a  (100)  and  d  (110), 
rare ;  Cleavage,  cubic  in  traces ;  Brittle ;  Fracture,  subconchoidal ; 


486 


MINERALOGY 


H.  =  5-5.5 ;  G.  =  2.22-2.29 ;  Color,  white  with  various  tints,  espe- 
cially reddish ;  Streak,  white ;  Luster,  vitreous  to  nearly  opaque ; 
n  =  1.487. 

B.B.  —  Fuses  easily  to  a  colorless  glass,  gelatinizes  with  HC1. 
Yields  water  in  the  closed  tube. 

General  description.  —  Crystals  simple  tetragonal  trisoctahe- 
drons;  rarely  is  it  in  combination  with  the  cube  or  the  rhombic 

dodecahedron.  Like  leucite, 
these  individual  crystals 
may  be  complexes,  and  the 
outward  form  is  only  pseudo- 
isometric,  also  massive. 

Analcite  is  formed  in  many 
instances  as  pseudomorphs 
after  leucite,  by  an  inter- 
change oi'_.  alkalies  and  hy- 
dration;  it  is  also  formed 
from  sedate  and  nepheline. 

Analcite  m  oiue  of  the  more 
common  geoftics  and  occurs 
in  rocks  containing  sodalite, 
nepheline,  or  teucite,  as  a 


FIG.   505.  —  Tetragonal-trisoctahedron    of 
Analcite,  from  Fassathal,  Tyrol. 


secondary  mineral,  as  well 
as  in  the  cracks  and  cavities 
of  the  traps,  diorites,  and 

basalts.  It  is  found  at  Bergen  Hill  and  Princeton,  New  Jersey ; 
in  the  Lake  Superior  region;  Golden,  Colorado;  various  points 
in  Nova  Scotia ;  and  in  Europe,  in  the  Fassathal,  Tyrol ;  Aussig, 
Bohemia ;  Andreasberg,  Harz ;  Faroe  Islands  and  Iceland. 

NATROLITE 

Natrolite.  —  Na2(A10)Al(SK)3)3 .  2  H20  ;  Orthorhombic ;  Type, 
Didigonal  Equatorial ;  a  :  b  :  c  =  .9785  : 1 :  .3536 ;  100  A  1 10  = 
44°  23';  001 A 101  =  19°  52';  001 A Oil  =  19°  28';  110 A  111  =  63° 
11';  Common  forms,  m  (110),  o  (111),  b  (010),  c(001);  Cleavage, 
prismatic  perfect,  b  imperfect ;  Brittle ;  Fracture,  uneven ;  H.  = 
5-5.5 ;  G.  =  2.20-2.25 ;  Color,  white,  gray,  yellow,  or  reddish ; 
Streak,  white ;  Luster,  vitreous ;  Transparent  to  translucent  ; 
a  =  1.475;  p  =  1.478;  -y  =  1.488;  <y  -  a  =  .013;  Optically  (  +  ) 
Axial  plane  =  010 ;  Bxa  =  c  ;  2  V  =  61°  56'. 


SILICATES,   TITANATES,   ETC.  487 

B.B.  —  Fuses  at  2  to  a  colorless  glass.  Yields  water  in  the 
closed  tube.  Gelatinizes  with  HC1 ;  the  solution  freed  of  silica  and 
aluminium  yields  little  or  no  precipitate  with  ammonium  carbonate 
(calcium). 

General  description.  —  Long  prismatic  in  habit,  the  individual 
crystals  are  combinations  of  nearly  a  square  prism  terminated  by 
the  unit  pyramid,  tetragonal  in  appearance ;  except  for  the  occur- 
rence of  the  brachypinacoid,  as  in  case  of  the  specimens  from  San- 


FIG.  506.  —  Natrolite.    Aussig,  Bohemia. 

doen,  Norway.  Often  in  long  acicular  crystals  joined  in  spherical 
or  divergent  masses  with  the  free  end  terminated ;  in  this  habit, 
it  is  very  similar  to  pectolite,  mesolite,  and  thomsonite.  Also 
granular  but  rarely  massive,  as  that  in  which  the  neptunite  and 
benitoite  is  imbedded,  occurring  in  San  Benito  County,  California. 

Natrolite  occurs  filling  amygdaloid  cavities  and  cracks  in  basalt 
and  other  related  igneous  rocks,  as  at  Bergen  Hill ;  Moore's  Sta- 
tion, New  Jersey;  at  various  points  in  Nova  Scotia  and  Magnet 
Cove,  Arkansas, 

THOMSONITE 

Thomsonite.  —  (NaaCa^A^CSiO^  .  5H2O ;  Orthorhombic ;  Type, 
Didigonal  Equatorial ;  a  :  b :  c  =  .9932 : 1 :  1.0066 ;  100  A 110  =  44° 
48';  001,101=44°  37';  001 A  Oil  =45°  11';  001 A 111  =  55°  ; 
001  A  012  =  26°  43';  Common  forms,  a  (100),  m(110),  c  (001), 
d  (401),  p  (111) ;  Cleavage,  b  perfect,  a  less  so ;  Brittle ;  Fracture, 
uneven ;  H.  =  5-5.5 ;  G.  =  2.5-3.4 ;  Color,  white,  reddish  to 
brown;  Streak,  white;  Transparent  to  translucent;  a  =  1.497; 
p  -  1.503 ;  -y  =  1.525 ;  <y  -  a  =  .028 ;  Optically  (+) ;  Axial 
plane  =  001 ;  Bxa  =  b ;  2  V  =  53°  50'. 


488  MINERALOGY 

B.B.  —  Intumesces  and  fuses  at  2  to  a  white  enamel ;  yields 
water  in  the  closed  tube.  Gelatinizes  with  HC1 ;  the  solution  freed 
of  silica  and  aluminium  yields  a  white  precipitate  with  ammonium 
carbonate  (calcium). 

General  description.  —  Simple  individual  crystals  are  rare, 
usually  elongated  and  joined  with  their  vertical  axis  parallel  or  in 
radiated  and  divergent  groups,  resembling  pectolite  and  natrolite. 
The  free  ends  of  the  long  acicular  crystals  are  usually  terminated 
by  the  base  and  not  by  the  pyramid,  as  in  natrolite.  It  also  occurs 
nodular,  concretionary,  or  massive ;  comptonite  is  a  massive  variety 
filling  amygdaloid  cavities  of  the  lavas  on  Monte  Somma,  Vesu- 
vius. A  radiated  variety  found  filling  cavities  in  a  rock  at  Grand 
Marais,  Lake  Superior,  is  known  as  lintonite.  It  is  often  polished 
and  used  for  buttons  and  small  ornaments,  showing  concentric 
rings  of  green,  red,  and  white.  Much  of  this  material  is  gathered 
on  the  beach  of  Lake  Superior,  as  water- worn  pebbles. 

Thomsonite  occurs  associated  with  other  zeolites  at  the  various 
points  in  Nova  Scotia ;  at  Kaden,  Bohemia ;  Kilpatrick,  Scotland ; 
Schneeberg,  Saxony.  It  is  rare  at  the  zeolite  localities  in  New  Jer- 
sey, but  recently  identified  at  Paterson.  It  is  associated  with  other 
zeolites  at  Magnet  Cove,  Arkansas,  and  Table  Mountain,  near 
Golden,  Colorado. 

MICAS 

Muscovite H2KAl3(SiO4)3. 

Paragonite H2NaAl3(Si04)3. 

Biotite HK(Mg .  Fe)2Al2(Si04)3. 

Phlogopite H2KMg3Al(SiO4)3. 

Lepiddomelane A  phlogopite  rich  in  iron. 

Lepidolite H2KAl3(SiO4)3  +  R'3AlF2Si308. 

Zinnwaldite Is  a  lepidolite  with  magnesia  and 

iron. 

The  above  list  includes  the  most  important  species  of  micas,  but 
between  these  are  varieties  representing  substitution  products  and 
mixtures  of  the  two  molecules  of  ortho-  and  tri-silicates,  in  various 
proportions,  as  their  compositions  vary  considerably. 

Chemically  the  micas  are  mostly  orthosilicates,  substitution 
products  derived  from  the  general  formula  Al4(SiO4)3,  in  which  the 
substitution  replaces  one  or  more  of  the  aluminium  atoms.  In  mus- 
covite  one  of  the  Al  atoms  is  replaced  by  H2K,  yielding  the  formula 


SILICATES,  TITANATES,  ETC.  489 

H2KAl3(SiO4)3.  In  paragonite  H2Na  takes  the  place  of  one  Al,  in 
biotite  HK(Mg .  Fe)2  is  substituted  for  two  Al,  and  in  phlogopite 
H2KMg3  takes  the  place  of  three  Al.  In  all  the  species  hydrogen 
appears  as  one  element,  either  as  basic  or  acid,  and  therefore  all 
micas  yield  water  at  a  high  temperature.  Their  formation  in  nature 
must  be  a  hydrothermal  reaction  under  pressure.  Mixed  with  the 
orthosilicate  in  some  species  is  a  trisilicate  (H^SisOg),  and  when  this 
appears  in  sufficient  amounts  the  percentage  of  silica  may  be  that 
of  a  metasilicate,  as  in  lepidolite. 

They  are  all  monoclinic  and  assume  a  pseudohexagonal  symmetry, 
as  the  angle  of  the  unit  prism  is  120°,  which  in  combination  with 
the  pinacoid  yields  a  basal  section  of  hexagonal  outline,  parallel  to 
which  is  the  characteristic  perfect  basal  cleavage  known  as  mica- 
ceous. 

MUSCOVITE 

Muscovite.  —  A  potash  mica;  H2KAl3(SiO4)3 ;  K2O  =  11.8; 
A12O3  =  38.5  ;  Si02  =  45.2 ;  H2O  =  4.5 ;  Monoclinic ;  Type, 
Digonal  Equatorial ;  a  :  b  :  c  =  .5773  ;  1 :  3.3128  ;  0  =  89°  54'  = 
001A100;  100A110  =  30';  001 A 101  =80°  12';  001A011  =  73°  12'; 
001 A  221  =85°  36';  Common  forms,  c  (001),  m  (110),  b  (010) ; 
Twinning  plane,  110;  Cleavage,  basal  micaceous;  Laminae 
flexible,  elastic,  and  tough;  H.  =  2-2.5;  G.  =  2.76-3;  Color,  gray 
and  shades  of  brown,  green,  rarely  red;  Streak,  white;  Luster, 
vitreous;  Transparent  to  translucent;  a  =  1.557;  p  =  1.587; 
y  =  1.590;  <y  -  a  =  .038;  Optically  (  -  );  Axial  plane  J.010; 
Bxa-L  to  the  base ;  2  V  =  64°-72°. 

B.B.  —  Fuses  quietly  at  5.5.  Yields  some  water  in  the  closed 
tube.  Not  decomposed  with  sulphuric  acid. 

General  description.  —  Crystals  are  short,  stout  prisms  or  tabu- 
lar combinations  of  the  prism,  base,  and  brachypinacoid,  pseudo- 
hexagonal  in  section  and  outline.  Cleavage  laminae  yield  per- 
cussion figures  indicating  a  secondary  cleavage  or  a  parting.  If  a 
blunt-pointed  instrument  is  placed  on  a  cleavage  piece  of  muscovite 
and  struck  a  sharp  blow,  a  six-pointed  star  will  be  produced  at  the 
point  of  contact,  formed  by  the  intersection  of  three  straight  cracks 
intersecting  each  other  at  an  angle  of  60°.  The  cracks  are  parallel 
to  the  six  sides  of  the  basal  section,  and  the  crack  parallel  to  the 
brachypinacoid  is  more  developed  or  larger  than  the  other  two,  indi- 
cating the  monoclinic  nature  of  the  mineral ;  for  if  it  were  hexag- 


490 


MINERALOGY 


FIG.  507.  —  Muscovite  Crystals  from  Woodstock, 
Maine. 


onal,  all  three  should  be  interchangeable.  On  the  other  hand,  the 
figure  produced  by  pressure,  or  pressure  figure,  will  have  the  three 
lines  at  right  angles  to  the  sides  of  the  basal  section.  Inclusions, 

often  oxides  of  iron,  in 
micas  are,  in  numerous 
instances,  symmetrically 
arranged  along  lines  in- 
tersecting at  60°,  thus 
forming  equilateral  tri- 
angles, the  sides  of  which 
are  parallel  to  the  rays 
of  the  above  figures,— 
more  often  to  the  rays 
of  the  pressure  figure. 
In  some  cases  both  sets 
may  be  present. 

The  chemical  compo- 
sition of  muscovite 
varies  greatly  from  the 

formula  given  above,  as  some  of  the  potassium  may  be  replaced  by 
sodium,  lithium,  or  caesium,  while  the  hydrogen  is  replaced  by 
fluorine,  and  all  micas  contain  small  amounts  of  fluorine.  Alu- 
minium is  at  times  replaced  by  chromium,  as  in  the  green  chrome 
mica,  fuchsite,  of  Pfitsch,  Tyrol ;  or  again  by  vanadium,  as  in  ros- 
ccelite,  a  vanadium  mica  occurring  in  some  of  the  gold-bearing 
veins  of  California.  Ferric  iron  may  also  take  the  place  of  alumin- 
ium, yielding  varieties  of  dark  color. 

Both  composition  and  physical  texture  are  taken  into  account 
in  separating  the  micas  into  species  and  varieties,  which  in  some 
cases  may  be  only  artificial  distinctions,  as  it  is  possible  that  they, 
like  the  plagioclase  feldspars,  are  isomorphous  mixtures  of  various 
salts  which  have  not  been  definitely  determined ;  if  so,  they  should 
in  their  composition  grade  into  each  other  with  no  sharp  line  of 
demarcation. 

In  rock  sections  muscovite  is  of  an  irregular  outline.  When  cut 
parallel  to  the  vertical  axis,  the  sections  show  fine  parallel  cleavage 
cracks,  very  close  together ;  the  relief  in  this  section  is  high,  and  the 
interference  colors  are  of  the  second  or  third  order.  When  the 
section  is  parallel  to  the  base  or  cleavage,  there  are  no  cracks  or 
relief,  and  the  interference  color  is  a  gray  of  the  first  order,  and  as 
the  cleavage  is  only  slightly  inclined  to  the  acute  bisectrix,  the  inter- 


SILICATES,   TITANATES,   ETC.  491 

ference  figure  is  symmetrical  in  the  field  showing  well-defined  dark 
shadows,  but  in  thin  sections  no  color  bands.  Pleochroism  is  not 
marked,  but  absorption  takes  place  in  those  sections  containing 
cleavage  cracks  and  is  the  strongest  in  sections  parallel  to  the  verti- 
cal axis  and  parallel  to  the  cleavage  cracks,  which  is  the  reverse  of 
that  found  in  tourmaline,  where  absorption  is  the  strongest  parallel 
to  the  vertical  axis. 

Muscovite  is  a  common  primary  mineral  of  granites,  syenites,  and 
pegmatites,  and  usually  separates  from  the  magma  directly  after 
zircon,  apatite,  and  magnetite  and  before  the  feldspars.  It  is 
particularly  characteristic  of  pegmatites,  in  which  the  sheets  may 
be  of  enormous  size,  as  in  the  Black  Hills,  where  sheets  a  yard  across 
have  been  found. 

It  is  also  widely  distributed  in  the  schists  and  gneisses,  where  the 
scales  are  parallel  in  position  and,  in  the  variety  sericite,  very  small 
and  silky  in  appearance.  As  a  secondary  mineral  it  is  a  product 
derived  from  the  alteration  of  numerous  aluminium  silicates,  as  the 
feldspars,  andalusite,  cyanite,  scapolite,  nepheline,  and  also  from 
corundum.  Muscovite  itself  is  very  stable  under  the  conditions 
of  weathering,  persisting  in  an  exceptionally  fresh  condition  in  the 
residual  products  of  such  rocks  as  granite  and  syenite,  long  after 
other  minerals  have  succumbed  to  oxidation  or  kaolinization.  Un- 
der the  influence  of  hot  aqueous  solutions  and  pressure,  the  reac- 
tions above  may  be  reversed,  and  muscovite  will  be  transformed  to 
leucite,  nepheline,  or  feldspars.  The  complex  nature  of  micas  is 
demonstrated  on  fusion,  for  then  they  break  down  on  cooling  to 
various  products,  among  which,  in  case  of  muscovite,  are  leucite, 
sillimanite,  glass,  and  water. 

Commercially  muscovite  is  mined  in  the  pegmatites  of  the  Black 
Hills,  South  Dakota ;  along  the  Blue  Ridge  in  North  and  South 
Carolina,  Virginia,  and  Georgia ;  at  Canon  City,  Colorado,  while 
several  other  states  are  producers  of  small  quantities.  The  product 
of  sheet  mica  in  the  United  States  is  about  one  million  pounds  an- 
nually. 

The  micas,  especially  muscovite,  are  used  for  nfeny  purposes, 
when  in  large  sheets,  as  for  stove  doors  and  lamp  chimneys  and 
shades.  The  value  varies  greatly  with  the  size  of  the  sheets.  Frag- 
mental  mica  is  used  as  an  insulator  in  various  electric  apparatus, 
and  when  finely  ground  it  is  used  as  a  paint  for  frosting,  and,  as  in 
the  case  of  graphite,  as  a  lubricant. 

The  synthetic  production  of  mica  is  uncertain.     Biotite  has  been 


492  MINERALOGY 

produced  by  the  fusion  of  a  mixture  of  KAlSiO4  and  Mg2Si04  with 
fluorides ;  by  a  variation  of  the  components,  phlogopite,  zinnwaldite, 
and  muscovite  have  also  been  produced,  but  none  of  these  arti- 
ficial products  contains  hydrogen,  which  all  natural  micas  do. 
Micas  have  been  found  as  a  crystalline  product  in  some  furnace 
slags. 

BIOTITE 

Biotite.  —  HK(Mg .  Fe)2Al2(SiO4)3 ;  K2O  =  11.21;  (Mg.Fe)O  = 
19.21;  A12O3  =  24.35;  SiO2  =  43.08;  H2O  =  2.14;  Monoclinic; 
Type,  Digonal  Equatorial ;  a  :  b  :  c  =  .5774  :  1 :  3.2743 ;  p  = 
90°  =  001  A  100 ;  100  A 110  =  30°  ;  001  A'l01  =  80°  ;  001 A  Oil  =  73°  1' ; 
001  A  221  =85°  38';  Common  forms,  c  (001),  b  (010),  M  (221), 
r(101),  o(112);  Twinning  plane,  110;  Cleavage,  basal  perfect; 
Laminae,  flexible,  elastic,  and  tough;  H.  =  2.5-3;  G.  =  2.7-3.1; 
Color/ greenish  black  to  black  in  thick  crystals;  Streak,  colorless; 
Luster,  splendent,  transparent  to  translucent;  a  =  1.504;  p  = 
1.589;  Y  =  1.589;  <y  -  a  =  .085;  Optically  (  -  );  Axial  plane 
J_  010  or  I!  010 ;  Bxa  -L  001 ;  2  V  =  0-40°. 

B.B.  —  Whitens  and  fuses,  some  with  difficulty,  while  specimens 
with  much  iron  fuse  easily  and  become  magnetic.  In  the  closed 
tube  yields  water.  Decomposes  with  sulphuric  acid  without  gelat- 
inizing. 

General  description.  —  In  crystalline  habit  twinning  and  cleav- 
age like  muscovite. 

Chemically  biotite  differs  frdm  muscovite  in  that  two  atoms  of 
aluminium  have  been  replaced  by  magnesium  and  iron.  Ferric 
iron  may  replace  the  Al  in  part ;  lepidomelane  is  a  black  mica  rich 
in  both  ferrous  and  ferric  iron. 

In  rock  sections  biotite  is  brown,  greenish,  or  reddish,  rarely  with 
distinct  outline.  Pleochroism  very  marked,  increasing  with  the 
depth  of  color ;  the  absorption  is  the  strongest  with  the  ray  vibrat- 
ing parallel  to  the  cleavage  cracks.  Relief  is  high  in  those  sections 
in  which  the  cleavage  cracks  appear,  and  is  very  low  in  the  basal 
section,  in  which  the  indices  of  refraction  are  but  little  above  that 
of  Canada  balsam.  Interference  colors  in  sections  parallel  to  the 
vertical  axis  are  high,  but  in  the  basal  section  where  the  two  in- 
dices are  so  nearly  equal,  the  section  reacts  like  a  uniaxial  crystal, 
appearing  as  if  dark  during  a  complete  revolution.  The  plane  of 
the  optic  axes  varies  in  its  position  from  perpendicular  to  the  plane 


SILICATES,   TITANATES,   ETC.  493 

of  symmetry,  010,  in  muscovite,  paragon! te,  lepidolite,  or  the  alkali 
micas,  and  some  biotites,  termed  anomites,  to  parallel  to  the  plane 
of  symmetry  in  most  biotites,  phlogopite,  zinnwaldite,  and  lepi- 
domelane.  The  former  are  the  micas  of  the  first  class,  while  the 
latter  are  the  micas  of  the  second  class.  The  angle  between  the 


FIG.  508.  —  Biotite  Crystals  from  Franklin,  New  Jersey. 

optic  axes  in  some  cases  is  so  near  zero  that  biotite  for  a  long  time 
was  thought  to  be  hexagonal,  but  2  V  may  vary  from  0,  to  in  some 
cases  nearly  60°. 

Biotite  is  probably  the  most  common  of  all  the  micas,  occurring 
as  a  primary  mineral  in  such  rocks  as  granite,  syenite,  diorites,  and 
the  basaltic  igneous  rocks,  in  which  latter  muscovite  is  rare.  It 
separates  from  the  magma  early,  crystallizing  before  the  feldspars, 
and  in  some  cases  is  intergrown  with  other  species  forming  crystals 
of  zonal  structure,  in  which  the  central  portion  will  be  the  more 
magnesian  varieties  of  phlogopites,  while  the  alkali  micas  form  the 
outer  zone.  In  this  way  lepidolite  often  forms  the  outer  zone  of 
muscovite.  Biotite  is  also  of  metamorphic  origin,  as  in  the  gneisses 
and  schists,  as  well  as  in  the  sedimentary  rocks.  As  a  secondary 
mineral  it  is  not  formed  from  the  material  furnished  by  the  altera- 
tion of  a  single  mineral,  as  is  the  case  of  the  feldspars  and  muscovite, 


494  MINERALOGY 

but  by  double  decomposition  and  precipitation  from  solution  of 
materials  furnished  by  magnetite  and  feldspars  or  amphibole  as 
an  illustration. 

Owing  to  its  iron  content,  biotite  is  much  easier  decomposed  by 
weathering  and  oxidation  than  muscovite,  the  first  step  in  which 
seems  to  be  the  absorption  of  water,  or  hydration,  and  loss  of  alkali, 
forming  hydrobiotite  or  vermiculite,  H2Mg2Al2(SiO4)3 .  3  H20.  The 
second  step  is  the  formation  of  chlorite,  H2(MgOH)4Al2(Si04)3  by 
further  hydration,  in  which  the  water  becomes  more  firmly  attached 
to  the  molecule.  When  the  biotite  is  rich  in  iron,  magnetite  is  a 


FIG.  509. — Biotite  and  Muscovite  in  a  Section  of  Augite-syenite,  between  Crossed 
Nicols.  The  Dark  Areas  are  Dark  Brown  Biotite,  Parallel  to  the  Cleavage.  The 
Fine  Striations  are  Traces  of  the  Micaceous  Cleavage. 

product.  A  large  number  of  minerals  have  been  described  as 
derived  from  biotite,  as  epidote,  serpentine, .  kaolinite,  gibbsite, 
etc. 

Biotite  when  fused  loses  its  water  and  breaks  down,  forming 
olivine,  leucite,  spinel,  and  glass.  Its  synthesis  is  as  in  muscovite. 

PHLOGOPITE 

Phlogopite.  —  Magnesian  mica,  H2KMg3Al(SiO4)3 ;  K2O  = 
8,40,  MgO  =  28.90,  A12O3  =  10.87,  SiO,  =44,81,  H2O  =  5.42, 
with  a  little  iron,  sodium,  and  fluorine;  Monoclinic;  Type, 


SILICATES,   TITANATES,   ETC. 


495 


Digonal  Equatorial ;  Crystalline  elements  and  habit  like  biotite ; 
Cleavage,  basal  perfect,  micaceous,  lamina?  tough  and  elastic; 
H.  =  2.5-3 ;  G.  =  2.78-2.85 ;  Color,  yellowish  brown,  with  me- 
tallic-like  reflections,  also  greenish  to  nearly  colorless ;  Streak, 
white;  Luster,  pearly;  Transparent  to  translucent;  a  =  1.562; 
P  =  1.606  ;  -y  =  1.606;  7  -  a  =  .044  ;  Optically  (-)  ;  Axial 
plane  =  010 ;  Bxa  J.  to  001 ;  2  E  =  0  -  40°. 

B.B.  —  Whitens  and  fuses  on  the  thin  edges.  Generally  yields 
no  iron  reactions  with  the  fluxes.  Decomposes  with  sulphuric 
acid,  leaving  the  silica  in  thin  flakes.  In  the  closed  tube  yields 
some  water. 


General     description.  —  In 

rough,  six-sided  crystals, 
which  are,  however,  better 
formed  than  in  biotite. 

It  occurs,  as  a  product  of 
metamorphism,  in  the  crys- 
talline limestones  and  dolo- 
mites, as  at  Gouverneur, 
New  York;  Newton,  New 
Jersey ;  Franklin,  New  Jer- 
sey ;  and  various  other  locali- 
ties in  the  Middle  States  and 
Ontario.  Crystals  from 
Sydenham,  Ontario,  measure 
five  or  six  feet  across  the 
base. 

In  decomposition  it  resembles  biotite,  and  commercially  it  finds 
the  same  uses  as  muscovite. 


FIG.  510. — Phlogopite.   Rossie,  New  York. 


LEPIDOLITE 

Lepidolite.  —  Lithia  mica,  H2KAl3(Si04)3 .  R3AlF(Si308) ;  Mono- 
clinic  ;  Type,  Digonal  Equatorial ;  Crystalline  elements  and  form 
as  in  muscovite;  Cleavage,  basal  perfect,  micaceous  laminae 
flexible,  elastic,  and  tough ;  H.  =  2.5-4 ;  G.  =  2.8-2.9 ;  Color, 
pale  pink,  violet,  gray,  or  yellowish;  Streak,  colorless;  Luster, 
pearly;  Transparent  to  translucent;  P  =  1.597;  -y  =  1.605; 
y  -  p  =  .008  ;  Optically  (-) ;  Axial  plane -L  to  010  ;  Bxa  A  c  =  5° ; 
2E  =  57°-85°. 


496  MINERALOGY 

B.B.  —  Intumesces  and  fuses  at  2.5  to  a  blebby  glass,  yielding  a 
lithium  flame,  especially  when  mixed  with  the  potassium  bisul- 
phate  flux.  Only  slightly  attacked  by  acids. 

General  description.  —  Crystals  are  small  tabular  or  scaly  ag- 
gregates or  massive.  Chemically  it  approaches  the  formula  of  a 
metasilicate,  or  it  may  be  a  mixture  of  the  ortho-  and  trisilicate  as 
the  formula  given  would  indicate,  in  which  R'  represents  lithium 
and  small  amounts  of  rubidium  and  caesium;  as  with  all  micas, 
fluorine  is  present. 

Polylithionite  is  a  lepidolite  from  Greenland,  in  which  the  silica 
is  in  the  proportion  of  a  metasilicate.  Zinnwaldite  is  a  lithia  mica, 
found  in  the  tin  veins  of  Bohemia,  containing  considerable  iron. 

Lepidolite  occurs  in  pegmatites  or  granitic  veins  associated  with 
tourmaline,  spodumene,  amblygonite,  beryl,  cassiterite,  feldspars, 
biotite,  and  quartz,  often  intergrown  with  the  biotite  forming  the 
margins  of  the  plates. 

In  the  United  States  it  occurs  at  Hebron  and  Paris,  Maine,  and 
at  various  points  in  Massachusetts  and  Connecticut ;  at  the  rubel- 
lite  locality  in  San  Diego  County,  California. 

In  weathering  it  hydrates  and  probably  forms  cookeite,  which  is 
associated  with  it  at  Paris  and  Hebron,  Maine,  and  at  Chester- 
field, Massachusetts. 

CLINTONITE  GROUP 

The  clintonites  or  brittle  micas  are  foliated  micaceous  minerals 
in  which  the  alkalies  of  the  true  micas  are  wanting  and  the  mag- 
nesia is  in  large  part  replaced  by  iron  and  calcium ;  manganese  may 
also  enter  the  molecule.  They  are  all  basic  and  more  complex  in 
their  nature  and  chemical  composition.  Their  formula  may  be 
expressed  R3R"R'"O2'(SiO4)  or  R«R//R/"O2(Si808)  or  a  mixture  of 
the  two,  in  which  R'"  is  either  aluminium  or  ferric  iron,  R"  is  Fe, 
Ca,  Mn,  or  Mg,  and  R'  is  H,  OH,  or  F.  They  are  all  easily  altered 
by  hydration,  and  it  is  difficult  to  determine  whether  the  specimen 
represents  the  true  and  unaltered  composition  or  not,  —  another 
difficulty  in  the  way  of  expressing  a  formula. 

Margarite,  H2CaAl4Si20i2,  a  calcium  mica,  associated  with 
corundum,  from  which  it  is  often  derived  as  a  secondary  product. 
It  has  a  hardness  of  3.4-4.5,  G.  =  2.99 ;  Color,  pink,  gray,  or  yel- 
low ;  Luster,  pearly. 

B.B.  —  Fuses  on  the  thin  edges,  yields  water  in  the  closed  tube. 
Crystals  are  rare.  It  is  found  associated  with  staurolite  and 


SILICATES,   TITANATES,   ETC.  497 

tourmaline  in  schists  at  Chester,  Massachusetts;  also  at  Union- 
ville,  Pennsylvania,  and  at  the  corundum  localities  of  North  Caro- 
lina and  Georgia. 

Ottrelite,  H2(Fe .  Mn)Al2Si2O9,  is  a  mica  in  which  there  is  con- 
siderable manganese ;  H.  =  6-7  ;  G.  =  3.3.  It  fuses  with  difficulty 
and  yields  iron  and  manganese  reactions  and  is  usually  dark  gray  to 
black. 

Clintonite  is  a  bronze-colored  mica  occurring  at  Amity,  New 
York. 

CHLORITES 

The  composition  of  the  chlorites  is  complex  and  uncertain. 
They  are  all  hydrated  decomposition  products,  probably  formed  of 
mixtures  of  several  isomorphous  salts  or  types  of  salts,  but  just 
what  these  types  are  is  still  unknown.  When  fused,  their  water  is 
driven  off  and  they  break  down  into  a  portion  which  is  soluble,  and 
into  an  insoluble  portion,  of  the  nature  of  spinel.  They  may  be 
derived  from  the  hydration  and  alteration  of  almost  any  ferro- 
magnesian  mineral  containing  aluminium,  as  they  are  all  basic 
orthosilicates  of  aluminium  and  ferrous  iron.  Some  ferric  iron 
may  replace  the  aluminium,  and  isomorphous  elements  replace  the 
ferrous  iron,  as  manganese.  When  crystallized,  they  are  mono- 
clinic  in  symmetry,  tabular  and  six-sided  in  habit,  with  a  perfect 
basal  cleavage.  The  laminae  are  flexible  and  tough,  but  inelastic. 
Their  color  is  usually  shades  of  green,  as  the  name  implies,  except 
where  manganese  is  present,  when  they  may  be  pink ;  H.  =  1-3.5 ; 
G.  =  2.65-2.96;  Double  refraction  .001  to  .009:  Optically  (±); 
BxaAc  =  0-8. 

B.B.  —  They  whiten  and  fuse  on  the  edges  to  a  black  slag  when 
much  iron  is  present,  or  a  yellowish  slag  when  it  is  absent.  Yield 
much  water  in  the  closed  tube,  about  12  per  cent. ;  decomposed 
by  sulphuric  acid. 

Clinochlore,  H8(Mg  .  Fe)5Al2Si30i8,  and  orochlorite  (ripidolite) 
H40(Mg .  Fe)23Ali4Sii3O9o,  are  well-crystallized  members  of  the 
group,  occurring  in  large  hexagonal  plates  or  curiously  curved 
prisms.  Their  crystals  are  often  implanted  on  the  edges  in  diver- 
gent groups.  They  yield  pressure  and  percussion  figures  as  in  the 
true  micas.  Large  rough  crystals  of  clinochlore  occur  at  West 
Chester,  Pennsylvania,  and  well-formed  tabular  crystals  at  Texas, 
Pennsylvania. 

2K      ' 


498  MINERALOGY 

Corundophilite,  H20(Mg .  Fe)iiAl8Si6O45,  is  associated  with  corun- 
dum at  Chester,  Massachusetts,  and  Asheville,  North  Carolina. 
Thuringite  is  a  massive  scaly  variety  occurring  at  French  Creek, 
Pennsylvania;  Hot  Springs,  Arkansas;  and  at  Harper's  Ferry 
inclosing  garnets. 

Stilpnomelane  is  an  iron  variety  having  the  metallic  luster  of 
brass  or  mosaic  gold,  occurring  at  the  Sterling  iron  mine,  New  York. 

Some  fifteen  other  species  and  varieties  could  be  added,  most  of 
which  are  rare  or  uncertain. 

Chlorites  occur  as  schist-forming  minerals,  and  lining  cavities  or 
filling  veins  in  all  kinds  of  igneous  rocks,  as  alteration  products. 
Most  schist-forming  chlorites  are  fine  and  scaly,  having  a  talclike 
or  soapy  feeling ;  but  chemically  they  are  easily  distinguished,  as 
chlorite  contains  much  aluminium. 

SERPENTINE 

Serpentine.  —  H4Mg3Si209,  or  H3Mg2(Mg  .  OH)(Si04)2;  MgO 
=  43.52,  SiO2  =  43.16,  H2O,  =  13.32;  Monoclinic,  never  in 
crystals ;  Cleavage,  basal  sometimes  distinct ;  H.  =  2.5-5.5 ; 
G.  =  2.50-2.65 ;  Color,  shades  of  green  and  yellow ;  Fracture, 
conchoidal  or  splintery ;  Streak,  white ;  Luster,  dull  to  resinous, 
or  greasy;  feels  smooth  or  greasy;  Translucent  to  opaque; 
a  =1.560;  0  =  1.570;  7  =  1.571;  7  — a  =  .011-013;  Opti- 
cally (-);  Chrysotile  (+) ;  Axial  plane  (?)  ;  Bxa  =  c,  In 
chrysotile,  i.  c ;  2  V  =  16°-98°. 

B.B.  —  Fuses  with  difficulty  on  very  thin  edges.  In  the  closed 
tube  yields  water.  After  ignition  with  cobalt  solution  usually 
flesh-colored  (Mg) ;  decomposes  with  HC1  without  gelatiniza- 
tion ;  the  solution  freed  of  silica  yields  little  or  no  precipitate  with 
ammonia  (Al)  (those  containing  iron  will  yield  a  brown  precipi- 
tate); the  filtrate  tested  with  sodium  phosphate  shows  magnesium. 

General  description.  —  Massive  with  a  microscopic  fibrous  or 
felted  structure,  also  foliated  or  slaty.  Chemically  serpentine  is  a 
basic  orthosilicate,  which  when  fused  breaks  down  to  olivine  and 
enstatite.  Iron  may  replace  the  magnesium  to  as  much  as  seven 
per  cent,  and  also  manganese  or  nickel  in  small  quantities.  It  is 
in  all  cases  a  secondary  product  formed  by  the  weathering  and 
hydration  of  a  large  number  of  minerals ;  in  fact,  any  silicate  con- 
taining considerable  magnesium  may  form  serpentine.  It  is 
especially  derived  from  the  weathering  of  olivine,  tremolite,  and 


SILICATES,  TITANATES,   ETC. 


499 


enstatite ;  large  masses  of  serpentine,  often  impure,  result  from  the 
alteration  of  rocks  containing  these  minerals.  Magnesium  sili- 
cates yield  much 
of  their  associated 
bases,  as  calcium 
and  iron,  to  per- 
colating water 
containing  carbon 
dioxide,  as  bicar- 
bonates,  the  mag- 
nesium remaining 
behind  in  the  basic 
form  as  serpen- 
tine. Pseudo- 
morphs  of  serpen- 
tine after  such 
minerals  as  oliv- 
ine,  amphibole,  

and    pyroxene   re-        FIG  51L_serpentine  with  Veins  of  Fibrous  Chrysotile. 
Suit    from     this  Vernon,  New  York. 

method  of  altera- 
tion. Magnesium  may  also  be  carried  in  solution  by  the  percolating 
waters  as  a  bicarbonate,  which  will  replace  calcium  or  iron  in  sili- 
cates, again  forming  serpentine;  pseudomorphs  formed  by  this 
method  after  minerals  which  contain  no  magnesium,  as  the  feld- 
spars and  even  quartz,  are  common. 

Serpentine  is  itself  decomposed  by  percolating  waters,  especially 
those  of  solfataric  origin,  yielding  its  magnesium  as  a  sulphate  or 
carbonate,  leaving  the  silica  free  as  quartz  or  opal.  Brucite  is 
also  a  product  of  the  decomposition  and  hydration  of  serpentine. 

From  the  many  sources  from  which  it  may  be  derived  serpentine 
is  necessarily  a  very  widely  distributed  mineral.  It  occurs  in  large 
bodies,  and  associated  with  it  in  many  instances  are  deposits  of 
chromite  and  nickel  ores,  as  at  Bare  Hill,  Maryland,  and  Texas, 
Lancaster  County,  Pennsylvania. 

Massive  serpentine  is  quarried  as  a  building  stone,  and  when 
polished  is  known  in  the  trade  as  serpentine  marble.  When  mixed 
with  carbonates,  which  it  often  is,  it  forms  the  mottled  green 
"  verdi  antique."  Serpentine  is  often  disseminated  through  dolo- 
mites, where  it  has  arisen  from  the  alteration  of  contained  silicates 
rather  than  from  the  magnesium  of  the  carbonates. 


500  MINERALOGY 

The  variety  chrysotile  commercially  known  as  asbestos  is  fibrous 
and  occurs  in  cross-fibred  veins  in  massive  serpentine.  When  the 
fibers  are  soft,  silky,  and  easily  separable,  they  are  spun  and  woven 
into  fireproof  cloth.  The  poorer  qualities  are  used  for  fireproofing 
and  as  non-conductor  coverings  for  steam  boilers  and  pipes. 

Chrysotile  is  mined  at  Black  Lake,  Thetford,  Quebec ;  in  the 
United  States  at  Casper,  Wyoming,  and  in  northern  Vermont  near 
the  Canada  locality.  Closely  related  to  serpentine  is  the  silicate 
of  nickel,  genthite,  H4Mg2Ni2(Si04)3 .  4  H2O ;  as  bright  green  or 
yellowish  green,  amorphous  crusts,  or  stalactitic,  associated  with 
chromite  and  serpentine,  at  Texas,  Pennsylvania,  and  Webster, 
North  Carolina.  It  is  a  secondary  mineral  derived  from  the  ser- 
pentine. Usually  contains  about  30  per  cent,  of  NiO. 

Garnierite,  also  a  green  amorphous  silicate  of  nickel  and  magne- 
sium, H2(Ni.Mg)SiO4,  is  associated  with  serpentine  at  Webster, 
North  Carolina,  and  at  Riddle,  Oregon.  In  New  Caledonia  it 
occurs  in  quantities  sufficient  to  be  worked  as  an  ore  of  nickel. 

TALC 

Talc. — A  basic  metasilicate  of  magnesium,  H2Mg3(Si03)4; 
MgO  =  31.7,  SiO2  =  63.5,  H2O  =  4.8;  Orthorhombic  or  mono- 
clinic,  well-developed  crystals  are  not  known ;  Cleavage,  basal ; 
H.  =  1-1.5;  G.  =  2.7-2.8;  Color,  gray,  or  shades  of  green  and 
yellow ;  Streak,  white ;  Luster,  pearly  to  greasy ;  Subtranslucent 
to  opaque;  a  =  1.539;  p  =  1.589;  \  =  1.589;  -y  -  a  =  .050; 
Optically  (-);  Axial  plane  =  100;  Bxa-L001;  2E  =  6°-40°. 

B.B. — Whitens,  exfoliates  somewhat,  and  fuses  with  difficulty 
on  the  very  thin  edges.     Ignited  in  the  forceps  with  cobalt  solu- 
tions becomes  flesh-colored  (Mg).     Insoluble  in  acids. 
• 

General  description.  —  Usually  massive,  granular,  or  foliated, 

rarely  does  it  occur  in  six-sided  scales.  The  massive  variety  is 
known  as  soapstone  or  steatite.  Rensselaerite  is  a  fibrous  variety, 
pseudomorphous  after  enstatite  or  amphibole,  preserving  the 
structure  of  the  parent  mineral. ' 

Chemically,  small  amounts  of  iron  or  nickel  may  replace  the 
magnesium.  Talc  is  the  end  product,  produced  in  the  weathering 
of  a  large  number  of  silicates  containing  magnesium,  especially 
amphiboles  and  pyroxenes  and  often  spinel.  It  is  very  stable 
under  conditions  of  weathering,  but  when  fused  it  loses  water  and 
on  cooling  forms  enstatite  and  quartz. 


SILICATES,   TITANATES,   ETC.  501 

Like  serpentine,  owing  to  the  many  sources  from  which  it  may 
originate,  it  forms  pseudomorphs  after  a  large  number  of  minerals, 
and  even  after  species  which  contain  no  magnesium,  as  quartz, 
topaz,  and  cyanite,  where  the  magnesium  is  furnished  by  perco- 
lating waters. 

Talc  is  an  abundant  mineral,  occurring  under  the  same  conditions 
as  the  chlorites  or  serpentine ;  it  forms  a  large  proportion  of  some 
schists,  which  are  termed  talcose  schists.  It  is  also  found  as  len- 
ticular masses  in  metamorphic  rocks,  and  magnetite,  chromite, 
hornblende,  serpentine,  and  chlorites  are  associated  minerals. 

Sepiolite  (meerschaum),  H4Mg2Si30i0,  is  a  magnesium  silicate 
closely  related  to  talc.  Commercially  talc  finds  many  and  varied 
uses ;  in  the  powdered  form  it  is  used  as  a  paper  filler ;  in  toilet 
powders,  paints,  lubricants,  and  soaps.  In  slabs  it  is  used  in  the 
manufacture  of  hearthstones,  table  tops,  vats,  ovens,  furnace 
linings,  stair  treads,  etc. 

All  the  Atlantic  coast  states  are  commercial  producers  of  talc. 
The  fibrous  variety  is  only  found  at  Gouverneur,  New  York. 


KAOLINITE 

Kaolinite.  —  Clay;  A  basic  orthosiiicate  of  aluminium,  H4A12- 
Si2O9 ;  A12O3  =  39.5,  Si02  =  46.5,  H2O  =  14.0 ;  Monoclinic ;  Type, 
Digonal  Equatorial ;  a  :  b  :  c  -  .5748  : 1 :  1.5997  ;  p  =  83°  11'  = 
100  A  001 ;  100  A 110  =  29°  43' ;  001 A 101  =  76°  22' ;  001 A  Oil  =  57° 
48';  Common  forms,  b  (010),  c  (001),  m(110),  n(lll);  Twins 
as  in  mica ;  Cleavage,  basal  perfect,  flexible,  inelastic,  plastic,  and 
unctuous;  H.  =  1-2.5;  G.  =  2.6-2.65;  Color,  when  pure,  white, 
otherwise  yellow,  brown,  red,  or  bluish ;  Streak,  white  or  pale ; 
Luster,  pearly  to  dull ;  Transparent  scales  to  translucent ;  n  = 
1.54;  y-a  =  .OQ8;  Axial  plane  ±010;  BxaA-L001  =  20° 
behind;  2E  =  0°-90°. 

B.B.  —  Infusible,  becomes  blue  with  cobalt  solution.  Yields 
water  in  the  closed  tube  and  a  silica  residue  in  the  S.  Ph.  bead. 
Insoluble  in  acids. 

General  description.  —  In  microscopic  six-sided  scales  with 
angles  of  nearly  120° ;  also  friable,  mealy,  or  massive.  When  wet 
it  is  plastic  and  has  a  peculiar  greasy  feel.  When  dry  it  absorbs 
moisture  from  and  sticks  to  the  tongue  and  emits  a  peculiar  argilla- 
ceous odor  just  after  being  moistened. 


502  MINERALOGY 

Kaolinite  represents  the  ultimate  product  in  the  weathering  and 
hydration  of  a  long  series  of  aluminium  silicates.  The  most  impor- 
tant source  is  in  the  alteration  of  the  feldspars,  known  as  kaoliniza- 
tion,-  which  is  the  result  of  percolating  waters  charged  with  carbon 
dioxide.  This  is  especially  true  of  the  alkali  feldspars,  as  anorthite 
does  not  form  kaolin.  The  potassium  is  yielded  as  a  carbonate, 
leaving  the  aluminium  silicate  in  the  hydrated  form  as  kaolin. 
It  is  therefore  a  secondary  mineral  widely  distributed  in  all  rock's 
which  have  suffered  alteration  and  in  all  soils.  It  forms  pseudo- 
morphs  after  many  minerals,  or  remains  behind  mixed  with  other 
products  of  weathering,  to  form  the  soils ;  or  it  is  carried  by  run- 
ning water  and  deposited  as  sedimentary  beds,  some  of  which  are 
very  pure  kaolin.  Owing  to  the  extreme  fineness  of  the  scales,  they 
remain  longer  in  suspension,  and  are  deposited  apart  from  heavier 
and  coarser  materials. 

Clay  is  a  large  component  of  all  sedimentary  rocks  except  the 
sandstones  and  carbonates.  Under  ordinary  conditions  of  weather- 
ing it  is  extremely  stable  and  is  not  broken  down,  but  under  the 
influence  of  heat  and  pressure  it  is  dehydrated,  yielding  its  alumina 
and  silica  for  the  formation  of  minerals  characteristic  of  meta- 
morphic  sediments. 

Allophane,  Al2Si05 .  5  H20,  is  an  amorphous  aluminium  sili- 
cate forming  mammillary  crusts  and  stalactites.  Hardness  3, 
harder  than  kaolin,  and  gelatinizes  with  HC1. 

Pyrophyllite,  H2Al2Si4Oi2,  is  a  silicate  of  aluminium  resembling 
talc  in  appearance,  as  to  color,  structure,  luster,  and  feeling.  It 
may  also  be  radiated  and  fibrous  as  well  as  massive,  but  when  heated 
it  swells  many  times  its  volume  and  becomes  blue  when  ignited 
with  cobalt  solution. 

There  are  other  silicates  of  aluminium  related  to  kaolinite, 
some  of  which  are  probably  mixtures,  and  the  composition  of  others 
is  uncertain. 

Commercially  clay  or  kaolinite  is  the  basis  of  all  porcelain  and 
chinaware,  and  the  impure  varieties  are  used  in  the  manufacture 
of  tile,  drainpipes,  and  bricks. 

The  purer  varieties  are  obtained  in  Chester  and  Delaware 
Counties,  Pennsylvania,  and  in  North  Carolina,  while  a  large  pro- 
portion of  the  ordinary  clays  is  dug  in  New  Jersey. 


SILICATES,  TITANATES,  ETC.  503 

CHRYSOCOLLA 

Chrysocolla.  —  A  hydrous  metasilicate  of  copper,  CuSiO3 .  - 
2  H20 ;  CuO  =  45.2,  SiO2  =  34.3,  H2O  =  20.5 ;  Cryptocrys- 
talline ;  Brittle ;  Fracture,  conchoidal ;  H.  =  2-4 ;  G.  =  2-2.4  ; 
Color,  shades  of  blue  and  green  passing  into  black  by  oxidation ; 
Streak,  white  when  pure ;  Luster,  vitreous  to  earthy ;  Transparent 
to  earthy. 

B.B.  —  Decrepitates,  infusible  but  yields  a  copper  flame.  Re- 
duced with  soda,  borax,  and  a  little  coal  dust,  yields  copper 
buttons.  In  the  closed  tube  blackens  and  yields  water.  Decom- 
posed with  HC1  with  the  separation  of  silica,  but  without  gelatiniz- 
ing. 

General  description.  —  Occurs  in  crusts,  botryoidal,  or  filling 
veins  and  seams  in  the  gangue  rock  of  copper  deposits.  It  is  often 
impure  from  an  admixture  of  silica,  when  it  will  appear  much  harder 
than  4.  It  becomes  black  from  the  formation  of  oxides. 

Chrysocolla  is  a  mineral  deposited  from  the  percolating  waters 
which  carry  copper  and  silica,  and  is  characteristic  of  the  zone  of 
oxidation.  It  is  therefore  associated  with  the  superficial  areas  of 
most  copper  deposits. 

Fine  specimens  have  been  obtained  at  Bisbee,  Arizona;  at 
Somerville,  New  Jersey,  and  in  the  Lake  Superior  copper  region. 

When  in  sufficient  quantity  it  is  an  excellent  copper  ore.  It  is 
often  polished  and  sold  as  an  imitation  turquoise. 

TITANITE 

Titanite.  —  Calcium  titanosilicate,  CaTiSiO6;  CaO  =  28.6, 
TiO2  =  40.8,  Si02  =  30.6 ;  Monoclinic ;  Type,  Digonal  Equa- 
torial ;  a :  b  :  c  =  .7546_:  1 :  .8543 ;  p  =  60°  11'  =  001 A 100 ;  100  A 
110  =  33°  14';  001 A 101  =  65°  57';  001 A  Oil  =  36°  34';  001 A 
102  =  21°;  001 A  111  =  38°  16';  Common  forms,  c  (001),  m  (110), 
n(lll),  x(102),  a  (100),  s  (021) ;  Twinning  plane  100,  both  con- 
tact and  penetrating,  other  twins  rare;  Cleavage,  prismatic  dis- 
tinct ;  Brittle ;  Fracture,  subconchoidal ;  H.  =  5-5.5 ;  G.  =  3.4- 
3.6 ;  Color,  brown,  yellow,  gray,  green,  rose-red  to  black ;  Streak, 
white  to  pale  brown;  Transparent  to  opaque;  a  =  1.887;  p  = 
1.894;  -y  =  2.009;  -y  -  a  =  .122;  Optically  (+) ;  Axial  plane 
=  010;  BxaAc  =  51°  in  front;  2V  =  27°  30';  2E  =  52°  30'. 


504  MINERALOGY 

B.B.  —  Fuses  at  3  with  intumescence,  in  most  cases.  The 
powdered  mineral  fused  with  soda  and  dissolved  in  strong  HC1 
and  heated  with  tin  yields  a  violet-colored  solution  (titanium). 
Only  slightly  attacked  by  HC1,  but  decomposed  by  sulphuric 
acid. 

General  description.  —  Crystals  are  tabular  parallel  to  the  base 
in  habit,  or  elongated  parallel  to  the  prism,  when  they  are  terminated 
by  the  unit  pyramid.  Usually  combinations  of  the  base,  unit 


FIG.  512.  —  Titanite.     Renfrew,  Canada. 

prism,  and  pyramid ;  other  faces,  as  x,  a,  and  s,  are  less  common. 
A  large  number  of  rare  forms  have  been  described. 

Chemically,  some  of  the  titanium  may  be  replaced  by  ferric 
iron  and  aluminium,  and  a  rose-red  manganese  variety  occurs  at 
St.  Marcel,  Piedmont,  associated  with  piedmontite,  the  man- 
ganese epidote. 

In  rock  sections  titanite  appears  in  wedge-shaped  out- 
line or  rounded  grains,  either  colorless,  pale  yellow,  or  brown. 
The  relief  is  very  marked ;  cleavage  in  two  directions  distinct, 
but  peculiar  from  the  fact  that  they  are  never  parallel  to  the 
crystal  outline.  The  extinction  due  to  dispersion  is  ill-defined. 
Interference  colors  vary  greatly  according  to  the  direction  of  the 
section  in  the  crystal,  from  gray  of  a  high  order  to  gray  of  the 
first  order,  when  the  two  rays  a  and  p,  which  are  nearly 
equal,  are  vibrating  in  the  section. 


SILICATES,   TITANATES,   ETC.  505 

Titanite  as  a  pyrogenetic  accessory  mineral  is  very  widely  dis- 
tributed in  all  igneous  rocks,  except  those  rich  in  silica  and  mag- 
nesia. As  a  metamorphic  mineral  it  occurs  in  the  schists  and  crys- 
talline limestones,  where  it  is  associated  with  such  minerals  as 
apatite,  scapolite,  zircon,  hornblende.  Especially  fine  crystals 
occur  in  a  limestone  at  Renfrew  County,  Ontario.  In  the  United 
States  it  occurs  at  numerous  localities  in  the  Atlantic  coast  states. 

At  St.  Gothard,  Switzerland,  clear  honey-yellow  crystals  occur, 
associated  with  adularia. 

Alteration  products  of  titanite  are  not  common.  At  times  it 
forms  perovskite,  CaTiO3,  an  isometric  mineral  crystallizing  in  cubes 
and  octahedrons;  by  the  elimination  of  both  calcium  and  silica, 
rutile  and  octahedrite  have  been  noted  as  being  formed  from 
titanite.  Under  favorable  conditions  the  reverse  of  this  may  occur 
and  titanite  result  from  rutile  or  octahedrite,  and  especially  from 
ilmenite,  when  the  alteration  product  is  known  as  leucoxene. 

The  synthesis  of  titanite  has  been  effected  by  the  simple  fusion  of 
its  chemical  constituents,  at  a  temperature  of  1400°.  If  the  melt  is 
low  in  silica,  perovskite  will  form. 

Benitoite,  BaTi(SiO3)3,  a  titanosilicate  of  barium,  is  an  interest- 
ing mineral,  as  it  represents  the  ditrigonal  equatorial  type,  which  up 
to  the  time  of  its  discovery  had  no  representative.  It  occurs  in  San 
Benito  County,  California,  imbedded  in  a  vein  of  massive  natrolite 
and  associated  with  another  rare  titanium  mineral,  neptunite, 
(Na.K)2(Fe.Mn)(Si.Ti)5Oi2,  heretofore  only  found  at  Narsarsuk, 
Greenland.  . 

COLUMBITE 

Columbite.  —  (Fe.Mn)(Cb03)2;  a  columbate  of  iron  and^ man- 
ganese ;  Orthorhombic  ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c  = 
.8285  : 1 :  .8897  ;  100  A 110  =  39°  38' ;  001 A 101  =  47°  2' ;  001  A 
Oil  =41° 40';  001  A  111  =  54°  21';  001.133  =43°  48';  Common 
forms,  a  (100),  b  (010),  c  (001),  m  (110),  o(lll),  u(133),  k  (103) ; 
Twinning  plane,  021,  contact  and  heart-shaped  twins  common; 
Cleavage,  a  distinct,  b  less  so  ;  Brittle ;  Fracture,  uneven ;  H.  =  6 ; 
G.  =  5.3-7.3 ;  Color,  iron-black  to  brownish ;  Streak,  dark  red  to 
black ;  Luster,  submetallic,  brilliant  to  resinous ;  Opaque,  at  times 
iridescent. 

B.B.  —  Infusible  or  fuses  with  great  difficulty.  The  S.  Ph. 
bead  saturated  and  reduced  with  tin  on  coal,  then  dissolved  in  a 


506  MINERALOGY 

little  dilute  HC1  and  boiled  with  tin,  yields  a  turbid  brown  color  ; 
when  boiled  with  zinc  yields  a  blue  color.  Fused  with  soda  in  O.  F. 
generally  yields  a  green  color  (Mn) .  Insoluble  in  acids. 

General  description.  —  Either  crystalline  or  massive ;  when 
crystalline,  in  short  stout  prisms  or  of  tabular  habit,  parallel  to  the 
macropinacoid,  at  times  acutely  terminated  by  the  pyramid  (133). 

It  often  contains  considerable  amounts  of  tantalum,  tungsten, 
and  tin.  Tantalum  and  columbium  form  isomorphous  compounds 
which  are  associated  in  nature;  tantalite  (Fe.Mn)(Ta03)2  is  a 
mineral  isomorphous  with  columbite,  and  it  is  rarely  that  one  is 
found  without  containing  an  admixture  of  the  other  crystallized 
with  it.  They  also  form  a  series  of  compounds  with  the  rare  earths, 
of  which  fergusonite  (Y.Er.Ce)(Cb.Ta)O4  is  the  most  important. 
It  occurs  at  Rockford,  Massachusetts;  in  North  Carolina;  and  in 
Llano  County,  Texas. 

Columbite  is  associated  with  cassiterite,  wolframite,  ilmenite, 
magnetite,  and  other  minerals  found  in  pegmatites.  It  occurs  in 
the  pegmatites  of  Maine ;  brilliant  crystals  occur  at  Standish  and 
Middletown,  Connecticut ;  at  various  localities  in  North  Carolina ; 
in  the  pegmatites  of  the  Black  Hills  in  South  Dakota,  where  crystals 
weighing  nearly  two  thousand  pounds  have  been  found ;  here  it  is 
also  associated  with  cassiterite  and  it  has  been  concentrated  by  the 
same  pneumatolytic  agents.  It  is  associated  and  contained  in  the 
cryolite  deposit  of  West  Greenland. 

Tantalum  has  been  used  in  the  production  of  incandescent  lamp 
filaments,  which  are  much  more  efficient  than  the  carbon  lamps. 
Little  or  no  columbite  is  mined  in  the  United  States. 


CHAPTER  XI 

COLUMBATES,    PHOSPHATES,    VANADATES,    INCLUDING 
NITRATES,   BORATES,   AND   URANATES 

MONAZITE 

Monazite.  —  (Ce.La.Di)PO4;  an  orthophosphate  of  the  rare 
earths,  cerium,  lamthanum,  and  didymium  ;  Monoclinic  ;  Type, 
Digonal  Equatorial  ;  Ce2,O3  =  37.26,  (Ce  .  La  .  Di)203  =  31.60,  P205 
=  29.32,  Th02  =  1.48;  SiO2  =  .32  (North  Carolina)  ;  a:  b:  c  = 
9693  ;  1  :  .9256  ;  p  =  76°  20'  =  001  A  100  ;  001  A  110  =  43°  17'  ; 
001  A  101  =  37°  7';  001  A  011=41°  58';  101  A  100  =  39°  12'; 
111  A  100  =_  48°  30';  Common  forms,  a  (100),  b  (010),  m  (110), 
w(101),  x(101),  r(lll),  v(lll),  e(011);  Twinning  plane,  100; 
Cleavage,  basal  perfect,  a  distinct,  b  difficult  ;  Brittle  ;  Fracture, 
uneven  ;  H.  =  5-5.5  ;  G.  =  4.9-5.3  ;  Color,  clove-brown  to  yellow- 
brown,  and  red;  Streak,  white  to  yellowish;  Luster,  somewhat 
resinous  ;  Translucent  to  opaque  ;  a  =  1.796  ;  p  =  .1796  ;  -y  =  1.841  ; 
-y  —  a  =  .045;  Optically  (+);  Axial  plane  perpendicular  to  010; 
C  =  2°  6'  30"  in  front;  2E  =  24°  36'. 


B.B.  —  Infusible  ;  powdered  and  fused  with  soda,  then  dis- 
solved in  HNOs,  yields  a  yellow  precipitate  with  ammonium 
molybdate,  or  the  powered  mineral  moistened  with  H2S04  yields 
a  green  flame.  In  O.  F.  reacts  for  the  rare  earths.  Page  572,  de- 
composed with  H2S04,  but  difficultly  soluble  in  HC1. 

General  description.  —  Crystals  are  tabular  parallel  to  the 
orthopinacoid  or  elongated  parallel  to  the  orthoaxis,  also  massive 
or  in  rounded  disseminated  grains. 

Monazite  is  an  accessory  mineral  in  certain  granites,  granitic 
gneisses,  and  pegmatites.  Chemically  it  always  contains  some 
thorium,  which  may  amount  in  some  cases  to  9  per  cent,  of  Th02. 
In  such  cases  it  becomes  a  source  of  this  rare  earth,  which  is  now 
used  in  considerable  quantities  in  the  manufacture  of  incandescent 
gas  mantles.  The  bulk  of  the  commercial  supply  of  monazite  is 
derived  from  Brazil,  where  it  is  separated  from  detrital  sands.  In 
the  United  States  the  supply  is  concentrated  from  sands  at  several 

507 


508 


MINERALOGY 


localities  in  North  Carolina.  It  occurs  at  Amelia  Court  House, 
Virginia,  in  a  granite  associated  with  rutile  and  hiddenite.  Often 
found  in  the  gold  gravels  of  the  West,  especially  in  Idaho.  At 
Arendal,  Norway,  it  is  inclosed  in  the  red  apatite  crystals. 


APATITE 

Apatite.  —  Fluo-phosphate  of  calcium,  Ca4(CaF)(PO4)3;  Ca  = 
55.5,  P205  =  42.3;  Hexagonal;  Type,  Hexagonal  Equatorial; 
c  =  .7346 ;  0001 A 1011  =  40°  18' ;  0001 A 1012  =  22°  59' ;  Com- 
mon forms,  c(0001),  m  (1010),  x(1011);  Twinning,  doubtful; 
Cleavage,  basal  imperfect ;  Brittle ;  Fracture,  uneven ;  H.  =  5 ; 
G.  =  3.17-3.23 ;  Color,  shades  of  green,  brown,  and  red,  also  white, 
gray,  and  violet;  Streak,  white  or  pale;  Luster,  vitreous;  a  = 
1.638;  €  =  1.634;  a>-€  =  .004;  Optically  (-). 

B.B.  —  Fuses  with  difficulty  at  5.  The  powder  moistened  with 
H2S04  yields  a  green  flame  in  O.  F.,  or  the  nitric  acid  solu- 
tion shows  phosphoric  acid 
with  ammonium  molyb- 
date.  The  concentrated 
HN03  solution  yields  a 
white  precipitate  with 
H2S04(Ca). 

General  description.  — 
Crystals  are  stout  prisms 
terminated  with  the  pyra- 
mid of  the  first  order,  or 
with  the  pyramid  in  com- 
bination with  the  base. 
Combinations  of  the  three 
orders  of  prisms,  which  as 
well  as  the  etch  figures  fix 
the  symmetry,  occur  on 
small,  brilliant,  colorless 
crystals  found  in  a  chloritic 

schist,  associated  with  epidote  and  adularia,  in  the  Untersulz- 
bachthal,  Austria.  All  three  orders  of  prisms  are  in  combination 
on  small  tabular  crystals,  with  small  prism  faces  striated  parallel 
to  the  vertical  axis,  occurring  in  veins  associated  with  fluorite, 


FIG.  513.  —  Apatite  from  Templeton,  Canada. 


COLUMBATES,   PHOSPHATES,   VANADATES  509 

cassiterite,  and  sulphides  at  Ehrenfriedersdorf,  Saxony.  Crystals 
rich  in  forms  have  also  been  described  from  Branchville,  Con- 
necticut, and  Alexander  County,  North  Carolina. 

Chemically  there  are  two  compounds,  a  chlor-apatite  and  a  fluor- 
apatite,  which  are  isomorphous  and  occur  in  the  same  crystals.  It 
is  rarely  that  one  occurs  without  the  other ;  in  addition  the  chlorine 
or  fluorine  may  be  replaced  by  hydroxyl  (OH) ;  such  specimens  will 
yield  a  little  water  in  the  closed  tube.  Phosphatic  rock  found  in 
the  South  and  West  is  of  the  nature  of  apatite,  but  of  organic  origin ; 
a  bone  phosphate,  phosphatic  nodules,  coprolites,  all  of  which  are 


FIG.  514.  —  Apatite.    Snarum,  Norway. 

phosphates  of  calcium,  but  not  crystalline,  and  therefore  their  com- 
position varies  greatly. 

Extensive  beds  of  these  phosphates  are  found  in  South  Carolina 
and  the  Gulf  states ;  after  treatment  with  sulphuric  acid  they  form 
the  superphosphates  of  the  fertilizer  industry. 

Apatite  occurs  in  rocks  of  all  descriptions  and  under  variable 
conditions.  In  igneous  rocks  it  is  always  well  crystallized,  elon- 
gated parallel  to  the  vertical  axis ;  one  of  the  very  first  minerals  to 
separate  from  the  magma;  it  appears  as  inclusions  in  all  others, 
even  penetrating  the  magnetite. 

In  rock  sections  it  is  colorless,  with  a  hexagonal  outline,  or  elon- 
gated when  cut  nearly  parallel  to  c ;  such  sections  usually  show  a 
tranverse  parting,  but  the  basal  cleavage  is  seldom  observed  in  sec- 
tions. The  relief  is  well  marked;  interference  colors  are  grays  of  the 
first  order.  Apatite  is  a  common  mineral  in  the  metamorphic  rocks 
and  crystalline  limestones,  where  it  is  associated  with  titanite,  scap- 


510 


MINERALOGY 


olite,  pyroxene,  and  vesuvianite.  At  Burgess,  Ontario,  hexagonal 
prisms  a  foot  in  length  occur  in  the  limestone.  These  large  crystals 
always  have  a  peculiar  vitrified  appearance,  their  edges  are  rounded 
as  if  they  had  been  partly  fused.  Apatite  occurs  commonly  along 
the  Atlantic  slope  from  Ontario  to  Georgia.  It  is  associated  with 
the  tin  veins  of  Bohemia  and  Cornwall,  where  its  origin  is  due,  as  is 
also  the  cassiterite,  to  the  chemical  interaction  of  volatile  fluorides 
and  chlorides. 

It  is  very  peculiar  that  apatite,  being  quite  soluble  in  acids  and  a 
salt  of  a  weak  acid,  decomposes  in  nature  with  difficulty.  Under  the 
action  of  percolating  waters  containing  carbon  dioxide  the  calcium 
phosphate  passes  into  solution,  to  be  again  separated  as  various 


FIG.  515.  —  Section  of  a  Mica-diorite,  showing  a,  Apatite;  6,  Hornblende; 
c,  Biotite  ;  e,  Quartz  ;  and  /,  Feldspar  partially  Altered. 

secondary  iron  phosphates,  as  vivianite,  Fe3(P04)2 . 8  H2O,  a 
common  mineral  of  clays;  also  as  dufrenite,  Fe2(OH)3P04; 
phosphosiderite,  2  FeP04 .  3£  H20 ;  strengite,  FePO4 .  2  H20 ;  as 
secondary  aluminium  phosphates,  wavellite,  A13(OH)3(P04)2 . 
5  H2O;  variscite,  A1P04 .  2  H20 ;  turquoise,  A12(OH)3P04 .  H2O. 

Artificial  apatite  has  been  formed  by  heating  calcium  and 
ammonium  chlorides  with  calcium  phosphate  in  a  sealed  tube  at  a 
temperature  as  low  as  150°  C.  It  has  been  reported  as  a  con- 
stituent of  some  slags,  but  this  has  never  been  confirmed  by  analy- 
sis. Either  chlor-  or  fluor-apatite  may  be  produced  in  the  dry  fusion 
of  sodium  phosphate  with  either  calcium  chloride  or  calcium 
fluoride  as  the  case  requires. 


COLUMBATES,  PHOSPHATES,  VANADATES  511 


PYROMORPHITE 

Pyromorphite.— Pb5Cl(P04)3;  Chloro-phosphate  of  lead;  PbO 
=  82.3,  Cl  =  2.6,  P205  =  15.7 ;  Hexagonal ;  Type,  Hexagonal 
Equatorial;  c  =  .7362;  0001  A  1011  =  40°  22';  0001  A  2021  =_59° 
32';  Common  forms,  c  (0001),  m(1010),  x(lOfl),  y  (2021) ; 
Cleavage,  m  and  x  in  traces;  Brittle;  Fracture,  conchoidal; 
H.  =  3.5-4 ;  G.  =  6.9-7 ;  Color,  shades  of  green,  yellow,  or  brown ; 
Streak,  white  or  pale ;  Luster,  resinous ;  Subtranslucent  to  nearly 
opaque;  o>  =  1.51;  €  =  1.45;  co  —  €  =  .006;  Optically  (  — ). 

B.B.  —  Fuses  easily  at  3.5.  With  soda  and  borax  in  R.  F.  on 
coal  yields  lead  buttons  and  a  lead  coat.  Dissolves  in  HNO3, 
yields  a  yellow  precipitate  with  ammonium  molybdate.  When  a 
S.  Ph.  bead  is  saturated  with  copper  oxide  and  heated  with  the 
powdered  mineral  it  shows  chlorine.  Some  specimens  may  con- 
tain arsenic. 


FIG.  516.  —  Pyromorphite  from  Baumbach,  Prussia. 

General  description.  —  Crystals  are  columnar,  striated  length- 
wise, usually  hexagonal  prisms  roughly  terminated  or  pitted  at  the 
termination.  In  rare  cases  they  are  terminated  by  the  pyramid 
and  base,  as  at  Causthal  in  the  Harz.  Crystals  from  Ems,  Nassau, 
are  terminated  by  the  base  only.  Also  in  parallel  growths,  irregu- 
lar aggregates,  or  granular ;  sometimes  in  amorphous  crusts  and 
concretions. 

Pyromorphite  is  a  secondary  mineral  formed  by  the  interaction 
of  water  containing  phosphates  in  solution  and  lead  ores.  It  is 


512  MINERALOGY 

characteristic  of  the  zone  of  oxidation  and  is  therefore  found  in 
the  superficial  workings  of  lead  mines.  While  it  is  a  valuable  ore  of 
lead,  it  occurs  only  in  small  quantities. 

Good  specimens  have  been  obtained  at  the  Wheatley  mine, 
Chester  County,  Pennsylvania.  It  occurs  in  small  quantities  at 
various  localities  in  New  England  and  North  Carolina. 

Mimetite,  Pb5Cl(AsO4)3,  a  chlorarsenate  of  lead,  is  very  similar 
in  habit  and  occurrence  to  pyromorphite,  with  which  it  is  iso- 
morphous.  In  color  it  is  yellow  to  greenish  white  and  often  in 
globular  or  barrel-shaped  crystals. 

Vanadinite,  Pb5Cl(P04)3,  a  chlorvanadate  of  lead,  another 
member  of  the  apatite  group,  in  which  V2O5  takes  the  place  of  P2O5 
or  As206.  It  is  similar  in  habit  and  crystallization,  but  usually  red 
to  light  yellow  in  color.  It  is  associated  with  lead  ores  at  various 
localities  in  Arizona  and  New  Mexico. 

Endlichite  is  a  light  yellow  variety  containing  arsenic,  occurring 
in  Sierra  County,  New  Mexico. 

AMBLYGONITE 

Amblygonite.  —  Li(AlF)PO4;  Li2O  =  10.1,  A1203  =  34.4,  P205 
=  47.9,  F  =  12.9 ;  Triclinic ;  Type,  Centrosymmetric ;  a  :  b  :  c  = 
.7334:1:  .7633;  a  =  108°  51';  p  =  97°48';  7  =  106°  27';  100  A  010  = 
69°  25';  100 A  001  =75°  30';  010 A  001  =  67°  38';  100AllO  =  29° 
35' ;  001  A  021  =  74°  40' ;  Common  forms,  c  (001),  a  (100),  m  (110), 
M(110),  e(021);  Twinning  plane,  101  and  101,  polysynthetic 
twins  common,  the  two  sets  of  striations  making  an  angle  of  89°  8' ; 
Cleavage,  basal  perfect,  at  times  e  also;  Brittle;  Fracture,  un- 
even; H.  =  6;  G.  =  3.01-3.09;  Color,  white,  gray,  or  pale  blue, 
green,  and  brown ;  Streak,  white ;  Luster,  vitreous ;  Translucent 
to  opaque;  a  =  1.579;  p  =  1.593;  -y  =  1.597;  -y-  a  =  .018; 
Optically  (-). 

B.B.  —  Fuses  easily  at  2  with  intumescence  and  yields  a  lithium 
flame,  especially  when  fused  with  the  fluorite  flux.  When  fused 
with  soda  and  dissolved  in  nitric  acid,  shows  phosphoric  acid  with 
ammonium  molybdate.  Usually  contains  some  water  from  the 
replacement  of  F  by  OH. 

General  description.  —  Crystals  are  coarse  and  not  well  formed, 
usually  in  cleavable  masses.  It  occurs  in  the  coarse  granites  and 


COLUMBATES,   PHOSPHATES,   VANADATES  513 

pegmatites  of  Maine,  as  at  Hebron,  Paris,  and  Auburn,  where  it  is 
associated  with  spodumene,  lepidolite,  and  tourmaline ;  also  in 
North  Carolina,  and  at  Pala,  San  Diego  County,  California,  with 
the  same  associated  minerals. 

Chemically  some  sodium  may  replace  the  lithium,  when  the 
flame  will  be  mixed  with  yellow.  There  has  been  a  variety  de- 
scribed in  which  sodium  occurs  alone,  without  lithium,  forming  a 
sodium  amblygonite. 

OLIVENITE 

Olivenite.  —  Cu2(OH)As04 ;  Basic  copper  arsenate  ;  CuO  = 
56.1,  As2O5  =  40.7,  H20  =  3.2;  Orthorhombic ;  Type,  Didigonal 
Equatorial ;  a  :  b  :  c  =  .9396  :  1 :  .6726  ;  100  A  110  =  43°  13' ;  001  A 
101  =  35°  36' ;  001  A  Oil  =  33°  55' ;  Common  forms,  a  (100),  b  (010), 
m  (110),  e  (Oil),  v  (101) ;  Cleavage,  m,  b,  and  e  in  traces  ;  Brittle ; 
Fracture,  uneven  ;  H.  =  3  ;  G.  =  4.1 ;  Color,  shades  of  dark  green 
and  brown ;  Streak,  green  or  brown ;  Luster,  vitreous  ;  Translucent 
to  opaque. 

B.B.  *—  Fuses  easily,  yielding  a  bluish  green  flame.  On  coal 
yields  an  arsenical  odor,  and  after  roasting  and  reducing  with  soda, 
borax,  and  coal  dust  yields  metallic  copper  ;  in  the  closed  tube  yields 
water.  Soluble  in  nitric  acid. 

General  description.  —  Crystals  small,  acicular,  prismatic,  or 
fibrous  aggregates  with  a  velvety  surface.  The  brown  varieties 
are  known  as  "  wood  copper."  Olivenite  is  a  secondary  mineral 
deposited  in  veins  or  cavities,  associated  with  quartz  in  the  oxidized 
zone  of  copper  mines.  Found  in  the  United  States  in  the  Tintic 
district  of  Utah. 

LIBETHENITE 

Libethenite.  —  Cu2(OH)PO4  is  the  phosphate  of  copper  isomor- 
phous  with  olivenite,  and  the  two  are  therefore  often  crystallized 
together.  It  differs  from  olivenite  in  that  the  cold  nitric  acid 
solution  yields  a  yellow  precipitate  with  ammonium  molybdate, 

(PA). 

Adamite,  Zn2(OH)As04,  is  the  zinc  member  of  the  series;  it  is 
associated  with  libethenite  in  the  old  zinc  mines  of  Laurium  in 
Greece. 

2L 


514  MINERALOGY 

DESCLOIZITE 

Descloizite.  —  ZnPb(OH)VO4;  A  basic  vanadate  of  lead 
and  zinc;  PbO  =  55.4,  ZnO  =  19.7,  V2O5  =  22.7,  H20  =  2.2 ; 
Orthorhombic  ;  Type,  Didigonal  Equatorial ;  a :  b  :  c  =  0.6368  : 
1:0.8045;  100  A  lib  =  32°  29' 40"  ;  001  A  101  =  51°  38';  001*011  = 
38°  49' ;  Common  forms,  a  (100),  m  (110),  b  (010),  o  (111),  f  (201) ; 
Cleavage,  none ;  Brittle ;  Fracture,  uneven  ;  H.  =  3.5  ;  G.  =  5.9  - 
6.2 ;  Color,  orange-red,  cherry-red,  also  shades  of  brown  to  black ; 
Streak,  orange  to  brownish ;  Luster,  greasy ;  Transparent  to  opaque. 

B.B.  —  Fuses  easily ;  when  reduced  with  soda,  etc.,  on  coal  yields 
malleable  lead  buttons  or  a  lead  coat.  Yields  a  green  bead  with  the 
fluxes  in  R.  F.  and  water  in  the  closed  tube.  Easily  soluble  in  cold 
dilute  nitric  acid  which  yields  tests  for  vanadium,  page  576. 

General  description.  —  Crystals  are  small  prisms  or  pyramids, 
forming  drusy  surfaces  on  crusts  ;  more  often  amorphous,  powdery 
or  earthy.  The  lead  and  zinc  may  be  replaced  by  manganese  or 
iron  and  at  times  some  of  the  V2O5  is  replaced  by  As208 ;  several  such 
compounds  have  received  separate  names. 

Descloizite  is  found  at  Tombstone  and  various  other  localities  in 
Arizona  and  New  Mexico ;  also  at  Leadville,  Colorado ;  and  small 
quantities  have  been  taken  from  the  Wheatley  mine  at  Phoenix- 
ville,  Pennsylvania.  Vanadium  minerals  are  at  the  present  time 
very  valuable,  as  the  vanadium  is  used  in  the  manufacture  of 
vanadium  steel.  A  very  small  quantity  of  vanadium  added  to  steel 
increases  the  toughness  and  the  elastic  limit  without  decreasing 
its  ductility.  Nickel  accomplishes  the  same  result,  but  vanadium 
is  nearly  twenty  times  as  effective. 

CARNOTITE 

Carnotite.  —  KUO2VO4 .  1J  H2O ;  a  potassium  uranyl  vanadate ; 
K2O  =  10.37,  U02  =  63.54,  V205  =  20.12,  H2O  =  5.95. 

A  light  canary-yellow  mineral,  disseminated  as  a  yellow  powder 
through  sandstones  in  Montrose,  San  Miguel,  and  Mesa  counties, 
Colorado,  and  the  adjacent  counties  of  Utah.  It  is  easily  soluble 
in  acids  and  yields  reactions  for  uranium  and  vanadium.  It  is  a 
valuable  mineral,  not  only  for  the  large  percentages  of  uranium  and 
vanadium  it  contains,  but  also  for  the  radium,  which  is  associated 
with  the  uranium. 


COLUMBATES,   PHOSPHATES,  VANADATES  515 

DUFRENITE 

Dufrenite.  —  Fe2(OH)3P04 ;  a  basic  orthophosphate  of  iron ; 
=  62.0,  P2O5  =  27.5 ;  H20  =  10.5 ;  Orthorhombic ;  Type,  ? 
a:b:c  =  .8734:1:. 4262;  100  A  110  =  41°  8';  Oil  A  Oil  =46° 
10';  001_A.101  =  26°  I']  001  A  Oil  =  23°  5';  Common  forms 
a  (100),  b(010),  m(110),  e(011);  Cleavage,  macropinacoidal 
distinct ;  Brittle ;  Fracture,  uneven ;  H.  =  3.5-4 ;  G.  =  3.2-3.4  ; 
Color,  dark  green  to  nearly  black,  brown  and  yellow  by  oxidation ; 
Streak,  green ;  Luster,  dull  to  silky ;  Subtranslucent  to  opaque. 

B.B.  —  Fuses  in  R.  F.  and  becomes  magnetic.  Powdered  and 
treated  with  H2SO4  yields  a  green  flame  in  O.  F.,  or  dissolved  in 
nitric  acid  shows  phosphoric  acid  with  ammonium  molybdate. 
Yields  water  in  the  closed  tube  and  only  iron  reactions  with  the 
fluxes. 

General  description.  —  Crystals  are  small  and  cubical  in  appear- 
ance, but  very  rare,  generally  occurring  as  radiated  or  fibrous 
masses  with  drusy  surfaces. 

Dufrenite  is  a  secondary  mineral,  precipitated  from  solutions 
and  associated  with  limonite  deposits,  as  at  Rockbridge  County, 
Virginia.  It  is  also  found  as  a  crust  in  the  green  sand  formations 
of  Allentown,  New  Jersey.  By  oxidation  it  becomes  brown  and 
by  loss  of  P206  forms  limonite. 

LAZULITE 

Lazulite.  —  (Fe .  Mg) A12(OH)2(PO4)2 ;  when  Fe  :  Mg:  :  1 :  2,  FeO 
=  7.7,  MgO  =  8.5,  A12O3  =  32.6,  P2O6  =  45.4,  H20  =  5.8  ; 
Monoclinic ;  Type,  Digonal  Equatorial ;  a  :  b  :  c  =  .9749  :  1 : 
1.6483  ;  p  =  89°  13'  =  001  A  100 ;  100  A  110  =  44°  16' ;  001  A  101  = 
58°  49';  001  A  Oil  =  58°  45';  Common  forms,  P(lll),  e  (111), 
t  (101) ;  Twinning  plane,  100 ;  Cleavage,  prismatic  distinct ;  Brit- 
tle; Fracture,  uneven ;  H.  =  5-6;  G.  =  3.06-3.12;  Color,  azure- 
blue;  Streak,  white;  Subtranslucent  to  opaque;  a  =  1.603; 
P  =  1.632;  «y  =  1-639;  -y  -  a  =  .036;  Optically  (-);  Axial 
plane  parallel  to  010;  BxaAC  =  9°  20'-9°  45'  behind;  2*E  = 
132° ;  Pleochroism  strong. 

B.B.  —  Swells,  whitens,  and  crumbles.  The  powdered  mineral 
moistened  with  H2SO4  yields  a  green  flame  (P2O5)  or  fused  with 


516  MINERALOGY 

soda  and  dissolved  in  HNO3  yields  a  yellow  precipitate  with  ammo- 
nium molybdate  (P208).  Becomes  blue  when  treated  with  cobalt 
solution  (Al).  Insoluble  in  acids. 

General  description.  —  Crystals  are  plus  and  minus  unit  pyra- 
mids, or  combinations  of  these  with  the  unit  orthodome ;  other 
forms  are  rare.  More  often  massive  or  granular,  and  associated 
with  quartz  and  cyanite  in  slates.  At  Crowder  Mountain,  Gaston 
County,  North  Carolina,  it  is  associated  with  corundum ;  at  Graves 
Mountain,  Georgia,  it  occurs  in  fine  sky-blue  crystals,  an  inch  or 
more  in  length,  associated  with  rutile  and  cyanite.  Crystals  six 
inches  long  occur  in  pockets  of  quartzite  in  Wermland,  Sweden. 

VIVIANITE 

Vivianite.  —  Fe3(PO4)2 .  8  H2O ;  Hydrous  ferrous  phosphate ; 
FeO  =  43.0,  P205  =  28.3,  H20  -  28.7 ;  Monoclinic ;  Type, 
Digonal  Equatorial ;  a  :  b  :  c  =  .7498  :  1 :  .7016  ;  p  =  75°  34'  = 
001  A  100;  100  A  110  =  35°  59';  001  A  101  =49°  46';  001*011=34° 
11' ;  Common  forms,  a  (100),  b  (010),  m  (111),  n  (101) ;  Cleavage, 
clinopinacoidal  perfect,  almost  micaceous,  very  thin  laminae 
slightly  flexible  and  sectile ;  H.  =  1.5-2;  G.  =  1.58-2.68;  Color, 
blue  to  green;  Streak,  white  to  blue,  darkens  on  exposure; 
Luster,  vitreous  to  pearly ;  Transparent  to  opaque. 


FIG.  517. — Vivianite.      Leadville,  Colorado.    The  smaller  specimen  is  from  Red 

Bank,  N.J. 


COLUMBATES,   PHOSPHATES,   VANADATES  517 

B.B.  —  Fuses  easily  at  1.5,  and  yields  a  green  flame  (P205).  In 
R.  F.  blackens  and  becomes  magnetic.  Yields  water  in  the  closed 
tube.  Dissolves  in  acids, 

General  description.  —  Crystals  are  prismatic  and  flattened 
parallel  to  the  orthopinacoid,  which  is  often  rounded  and  striated 
lengthwise;  also  in  interpenetrating  stellate  groups,  or  radiated, 
encrusted,  friable,  and  earthy ;  at  times  replacing  shells  and  roots 
and  fossil  bones,  and  is  then  known  as  bone  turquoise. 

Vivianite  is  a  secondary  mineral  formed  by  the  action  of  solu- 
tions containing  ferrous  iron  on  apatite  or  other  calcium  phosphates 
of  organic  origin.  It  occurs  in  the  clays  and  gravel  beds  as  blue 
nodules  at  several  localities  in  Monmouth  County,  New  Jersey. 
It  is  also  associated  with  limonite,  as  at  Stafford  County,  Virginia ; 
replacing  roots  in  clay  at  Eddyville,  Kentucky.  Groups  of  large 
crystals  are  obtained  at  Leadville,  Colorado. 


ERYTHRITE 

Erythrite.  —  Red  Cobalt ;  Cobalt  bloom ;  Hydrous  arsenate  of 
cobalt,  Co3(A04)2 . 8  H20 ;  CoO  =  37.5,  As2O5  =  38.4,  H2O  = 
24.1;  Monoclinic;  Type,  Digonal  Equatorial ;  a  :  b  :  c  =  .7937  : 
1 :  .7356 ;  p  =  74°  51' ;  Common  forms,  a  (100),  b  (010),  m  (110)  ; 
Cleavage,  b  perfect,  a  and  w  (101)  distinct,  very  thin  laminae  flex- 
ible and  sectile ;  H.  =  1.5-2.5;  G.  =2.95;  Color,  crimson  or  shades 
of  red  and  pink,  sometimes  grayish ;  Streak,  pale ;  Luster,  pearly 
on  cleavage  faces ;  Transparent  to  translucent. 

B.B.  —  Fuses  on  coal  to  a  dark  globule  and  yields  an  arsenic 
odor.  In  the  closed  tube  darkens  and  yields  water.  With  borax 
yields  a  blue  bead  (Co).  Dissolves  in  HC1  to  a  rose-colored  solu- 
tion. 

General  description.  —  Crystals  prismatic,  in  stellate  or  radiated 
aggregates;  in  drusy  crusts  or  earthy.  Associated  with  cobalt 
ores  as  an  oxidation  product.  Beautiful  radiated  specimens  are 
obtained  at  Schneeberg  in  the  Harz  and  at  Freiberg  in  Saxony. 
It  is  found  as  drusy  crusts  and  as  an  eartHy  powder  associated  with 
the  cobalt  ores  of  Cobalt,  Ontario. 

The  corresponding  nickel  mineral,  Annabergite,  Ni3(As04)2 .  8H2O, 
is  apple  green  and  is  also  found  associated  with  the  ores  of  cobalt 
and  nickel  as  an  oxidation  product. 


518  MINERALOGY 

WAVELLITE 

Wavellite.  —  A13(OH)3(PO4)2 .  5  H2O ;  hydrous  basic  aluminium 
orthophosphate;  A1203  =  38.0,  P2O5  =  35.2,  H20  =  26.8;  Or- 
thorhombic ;  Type,  Didigonal  Equatorial  a  :  b  :  c  =  .5573  :  1  : 
.4084 ;  100  A  110  =  26°  47' ;  001  A  101  =  36°  36' ;  001  A  Oil  =  20° 
33';  Common  forms,  m  (110),  b  (010),  p  (101) ;  Cleavage,  p  and  b 
quite  perfect;  Brittle;  Fracture,  uneven;  H.  =  3.25^;  G.  = 
2.3-2.33;  Color,  shades  of  green,  yellow,  white  to  gray,  when 
impure  brown  to  black;  Streak,  white;  Translucent  to  opaque; 
Luster,  vitreous  to  pearly ;  -y  —  a  =  .025 ;  Optically  (  +  ) ; 
Axial  plane  =  100;  Bxa  =  c;  2E  =  127°  2'. 

B.B.  —  Infusible,  but  whitens  and  crumbles  somewhat ;  becomes 
blue  with  cobalt  solution.  The  nitric  acid  solution  yields  a  yellow 
precipitate  with  ammonium  molybdate  (P205).  Soluble  in  hot 
strong  acids. 

General  description.  —  Separate  crystals  are  very  rare ;  usually 
occurs  in  concretionary  masses  with  drusy  surfaces  and  radiated 
structure ;  also  stalactitic  or  in  crusts.  Small  amounts  of  iron  or 
manganese  may  replace  the  aluminium,  and  while  it  is  not  recog- 
nized in  the  formula,  fluorine  is  nearly  always  present. 

Evansite,  A13(OH)6P04 .  6  H20,  peganite,  A12(OH)3PO4 .  1|  H20, 
and  sphaerite,  A15(OH)9(P04)2 .  3^  H2O,  are  other  phosphates  of 
aluminium  with  varying  amounts  of  water,  very  closely  related  to 
wavellite. 

They  are  all  secondary  minerals  produced  by  the  interaction  of 
percolating  waters,  containing  phosphates  in  solution,  with  argil- 
laceous shales  and  slates,  in  the  cracks  and  cavities  of  which  they 
occur,  never  forming  deposits  of  great  extent. 

Wavellite  is  associated  with  limonite  at  White  Horse  Station, 
Pennsylvania;  at  Magnet  Cove,  Arkansas,  in  fine  green  radiated 
aggregates. 

TURQUOISE 

Turquoise.  —  CuO  .  3  A1203 .  2  P205 .  9  H20 ;  CuO  =  9,  A120  = 
36.50,  P2O5  =  34.13,  H2O  =  20.12;  Triclinic  £  a :  b :  c  =  .791: 1:  .605; 
a  =  92°  58',  P  =  9_3°  30',  y  =  107°  41';  010  A  100  =  44°  50';  100 
A  110  =  31°  10;  011*110  =  105°  36';  Forms  b  (010),  a  (100), 
m(110),  M(110),  k(011);  Cleavage,  marked;  Brittle;  Fracture, 


COLUMBATES,  PHOSPHATES,  VANADATES  519 

conchoidal ;  H.  =  6 ;  G.  =  2.6-2.86 ;  Color,  sky-blue  to  apple- 
green  ;  Streak,  white  to  pale  green ;  Luster,  waxy ;  Subtranslucent 
to  opaque;  a  =  1.61,  v  =  1.65,  y  —a,  =  .04. 

B.B.  —  Infusible,  becoming  brown  and  glassy,  yields  a  green 
flame.  The  S.  Ph.  bead  reduced  with  tin  shows  copper.  In  the 
closed  tube  yields  water. 

General  description. — Crystals  of  Turquoise  were  discovered  for 
the  first  time  at  Lynch  Station,  Campbell  County,  Virginia,  and  de- 
scribed within  the  year.  They  were  very  small  and  scarce.  Usually 
occurs  in  amorphous  masses  filling  small  veins  in  altered  porphy- 
ries. In  the  United  States  turquoise  is  mined  extensively  at 
Mineral  Park,  Mohave  County,  Arizona;  Washoe  County,  Ne- 
vada ;  Burro  Mountains,  New  Mexico. 

The  blue  color  is  due  to  copper,  and  in  many  specimens  fades  on 
exposure.  The  finest  gem  turquoise  is  found  near  Mishapur, 
Persia.  Turquoise  has  been  used  as  a  gem  from  remote  ages,  as 
the  bracelets  discovered  at  El  Mehesna,  the  oldest  known  jewels, 
contained  beads  of  turquoise  alternated  with  beads  of  gold. 

The  American  material  is  slightly  soft  and  porous,  which  affects 
the  polish. 

Variscite.  —  A1P04  .  2  H2O ;  a  crystalline  hydrous  phosphate  of 
aluminium,  very  similar  to  turquoise  in  color,  but  lighter  green  to 
deep  emerald  green.  It  contains  no  copper  and  occurs  in  south- 
western Utah,  where  it  is  found  as  nodules  contained  in  a  lime- 
stone, associated  with  jade,  chalcedony,  and  limonite. 

TORBERNITE 

Torbernite.  —  Hydrous  uranyl  phosphate  of  copper,  Cu(U02)2- 
(PO4)2 .  8  H20;  Cu  =  8.4,  U03  =  61.2,  P205  =  15.1,  H2O  = 
15.3;  Tetragonal;  6  =  2.9361;  001*101=71°  11';  Forms, 
c(001),  m(110),  a  (100),  e  (101) ;  Cleavage,  basal  micaceous; 
Brittle;  H.  =  2-2.5;  G.  =  3.4-3.6;  Color,  emerald  or  siskin-green ; 
Streak,  pale  green ;  Luster,  pearly ;  Transparent  to  opaque. 

B.B.  —  Fuses  at  2.5  to  a  black  slag  and  yields  water  in  the  closed 
tube.  Reduced  with  soda,  etc.,  on  coal,  yields  copper  buttons. 
The  nitric  acid  solution  shows  phosphoric  acid  with  ammonium 
molybdate,  also  yields  tests  for  uranium,  page  576. 


520  MINERALOGY     • 

General  description.  —  Crystals  are  thin  plates  or  tabular;  also 
in  foliated  and  micaceous  aggregates.  Chemically  some  arsenic  may 
replace  the  phosphoric  acid.  Zeunerite,  Cu(UO2)2(As04)2 . 8  H2O, 
is  the  arsenic  mineral  isomorphous  with  torbernite  and  very  simi- 
lar to  it,  except  in  color,  which  is  lemon  or  sulphur  yellow. 

Autunite,  Ca(U02)2(PO4)2 .  8  H2O,  is  a  member  of  the  same 
group,  but  orthorhombic  and  lemon  or  sulphur  yellow  in  color. 

All  three  minerals  are  secondary  oxidation  products  associated 
with  uranium  deposits  of  Joachimsthal,  Bohemia.  Torbernite  is 
associated  in  small  amounts  with  the  carnotite  at  Richardson, 
Utah. 

NITRATES 

SODA   NITER 

Soda  Niter.  —  Chili  Saltpeter,  NaNO3 ;  Nitrate  of  soda ;  Na2O 
=  36.5,  N2O5  =  63.5;  Hexagonal;  Type,  Dihexagonal  Alternat- 
ing; c  =  .8276;  0001  A  1011  =  43°  42';  r*r'=  73°  30';  Cleavage, 
rhombohedral  perfect ;  Fracture,  conchoidal ;  Brittle  ;  H.  =  1.5-2 ; 
G.  =  2.24-2.29 ;  Color,  white,  gray,  red,  brown,  or  yellow ;  Trans- 
parent; CD  =  1.587;  €  =  1.336;  CD  -  €  =  .251;  Optically  (-). 

B.B.  —  Deflagrates  on  coal,  colors  the  flame  intensely  yellow 
(Na).  Has  a  cooling  taste,  easily  soluble  in  water  and  yields  reac- 
tions for  nitrogen,  page  590. 

General  description.  —  Crystals  are  rare,  usually  in  beds,  crusts, 
or  granular.  It  is  isomorphous  with  calcite,  though  differing 
from  it  entirely  chemically.  All  nitrates  are  very  soluble  in  water, 
and  their  occurrence  in  nature  is  therefore  restricted  to  arid  regions 
or  to  caves  where  little  water  has  access.  Nitrates  are  formed  in 
the  soils  by  the  oxidation  of  organic  matter  through  the  action  of 
certain  bacteria.  Nitrates  are  carried  in  the  ground  water  and 
serve  as  a  supply  for  growing  vegetation.  Nitrogen  is  one  of  the 
most  expensive  as  well  as  the  most  important  plant  foods,  and  for 
this  reason  the  nitrate  deposits  of  Chili  are  of  enormous  commercial 
importance,  as  they  are  the  only  extensive  deposits  of  nitrogen 
salts  in  the  world.  The  origin  of  these  beds  has  as  yet  not  been 
satisfactorily  explained.  They  may  have  been  deposited  by 
evaporating  solutions,  by  volcanic  action,  or  by  decaying  organic 
materials.  They  extend  over  an  area  of  many  square  miles  in 


COLUMBATES,   PHOSPHATES,   VANADATES  521 

northern  Chili,  southern  Peru,  and  Bolivia.  The  nitrate  is  asso- 
ciated with  salt,  gypsum,  glauber  salts,  and  generally  borax. 
Small  amounts  occur  in  Humboldt  County,  Nevada,  and  in  San 
Bernardino  County,  California. 


NITER 

Niter.  —  Saltpeter,  Potassium  Nitrate,  KNO3 ;  K20  =  46.5, 
N206  =  53.5 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  & :  B  :  c 
=  .5843  :  1 :  .7028 ;  100  A  110  =  39°  35' ;  001  A  101  =  49°  30' ;  001  A 
Oil  =  35°  2';  Common  forms,  b  (010),  m(110),  t  (021),  o(lll), 
q(011);  Cleavage,  Oil  perfect,  010  and  110  imperfect;  Brittle; 
Fracture,  uneven;  H.  =  2;  G.  =  2.09-2.14;  Color  and  streak, 
white;  Luster,  vitreous;  Subtranslucent ;  a  =  1.334;  p  =  1.505; 
7  =  1.506;  "y  -  a  =  .172;  Optically  (-);  Axial  plane  =  100; 
Bxa  =  b;  2E  =  8°  40'. 

B.B.  —  Deflagrates  on  coal,  yielding  a  violet  flame  (K),  has  a 
cooling  saline  taste.  Easily  soluble  in  water,  the  solution  yields 
reactions  for  nitrogen,  page  590. 

General  description.  —  Crystals  are  acicular,  forming  crusts  or 
silky  tufts,  as  efflorescent  crusts  in  dry  regions.  It  is  an  oxidation 
product  found  in  soils,  the  result  of  the  action  of  certain  nitrifying 
bacteria;  the  nitric  acid  thus  formed  combines  with  the  bases, 
with  potassium  to  form  niter,  or  with  calcium  to  form  nitrocal- 
cite,  Ca(NO3)2 .  n  (H20). 

Niter  is  dimorphous,  and  in  each  form  is  isomorphous  with  the 
two  forms  of  calcium  carbonate,  calcite  and  aragonite.  The  or- 
thorhombic  form  here,  however,  is  stable  at  ordinary  temperatures. 
The  hexagonal  phase  of  soda-niter  is  stable  at  ordinary  tempera- 
tures and  forms  the  natural  occurring  salt. 

Niter  is  of  great  commercial  importance  both  as  a  fertilizer  and 
in  the  manufacture  of  gunpowder,  the  natural  supply  being  so 
limited  that  the  salt  is  formed  from  Chili  saltpeter  by  the  interac- 
tion of  potassium  chloride. 

Niter  is  associated  to  some  extent  with  the  sodium  salts  in  the 
Chili  nitrate  beds.  It  also  occurs  as  an  impregnation  in  the  earth 
on  the  floors  of  some  of  the  caves  in  Kentucky  and  Tennessee.  It 
is  often  obtained  by  lixiviating  such  soils. 


522  MINERALOGY 

BORACITE 

Boracite.  —  Mg7Cl2Bi603o ;  A  chloride  and  borate  of  magnesium ; 
MgO  =  31.4,  Cl  =  7.9,  B2O3  =  62.5;  Pseudo-isometric;  Type, 
Ditesseral  Polar;  Common  forms,  a  (100),  d(110),  ±o(lll); 
Twinning  plane,  111;  Cleavage,  111  traces;  Fracture,  conchoid  al ; 
Brittle ;  H.  =  7 ;  G.  =  2.9-3 ;  Color,  white,  gray,  pale  yellow,  or 
green;  Streak,  white;  Transparent  to  translucent;  Double  re- 
fraction below  265° ;  a  =  1.662;  p  =  1.667;  <y  =  1.673;  <y  -  a  = 
Oil. 

B.B.  —  Fuses  easily  at  2,  intumesces  and  yields  a  green  flame. 
With  cobalt  solution  a  flesh  color  (Mg).  Yields  a  chlorine  reaction 
with  CuO,  soluble  in  HC1.  The  massive  variety  yields  water. 

General  description.  —  Crystals  cubic  or  tetrahedral  in  habit ; 
generally  isolated  simple  crystals,  or  massive  and  granular.  The 
faces  of  the  plus  tetrahedron  are  bright  and  highly  polished,  while 
those  of  the  minus  form  are  dull.  The  polar  nature  of  boracite  is 
brought  out  on  heating,  when  the  plus  and  minus  tetrahedrons  show 
opposite  polarity. 

Boracite  is  dimorphous,  the  orthorhombic  form  is  stable  at  ordi- 
nary temperatures  and  passes  over  to  the  isometric  form  at  265°. 
The  beautifully  formed  isometric  crystals  at  ordinary  temperatures 
are  complex  aggregates  of  orthorhombic  twins  and  twinning  lamel- 
lae, as  is  shown  by  their  double  refraction,  all  of  which  disappear 
when  heated  to  265°,  at  which  temperature  the  mineral  becomes 
isotropic,  and  even  the  polarity  disappears.  Chemically  magne- 
sium may  be  replaced  by  ferrous  iron,  as  in  the  green  boracites. 
Artificial  boracite  has  been  formed  in  which  Zn,  Cd,  or  Ni  enter  as 
the  base,  while  the  chlorine  has  been  replaced  with  iodine. 

Boracite  occurs  in  the  Stassfurt  salt  deposits,  imbedded  in  the 
gypsum,  anhydrite,  and  carnallite. 

Stassfurtite  is  a  massive  variety,  which  is  softer  than  the  crys- 
talline and  yields  water  in  the  closed  tube. 

BORAX 

Borax.  —  Na^Oy .  10  H2O ;  Hydrous  sodium  pyroborate ; 
Na/)  =  16.23,  B2O3  =  36.65,  H2O  =  47.12 ;  Monoclinic ;  Type, 
Digonal  Equatorial ;  a  :  b  :  c  <=  1.0995  :  1 :  .5632 ;  p  =  73°  25'  = 


COLUMBATES,  PHOSPHATES,  VANADATES  523 

001  A  100 ;  1001  A  10  =  46° 30' ;  001  A  101  =  29° 54' ;  001  A  Oil  =  28° 
21';  Common  forms,  a  (100),  c  (001),  b  (010),  n  (110),  o  (111), 
z(221);  Twinning  plane  a;  Cleavage,  orthopinacoidal  perfect, 
m  less  so;  Brittle;  Fracture,  conchoidal ;  H.  =  2-2.5 ;  G.  =  1.69- 
1.72;  Color,  white,  gray,  or  pale  blue  or  green;  Streak,  white; 
Luster,  vitreous  to  earthy;  Transparent  to  opaque;  a  =  1.446; 
p  =  1.469;  7  =  1.472;  V  -  a  =  .026;  Optically  (  -  );  Axial 
plane  -L  010 ;  Bx  A  c  =  71°  35'  in  the  obtuse  angle ;  2  E  =  59°  18'. 

B.B.  —  Fuses  easily  with  intumescence  to  a  clear  bead.  With 
Turner's  flux  yields  a  green  flame.  Soluble  in  water,  the  solution 
reacts  for  boric  acid  with  turmeric  paper.  Has  a  sweetish,  alkaline 
taste. 

General  description.  —  Crystals  prismatic  in  habit,  often  large, 
some  from  the  California  lake  regions  weigh  as  much  as  a  pound 
each.  More  often  granular,  earthy,  and  impure.  This  crude, 
impure  borax  is  sold  in  the  trade  as  tincal,  and  was  first  brought 
from  the  borax  lakes  of  Tibet.  Owing  to  its  solubility  in  water, 
borax  is  found  associated  with  deposits  formed  by  the  concentration 
of  the  waters  of  certain  lakes.  Such  lakes  are  found  in  the  West,  as 
in  San  Bernardino  County,  California,  where  the  borax  is  associated 
with  gypsum,  anhydrite,  thenardite,  glauberite,  hanksite,  halite, 
colemanite,  and  trona.  It  is  also  found  in  the  saline  lakes  and 
marshes  of  Nevada  and  Oregon,  and  at  other  localities  in  Cali- 
fornia ;  associated  with  the  soda  niter  in  Chili  and  the  salt  deposits 
of  Stassfurt,  Germany ;  in  the  hot  springs  of  the  Yellowstone  Park 
and  in  sea  water.  The  origin  of  the  borax  in  all  these  deposits  is 
uncertain ;  at  Stassfurt  it  has  been  by  concentration  of  the  solu- 
tions, while  in  numerous  other  localities  its  origin  may  be  traced 
tq  volcanic  and  fumarole  action.  Sassolite,  H3BO3,  orvthe  ortho- 
boric  acid,  is  found  in  solution  in  the  waters  issuing  from  the 
fumaroles  of  Tuscany;  from  these  solutions  it  is  recovered  by 
evaporation. 

Owing  to  the  varied  properties  of  borax,  it  finds  many  and  widely 
different  uses.  When  fused  it  dissolves  the  oxides  of  the  metals, 
yielding  characteristically  colored  glasses  which  form  the  color  base 
in  stained  glass.  It  is  used  in  soldering  to  dissolve  the  oxides  from 
the  surface  of  the  metals ;  as  a  flux  in  the  melting  and  purification 
of  the  precious  metals ;  in  the  manufacture  of  enamel  and  granite 
ware  and  in  encaustic  tiles.  From  its  cleansing  powers  it  is  used 


524  MINERALOGY 

in  making  soap,  and  from  its  preservative  properties  it  is  used  in  the 
canning  of  meats  and  vegetables  and  to  prevent  fermentation  in 
milk. 

ULEXITE 

Ulexite.  —  NaCaB507 .  8  H20 ;  a  hydrous  sodium  calcium  bo- 
rate;  Na^O  =  7.7,  CaO  =  13.8,  B2O3  =  43.0,  H2O  =  35.5; 
Monoclinic,  elements  or  angles  not  determined;  H.  =  1 ;  G.  =  1.65; 
Color  and  streak,  white ;  Luster,  silky ;  Translucent. 

B.B.  —  Fuses  easily  with  intumescence  to  a  clear  glass,  and 
colors  the  flame  yellow  (Na).  With  Turner's  flux  yields  a  green 
flame  (B).  In  the  closed  tube  yields  water,  and  after  ignition  reacts 
alkaline  with  turmeric  paper.  Insoluble  in  cold  water,  but  slightly 
soluble  in  hot  water,  yielding  an  alkaline  solution. 

General  description.  —  Crystals  very  fine  and  fibrous,  in  round 
masses,  with  a  loose,  porous  texture,  commonly  known  as  snowball 
or  cotton  mineral  by  the  prospectors.  It  forms  around  the 
cracks  or  small  holes  where  the  solutions  escape  at  the  surface  by 
evaporation.  It  is  the  common  borate  at  Teel's,  Rhodes',  and 
Columbus  marshes,  Nevada;  also  in  Inyo  and  San  Bernardino 
counties,  California,  which  are  practically  a  continuation  of  the 
Nevada  field.  Ulexite  is  also  associated  with  soda  niter  in  the 
deposits  of  the  dry  deserts  of  Tarapaca  and  Atacama,  northern 
Chili.  It  is  probably  formed  by  the  interaction  of  solutions  con- 
taining borax  and  calcium  bicarbonate. 

In  commerce  it  is  used  in  the  manufacture  of  borax  and  boric 
acid. 

COLEMANITE 

Colemanite.  —  CasBgOii .  5  H2O ;  a  hydrous  calcium  borate; 
CaO  =  27.2,  B203  =  50.9,  ^  H2O  =  21.9 ;  Monoclinic ;  Type, 
Digonal  Equatorial ;  a  :  b  :  c  =  .7748  :  1 :  .5410 ;  p  =  69°  51' 
=  001*_100;  100 A  110  =  36°  2';  001  A  101  =  42°;  001  A  Oil  =  26° 
55' ;  001  A  111  =  33°  45' ;  001  A  110  =  73°  49' ;  Common  forms, 
a  (100)r  b  (010),  c(001),  m(110),  P  (111),  h  (201) ;  Cleavage,  b 
perfect,  c  distinct ;  Brittle ;  Fracture,  uneven ;  H.  =  4-4.5  ; 
G.  =  2.42 ;  Color,  white,  yellowish,  or  gray ;  Streak,  white  ; 
Luster,  vitreous;  Transparent  to  translucent;  a  =  1.586;  p  = 
1.592;  7  =  1.614;  ?  -  a  =  .028;  Optically  (  +  ) ;  Axial  plane 
J-010;  BxaAC  =  83°  in  front;  2E  =  95°  15';  2V  =  54°  52'. 


COLUMBATES,  PHOSPHATES,  VANADATES  525 

B.B.  —  Exfoliates,  decrepitates,  and  colors  the  flame  green. 
Soluble  in  HC1,  yielding  a  boric  acid  reaction  with  turmeric  paper. 

General  description.  —  Crystals  prismatic  combinations  of 
a,  b,  c,  and  m,  other  forms  less  prominently  developed.  Beautiful 
groups  of  highly  lustrous  crystals  associated  with  quartz  occur  in 
geodes  of  the  massive  mineral,  to  which  the  terms  priceite  and 
pandermite  have  been  applied. 

Colemanite  was  first  discovered  in  the  Death  Valley,  California ; 
it  also  occurs  in  Nevada,  Oregon,  and  Chili. 

It  is  the  most  abundant  borate  in  the  California  locality,  and  is 
the  source  from  which  borax  and  borates  are  derived.  The  product 
or  pulp  is  treated  with  sulphuric  or  sulphurous  acid,  precipitating 
the  calcium,  leaving  the  boric  acid  in  solution,  which  is  recovered 
in  the  crude  form  by  evaporation. 

URANINITE 

Uraninite.  —  Pitchblende,  U3O8  or  (U02)(U03)2;  Uranoso- 
uranic  oxide ;  Composition,  variable ;  Isometric ;  Type,  Ditesseral 
Central;  Common  forms  o(lll),  d(110),  sometimes  a  (100) ; 
Cleavage,  none ;  Brittle ;  Fracture,  conchoidal ;  H.  =  3-6 ;  G.  = 
9-9.7 ;  Color,  pitch-black  or  greenish ;  Streak,  olive-green,  brown 
to  gray ;  Luster,  greasy,  pitchy,  or  dull  submetallic ;  Opaque. 

B.B. — Infusible ;  with  borax  or  S.  Ph.  a  green  bead  in  R.  F.  (U). 
Yields  tests  for  uranium,  page  576. 

General  description.  —  Crystals  are  combinations  of  the  octa- 
hedron, rhombic  dodecahedron,  and  at  times  the  cube ;  they  are 
rare ;  it  usually  occurs  massive  or  amorphous,  pitchlike  in  appear- 
ance. Its  chemical  composition  is  variable,  some  lead  is  almost 
always  present,  and  calcium,  copper,  iron,  arsenic,  bismuth,  tho- 
rium, cerium,  and  the  yttrium  earths  may  also  enter  into  its  com- 
position. Cleveite,  a  variety  from  Arendal,  Norway,  contains  10 
per  cent,  of  the  yttrium  earths. 

It  was  in  uraninite  that  Klaproth,  in  1789,  discovered  uranium, 
and  Pe*ligot  separated  the  metal  in  1842,  Klaproth's  product  being 
the  oxide  UO2.  It  was  also  from  this  mineral  that  Madam  Curie 
separated  radium,  and  it  has  since  been  proven  that  radium  and 
helium  contained  in  the  mineral  are  the  results  of  the  breaking 


526  MINERALOGY 

down  of  uranium ;  pitchblende  is  therefore  radioactive,  and  is  at 
present  the  source  of  radium. 

Crystals  of  uraninite  occur  in  the  granites  and  pegmatites  of 
Moss  and  Arendal,  Norway ;  Cornwall,  England ;  and  in  the  feld- 
spar quarries  of  Middletown,  Connecticut.  The  massive  mineral 
is  associated  with  ore  veins  at  Joachimsthal  and  Schneeberg.  In 
the  United  States  it  is  found  in  Gilpin  County,  Colorado ;  in  the 
Black  Hills,  South  Dakota,  and  in  Llano  County,  Texas. 

It  is  the  present  source  of  radium  and  uranium. 

Uranium  compounds  are  used  in  the  laboratory  in  the  determi- 
nation of  phosphorus  and  zinc ;  in  photography ;  in  pottery  glazes  ; 
for  coloring  glass ;  in  the  manufacture  of  special  steels  for  use  in 
gun  barrels,  and  as  a  pigment. 

Gummite  is  an  alteration  product  of  uraninite,  of  a  light  reddish 
yellow  or  yellowish  brown  color,  containing  considerable  water. 
It  is  associated  with  the  pitchblende  of  Flat  Rock,  Mitchell  County, 
North  Carolina.  Some  of  the  rounded  masses  of  orange-red  color 
still  contain  a  nucleus  of  unaltered  uraninite. 


CHAPTER  XII 

SULPHATES,    CHROMATES,    TUNGSTATES,    AND 
MOLYBDATES 

THENARDITE 

Thenardite.  —  Anhydrous  sodium  sulphate,  Na2SO4 ;  Na^O  = 
56.3,  SO3  =  43.7 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ; 
a  :  b  :  c  =  .5976  :  1 :  1.2524 ;  100  A 110  =  30°  52' ;  001 A 101  =  64° 
30';  001 A  Oil  =  51°  24';  Common  forms,  c  (001),  a  (110),  m  (110), 
o(lll),  t(106),  r(101),  e(011);  Twinning  plane,  101,  also  Oil, 
crossed  twins  common;  Cleavage,  basal  distinct;  Fracture, 
uneven  ;  Brittle ;  H.  =  2-3  ;  G.  =  2.68-2.69 ;  Color,  white,  gray 
or  brownish ;  Streak,  white ;  Transparent  to  translucent ;  Opti- 
cally (+) ;  Axial  plane,  001 ;  Bxa  =  b  ;  2V  =  83°  32'. 

B.B.  —  Fuses  easily  and  colors  the  flame  yellow  (Na).  Easily 
soluble  in  water,  the  solution  yielding  a  heavy  white  precipitate 
with  barium  chloride  (BaS04).  After  ignition  reacts  alkaline 
with  turmeric  paper. 

General  description.  —  Crystalline  habit  is  pyramidal  or  short 
prismatic,  usually  combinations  of  the  unit  pyramid,  prism,  base, 
and  dome.  Owing  to  its  great  solubility  in  water  its  occurrence  is 
restricted  to  desert  lake  regions,  as  those  of  Siberia,  Chili,  Arizona, 
Nevada,  and  California. 

Sodium  sulphate  is  the  last  of  the  sulphates  to  crystallize  from 
solution  upon  evaporation.  The  anhydrous  sulphate  is  separated 
above  a  temperature  of  32°  C. ;  below  this  temperature  the 
decahydrate  mirabilite,  Na2S04 .  10  H20,  is  in  equilibrium  and 
separates  from  the  saturated  solution.  In  a  dry  atmosphere  this 
effloresces,  losing  its  water  and  passing  into  the  anhydrous  salt 
thenardite. 

Thenardite  is  associated  with  halite,  trona,  gypsum,  hanksite, 
borax,  and  other  minerals  characteristic  of  the  desert  lake  de- 
posits. 

527 


528  MINERALOGY 

GLAUBERITE 

Glauberite.  —  Na2Ca(SO4)2 ;  Sodium  calcium  sulphate;  Na2O  = 
22.3,  CaO  =  20.1,  S03  =  57.6;  Monoclinic;  Type,  Digonal 
Equatorial ;  a  :  b  :  c  =  1.2199  :  1 :  1.0275 ;  p  =  67°  49'  =  001 
100;  100A110  =  48°  29';  001 A 101  =  30°  37';  001 A011  =  43°  34'; 
110A111=32°  29';  Common  forms,  a  (100),  c(001),  m(110), 
s(lll);  Cleavage,  basal  perfect;  Fracture,  conchoidal;  Brittle; 
H.  =  2.5-3 ;  G.  =  2.7-2.85 ;  Color,  pale  yellow,  gray,  or  red  ; 
Streak,  white;  Luster,  vitreous;  Transparent  to  translucent; 
Optically  (-);  Axial  plane  J.  010;  Bxac  =  30°  46'  in  front; 
2E  =  11°. 

B.B.  —  Whitens,  decrepitates,  and  fuses  at  1.5,  coloring  the  flame 
intensely  yellow  (Na).  Dissolves  in  HC1;  the  solution  yields  a 
white  precipitate  with  barium  chloride  (BaS04).  Whitens  in 
water,  depositing  gypsum,  which  also  dissolves  in  an  excess. 
Has  a  bitter,  salty  taste. 

General  description.  —  Crystals  tabular  in  habit,  combinations 
of  c,  s,  m,  and  a ;  at  times  the  pyramid  predominates.  The  faces 
c  and  s  are  often  striated  parallel  to  their  intersection. 

Glauberite  is  associated  with  the  deposits  of  other  soluble  sodium 
salts,  as  at  Stassfurt,  Germany ;  Tarapaca,  Chili ;  and  the  various 
borax  and  salt  lakes  of  Nevada  and  California. 

BARITE 

Barite.  —  Heavy  spar ;  Barium  sulphate,  BaS04 ;  BaO  =  65.7, 
S03  =  34.3 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  a  :  b  :  c 
=  .8152:1:1.3136;  100  A 110  =  39°  11';  001 A 101  =  58°  11'; 
001 A Oil  =  52°  43';  111A001  =  64°  19';  102  A 102  =  77°  43'; 
Common  forms,  a  (100),  b  (010),  c  (001),  m  (110),  o  (Oil),  d  (102)  ; 
Twinning  planes,  110  and  601  both  developed  as  lamellae;  Cleav- 
age, basal  and  prismatic  perfect,  b  imperfect ;  Fracture,  uneven  ; 
Brittle ;  H.  =  2.5-3.5 ;  G.  =  4.3-4.6 ;  Color,  white,  or  pale  shades 
of  yellow,  green,  blue,  brown,  or  red;  Streak,  white;  Luster, 
vitreous;  Transparent  to  translucent;  a  =  1.637;  p  =  1.638; 
v  =  1.649;  Y-a  =  .012;  Optically  (+) ;  Axial  plane,  010 ; 
Bxa  =  c;  2E  =  63°  12'. 

B.B. —  Often  decrepitates,  whitens,  and  fuses  at  3,  coloring 
the  flame  yellowish  green  (Ba).  After  ignition  reacts  alkaline  with 


SULPHATES,   CHROMATES,   ETC. 


529 


turmeric  paper.     Fused  with  soda  and  a  little  coal  dust  in  R.  F. 
yields  a  sulphur  reaction  on  silver.     Insoluble  in  acids. 

General  description.  —  Crystals  are  tabular  parallel  to  the  base, 
when  they  are  combinations  of  m,  c,  o,  and  d.  The  perfect  cleav- 
ages parallel  to  m  and  b,  which  are  usually  to  be  observed,  serve  to 
orient  the  crystals.  It  is  also  elongated  in  habit  parallel  to  the 
brachyaxis;  again  parallel  to  the  macroaxis;  but  rarely  are  the 
crystals  elongated 
parallel  to  the 
vertical  axis,  as  at 
Betler,  Hungary. 
Parallel  growths 
are  common  when 
tabular  in  habit; 
they  are  joined  by 
the  base,  yielding 
at  times  a  cocks- 
comb-like surface 
or  radiated,  with 
deep  reentrant 
angles  separating 
the  individuals. 
It  also  occurs  in 
beds,  massive, 
granular,  radi- 
ated, in  banded 

nodules      as    Well 

as  stalactitic.  "* 

Barite  is  easily  distinguished  from  other  white  minerals  by  its 
weight,  from  which  it  takes  its  common  miner's  name  of  heavy 
spar.  The  various  shades,  other  than  white,  are  due  to  impuri- 
ties. It  often  contains  calcium  or  strontium  sulphates,  with 
which  it  is  isomorphous. 

Barite  is  a  secondary  mineral  associated  with  sedimentary  rocks 
and  ore  veins.  In  ore  veins  it  is  more  often  associated  with  lead 
and  zinc  ores  and  with  veins  containing  sulphides.  In  such  veins  it  is 
deposited  through  the  interaction  of  percolating  waters,  carrying 
barium  in  solution,  either  as  the  chloride  or  the  bicarbonate,  with 
soluble  sulphates,  as  gypsum,  or  with  sulphates  furnished  by  the 
oxidation  of  sulphides,  as  pyrite.  Beautiful  crystals  of  barite  are 

2M 


—  Barite  and  Dolomite  from  Cumberland,  Eng- 


530  MINERALOGY 

found  on  the  walls  of  cavities  in  the  lead  mines  of  Cornwall,  Eng- 
land. Crystals  elongated  parallel  to  &  are  associated  with  the  iron 
mines  of  West  Cumberland,  England,  in  cavities  of  dolomite. 

Wonderfully  developed  tabular  crystals,  associated  with  and 
penetrated  by  acicular  crystals  of  stibnite,  are  characteristic  of 
Felsobanya,  Hungary.  Barite  has  also  been  observed  as  the  ce- 
menting material  in  some  sandstones,  where  also  it  must  have  been 
deposited  by  double  decomposition.  It  occurs  around  some  springs 
as  a  sinter  and  is  also  deposited  in  pipes  in  mines,  being  precipi- 
tated from  the  mine  waters.  It  is  associated  with  limestones  as 
lenticular  deposits,  and  it  is  from  such  occurrences  that  all  the 
commercial  barite  is  mined.  In  the  United  States  barite  is  mined 
in  the  lead  regions  of  Missouri,  also  in  Tennessee,  Virginia,  and 
North  Carolina ;  while  in  small  amounts  it  is  widely  distributed. 
Good  crystals  are  found  at  Sterling,  Colorado ;  at  Cheshire,  Con- 
necticut ;  at  the  Perkiomen  lead  mine,  Pennsylvania. 

Uses.  —  Ground  barite  when  pure  white  is  used  as  a  paint,  as  a 
filler  for  paper,  and,  owing  to  its  insolubility,  in  rubber  goods.  It 
is  also  the  principal  source  of  the  barium  salts.  The  banded 
varieties  are  cut  and  polished  as  vases,  mantles,  and  for  other 
ornamental  purposes. 

CELESTITE 

Celestite.  —  Strontium  sulphate,  SrSO4 ;  SrO  =  56.4,  S03  = 
43.6 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  & :  b  :  c  = 
.7789  : 1 : 1.2800 ;  100  A  110  =  37°  55' ;  001  A  101  =  58°  40' ;  001  A 
Oil  =  52°;  102A  102  =  78°  49';  001  A  111  =  64°  21';  Common 
forms,  a  (100),  b  (010),  c(001),  m(110),  o  (102),  p(011),  z(lll); 
Cleavage,  basal  and  prismatic  perfect,  b  less  so ;  Brittle ;  Fracture, 
uneven;  H.  =  3-3.5;  G.  =  3.95-4;  Color,  faint  bluish  white, 
rarely  red;  Streak,  white;  Luster,  vitreous;  Transparent  to 
translucent;  a  =  1.622;  p  =  1.623;  -y  =  1.629;  -y  -  a  =  .007; 
Optically  (  +  );  Axial  plane  =  010;  Bxa  =  £;  2E  =  88°  38'. 

B.B.  —  Often  decrepitates,  whitens,  and  fuses  at  3,  yielding  a 
deep  red  flame ;  after  ignition  reacts  alkaline  with  turmeric  paper. 
Fused  with  soda  and  a  little  coal  dust  in  R.  F.  yields  a  sulphur 
reaction  on  silver.  Insoluble  in  acids. 

General  description.  —  In  crystalline  habit  and  general  appear- 
ance like  barite,  with  which  it  is  isomorphous.  It  is  not  as  common 
in  its  occurrence  as  barite  and  is  less  often  associated  with  ore  de- 


SULPHATES,   CHROMATES,   ETC.  531 

posits;  but  is  more  often  associated  with  gypsum  and  halite 
deposits,  and  fills  cavities  in  limestones  and  dolomites,  where  it  has 
been  deposited  by  percolating  waters.  Sulphur  and  celestite  are 
common  associates  in  the  region  of  volcanoes  or  solfataras.  The 
most  noted  occurrence  of  this 
character  is  that  of  Girgenti, 
Sicily,  which  furnishes  beautiful 
crystals  of  both  celestite  and  sul- 
phur. At  many  localities,  as  at 
Tyrone,  Pennsylvania,  a  fibrous 
blue  celestite,  filling  veins,  oc- 
curs. A  red  variety  occurs  in 
Brown  County,  Kansas.  Very 
large  crystals  are  found  on  Stron- 
tian  Island,  Lake  Erie.  Clear 
crystals  associated  with  cole-  FIG  519._Celestite.  Put-in-Bay,  Ohio, 
manite  occur  in  Death  Valley, 

California;  also  large  crystals  at  Lampasas,  Texas.     Celestite  is 
the  source  of  strontium  salts ;   for  their  uses  see  strontianite. 

ANHYDRITE 

Anhydrite.  —  Calcium  sulphate,  CaS04;  CaO  =  41.2L  SO3 
=  58.8 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  a :  b  :  c  = 
.8932  : 1 : 1.0008  ;  100  *  110  =  41°  46' ;  001  A  101  =  48°  15' ;  001  A 
Oil  =  45°  1';  Common  forms,  a  (100),  b  (010),  c(001),  m(110), 
z(lll);  Twinning  plane,  012 ;  Cleavage,  basal  and t  brachypina- 
choidal  perfect,  a  less  so ;  Brittle ;  Fracture,  uneven ;  H.  =  3-3.5  ; 
G.  =  2.9-3;  Color,  white,  gray,  or  pale  shades  of  yellow,  blue,  or 
red;  Streak,  white;  Luster,  vitreous;  Transparent  to  translu- 
cent; a  =  1.569;  P  =  1.575;  y  =  1.613;  <y  -  a  =  .044;  Opti- 
cally (+) ;  Axial  plane,  010;  Bxa  =  a;  2E  =  70°  18'. 

B.B.  —  Whitens  and  fuses  at  3,  yielding  a  yellowish  red  flame 
(Ca);  after  ignition  reacts  alkaline  with  turmeric  paper.  Fused 
with  a  little  coal  dust  in  R.  F.  on  coal  yields  a  sulphur  reaction 
with  silver.  Soluble  in  HC1.  Yields  no  water  in  the  closed  tube. 

General  description.  —  Crystals  are  *not  common ;  when  well 
formed  they  are  stout,  tabular  combinations  of  the  three  pinacoids, 
in  combination  with  a  pyramid  or  two,  as  at  Aussee  in  Styria.  A 
simple  combination  of  the  two  unit  domes  elongated  parallel  to  the 


532  MINERALOGY 

macroaxis  occurs  on  crystals  from  Stassfurt.  It  is  usually  granu- 
lar, massive,  lamellar,  or  fibrous.  While  it  is  classed  with  the 
barite  group  of  sulphates,  it  differs  from  other  members  of  the 
group  widely  in  its  cleavage  and  axial  ratio.  Anhydrite  is  also 
the  most  soluble  of  the  series,  and  is  therefore  carried  in  solution 
longer  and  farther  than  the  others,  and  is  often  deposited  by  the 
evaporation  of  these  natural  waters. 

It  occurs  in  beds  connected  with  salt  deposits ;  in  the  evapora- 
tion and  deposition  of  the  soluble  salts  from  saturated  solutions, 
anhydrite  is  normally  on  the  bottom  of  such  deposits,  with  the  more 
soluble  magnesium,  sodium,  and  potassium  compounds  above  it. 

In  the  normal  concentration  of  a  calcium  sulphate  solution  at 
ordinary  temperatures,  gypsum  is  deposited,  but  when  the  solution 
contains  sodium  chloride  or  magnesium  salts,  anhydrite  is  deposited 
at  temperatures  as  low  as  25°  C.  Gypsum  in  a  concentrated  solution 
of  sodium  chloride  at  30°  passes  over  to  anhydrite,  and,  while  gypsum 
is  the  first  mineral  to  separate  on  concentration,  very  often  when 
concentration  has  advanced  the  sulphate  first  crystallized  as  gypsum 
forms  anhydrite.  Under  heat  and  pressure  gypsum  may  lose  its 
water  and  be  transformed  to  anhydrite,  or  the  reverse  of  this  is 
also  possible  and  anhydrite  by  hydration  in  many  cases  forms 
gypsum.  The  two  minerals  are  often  mixed,  areas  of  anhydrite 
occur  containing  considerable  gypsum,  which  is  probably  secondary 
to  the  anhydrous  sulphate. 

Anhydrite  occurs  at  the  borax  lakes  and  salt  deposits  of  California 
and  Nevada,  at  the  salt  deposits  of  Michigan ;  at  Lockport,  New 
York;  in  Nova  Scotia,  at  the  mouth  of  the  Avon  and  St.  Croix 
rivers.  Commercially  anhydrite  is  of  little  value;  when  attrac- 
tively colored  and  veined  it  is  polished  as  an  ornamental  stone. 
Artificial  crystals  may  be  formed  by  fusing  calcium  sulphate  and 
sodium  chloride. 

ANGLESITE 

Angles! te.  —  Sulphate  of  lead,  PbSO4;  PbO=73.6;  S03  = 
26.4 ;  Orthorhombic ;  Type,  Didigonal  Equatorial ;  a :  b  :  c  = 
.7852  :  1 :  1.2894 ;  100  A  110  =  38°  8' ;  001  A  101  =  58°  40' ;  001  A 
Oil  =  52°  12' ;  102  A  102  =  78°  47' ;  001  A  111  =  64°  24' ;  Common 
forms,  a  (100),  b(010),  c(001),  m(110),  d(102),  z(lll);  Cleav- 
age, basal  and  prismatic  distinct ;  Fracture,  conchoidal ;  Brittle  ; 
H.  =  2.75-3  ;  G.  =  6.1-6.4 ;  Color,  white,  gray  or  pale  yellow,  or 
blue;  Streak,  white;  Luster,  adamantine  to  resinous;  Trans- 


SULPHATES,   CHROMATES,   ETC.  533 

parent  to  opaque ;   a  =  1.877;    p  =  1.882;  \  =  1.894;    <y-  a  = 
.017;  Optically  (  +  ) ;  Axial  plane,  010;  Bxa  =  ft;  2V  =66°  47'. 

B.B.  —  Decrepitates  and  fuses  at  1.5.  In  R.  F.  on  coal  reduces 
to  lead  and  yields  a  lead  coat ;  fused  with  soda  and  a  little  coal  dust 
yields  a  sulphur  reaction  with  silver.  Insoluble  in  acids. 

General  description.  —  Crystals  often  tabular  in  habit,  combi- 
nations of  c,  m,  and  d,  or  elongated  similar  to  barite,  with  which  it  is 
isomorphous.  Transparent  crystals  have  a  particularly  high 
luster,  as  the  specimens  from  Monte  Poni,  Sardinia,  where  they 
occur  implanted  on  galena.  Anglesite  is  named  for  the  island  of 
Anglesea,  where  it  was  first  observed  associated  with  an  earthy 
limonite. 

Anglesite  is  an  oxidation  product  associated  with  lead  ores, 
particularly  with  galena,  and  when  in  sufficient  quantities  it  is  a 
valuable  lead  ore. 

CROCOITE 

Crocoite.  —  Lead  chromate,  PbCr04;  PbO  =^68.9,  Cr03  = 
31.1;  Monoclinic;  Type,  Digonal  Equatorial ;  a:b:  c  =  .9603: 
1 :  .9158 ;  p  =  77°  33'  =  001 A 100 ;  100  A 110  =  43°  10' ;  001 A 101  = 
37°  41';  001 A  Oil  =  41°  48';  111  A  111  =  60°  50';  Common  forms, 
b(010),  c(001),  m(110),  t(lll),  v(lll);  Cleavage,  m  distinct, 
a  and  c  less  so ;  Brittle ;  Fracture,  uneven ;  H.  =  2.5-3 ;  G.  =  5'.9- 
6.1;  Color,  bright  hyacinth-red;  Streak,  orange-red;  Luster, 
adamantine  to  vitreous;  Transparent  to  translucent;  p  =  2.42; 
Optically  (  +  ) ;  Axial  plane,  010 ;  Ex*  Ac  =  5°  30' ;  2  V  =  54°  3'. 

B.B.  —  Fuses  at  1.5,  and  reduced  on  coal  with  soda  yields  lead 
buttons  and  a  lead  coat.  In  S.  Ph.  yields  a  green  bead  in  both 
flames  (Cr).  In  the  closed  tube  blackens  and  decrepitates,  but 
recovers  its  original  color  on  cooling. 

General  description.  —  Crystals  are  elongated  parallel  to  the 
vertical  axis  with  striations  on  the  prism  faces  lengthwise.  At 
times  apparently  rhombohedral  or  granular. 

Crocoite  was  described  as  a  new  mineral  from  the  Urals  in  Russia 
by  Lehmann  in  1762,  where  it  occurs  associated  with  quartz  crys- 
tals. It  was  in  crocoite  that  Klaproth  and  Vauquelin  independ- 
ently in  1797  discovered  the  metal  chromium. 

Beautiful  long,  slender  crystals  with  nearly  a  square  cross  sec- 
tion have  been  found  at  Dundas,  Tasmania.  It  also  occurs  in 


534  MINERALOGY 

Brazil,  the  Philippine  Islands,  and  in  Maricopa  County,  Arizona, 
where  it  is  associated  with  vanadinite  and  wulfenite. 


FIG.  520.  —  Crocoite  from  Dundas,  Tasmania. 

Lead  chromate  is  used  as  an  oxidizer  and  as  a  pigment,  but  this 
is  the  artificial  salt,  as  crocoite  is  too  scarce  and  not  sufficiently 
pure. 

KAINITE 

Kainite.  —  MgS04 .  KC1 .  3  H2O ;  K20  =  18.9,  MgO  =  16.1, 
Cl  =  14.3,  SO3  =  32.1,  H2O  =  21.8;  Monoclinic;  Type,  Di- 
gonal  Equatorial ;  a :  b  :  c  =  1.2187 :  1  :  .5863 ;  P  =  85°  6'  = 
001 A 100;  100  A  110  =  50°  32';  001 A 101  =24°  43';  001,011  =  30° 
18';  111*111  =  54°  1';  Common  forms,  a(100),  b(010),  c(001), 
m(110),  o(lll),  o>(lll);  Cleavage,  a  distinct,  and  b  less  so; 
H.  =  2.5-3;  G.  =  2.06-2.2;  Color,  white  to  dark  flesh-red  or 
yellow;  Streak,  white;  Luster,  vitreous;  Transparent  to  trans- 
lucent; Optically  (-);  Axial  plane,  010;  Bxa  A  c  =  10°  45'  in 
front;  2V  =  84°  33'. 

B-B-  ~-  Yields  a  potassium  flame.  Soluble  in  water,  the  solu- 
tion yields  a  white  precipitate  with  barium  nitrate  (BaS04),  filtered 
and  acidified  with  nitric  acid  yields  a  white  precipitate  with  silver 
nitrate  (Cl),  or  reacts  for  chlorine  with  copper  oxide.  Will  not 
effervesce  with  acids.  Has  a  bitter,  saline  taste. 

General  description.— Crystals  are  combinations  of  the  plus  and 
minus  unit  pyramids  and  prism,  with  the  pinacoids  at  times. 


SULPHATES,   CHROMATES,   ETC.  535 

The  base  is  usually  rough  and  uneven.  It  is  more  often  granular 
or  massive. 

Kainite  being  very  soluble  in  water  has  been  deposited  from  con- 
centrated sea  water ;  when  this  concentration  has  reached  the 
stage  where  the  sulphates  have  been  deposited  and  the  mother 
liquor  is  saturated  in  respect  to  the  chlorides  and  sulphates, 
double  salts  are  separated,  of  which  kainite  is  an  example.  This 
mineral,  however,  may  have  been  formed  by  the  interaction  of  car- 
nallite  (KC1 .  MgCl2)  .6  H2O  and  kieserite  (MgSO4)  .  H2O  as  a 
secondary  mineral. 

Kainite  is  found  in  quantities  at  the  unique  salt  deposits  of 
Stassfurt  and  in  small  deposits  of  the  same  character  in  Galicia. 
These  two  deposits  are  of  great  commercial  value,  as  they  furnish 
the  potash  supply  to  the  world,  and  which  is  essential  as  one  of  the 
necessary  plant  foods,  so  apt  to  be  early  exhausted  from  the  soil. 

HANKSITE 

Hanksite.  —  4  Na2SO4 .  Na2C03.  A  double  salt  found  under  the 
same  conditions  as  kainite,  and  interesting  as  one  of  the  few  miner- 
als illustrating  the  dihexagonal  equatorial  type.  Its  crystals  are 
tabular  combinations  of  the  base,  unit  pyramid,  and  prism.  It  is 
found  associated  with  the  borax  lake  deposits  of  California. 

MIRABILITE 

Mirabilite.  —  Hydrous  sodium  sulphate,  Na2S04 . 10  H20; 
Na2O  =  19.3,  SO3  =  24.8,  H2O  =  55.9;  Monoclinic;  Type, 
Digonal  Equatorial ;  a:b:c  =  1.1158:  1:  1.2372  ;~~p  =  72°  15'  = 
001 A 100 ;  100  A 110  -  46°  44' ;  001 A 101  =  57°  55' ;  001 A  Oil  =  49° 
41' ;  Common  form,  a  (100),  b  (010),  c  (001),  m  (110) ;  Cleavage, 
a  perfect,  c  and  b  in  traces;  H.  =  1.5-2;  G.  =  1.48;  Color  and 
streak,  white ;  Luster,  vitreous  ;  Transparent  to  opaque ;  p  =  1.44 ; 
Optically  (-);  Axial  plane  J_  010;  Bxa  =  b  ;  2E  =  122°  48'. 

B.B.  —  Boils  and  yields  a  yellow  flame  (Na).  In  the  closed 
tube  yields  much  water.  After  ignition  leaves  an  alkaline  residue. 
Very  soluble  in  water,  the  solution  yields  a  white  precipitate  with 
barium  chloride  (BaSO4).  Has  a  cooling,  bitter  taste. 

General  description.  —  Occurs  as  crusts  or  in  beds  in  the  de- 
posits formed  by  the  evaporation  of  salt  lakes.  Sodium  sulphate 
is  contained  in  varying  amounts  in  all  natural  waters ;  upon  con- 


536  MINERALOGY 

centration  mirabilite  separates  from  the  saturated  solution  when 
the  temperature  is  below  32°  ;  when  the  temperature  is  higher,  the 
anhydrous  salt,  thenardite,  is  separated.  '  Its  solubility  varies 
greatly  with  the  temperature,  thus  at  Great  Salt  Lake,  Utah,  on 
cold  days  in  winter  large  amounts  of  sodium  sulphate  are  thrown 
up  on  the  shore  by  the  waves,  only  to  be  redissolved  when  the 
temperature  rises.  In  a  dry  atmosphere  it  loses  its  water  of 
crystallization,  falling  down  as  a  fine  white  powder.  It  also  occurs 
as  an  efflorescence  on  rocks  or  near  springs,  where  much  water  is 
quietly  evaporating. 

GYPSUM 

Gypsum.  —  CaSdi .  2  H2O ;  Hydrous  calcium  sulphate ;  CaO  = 
32.5,  SO3  =  46.6,  H20  =  20.9;  Monoclinic;  Type,  Digona! 
Equatorial;  a:b:  c=  .6899 :  1 :  .4124;  p  =  80°  42'  =  001  A 
100;  100,110  =  34°  15';  001*101  =  28°  17';  001  A Oil  -22°  9'; 
111*111  =36°  12';  Common  forms,  b(010),  c  (001),  m  (110), 
l(lll),  n(lll);  Twinning  plane,  100,  contact  and  crossed  pene- 
trating, also  101  less  common  ;  Cleavage,  clinopinacoidal  perfect, 
a  less  so,  and  111  fibrous;  laminae  flexible  parallel  to  the  fibrous 
cleavage;  H.  =  1.5-2;  G.  =  2.3-2.33,  when  pure;  Color,  white, 
pale  yellow,  red,  brown  to  black  when  organic  matter  is  present; 
Streak,  white ;  Luster,  silky,  pearly,  vitreous  to  dull ;  Transparent 
to  opaque;  a  =  1.5204;  (*  =  1.5229;  -y  =  1.5296;  y  -  a  = 
.009 ;  Optically  (  +  ) ;  Axial  plane  =  010 ;  Bxa  A  c  =  52°  30'  in 
front;  2V  =  58°  8'. 

B.B.  —  Whitens  and  fuses  to  an  opaque  white  mass  at  3  ;  colors 
the  flame  yellowish  red  ;  after  ignition  reacts  alkaline  with  turmeric 
paper.  In  the  closed  tube  yields  water.  Fused  with  soda  and  a 
little  coal  dust  in  the  R.  F.  reacts  for  sulphur  with  silver.  Soluble 
in  HC1. 

General  description.  —  Crystals  usually  simple  combinations  of 
the  plus  and  minus  unit  pyramids  with  the  unit  prism  and  the 
clino-  and  basal  pinacoids.  Crystals  six  feet  in  length  have  been 
found  in  Wayne  County,  Utah.  The  faces  m  and  b  often  striated 
parallel  to  their  intersection  with  the  base.  Parallel  growths  and 
rounded  stellate  aggregates  are  common,  as  at  St.  Mary's  River, 
Maryland,  and  Postelberg,  Bohemia.  Twinning  in  which  the 
composition  plane  is  parallel  to  the  orthopinacoid,  forming  the 
well-known  swallow-tail  twins,  and,  when  the  crystals  are  rounded, 


SULPHATES,   CHROMATES,   ETC. 


537 


the  arrowhead  twins,  is  common  at  Montmartre  near  Paris. 
Simple  crystals  are  more  often  found  in  clays,  as  at  Poland,  Ohio. 
All  crystalline  gypsum  which  shows  the  perfect  cleavage  is  known 
as  selenite.  The  fibrous  variety  with  a  satiny  or  pearly  luster  and 
a  fibrous  fracture  is  satin  spar,  while  the  granular  massive  variety 
is  alabaster.  Rock  gypsum  is  an  impure  granular  form,  often 
earthy.  Gypsum  is  deposited  from  solution  and  is  associated  with 
sedimentary  rocks,  limestones,  and  clays,  from  which  the  soluble 
calcium  sulphate  has  been  leached  out.  It  is  also  associated  with 
salt  deposits,  being  deposited  from  the  concentrated  brines  before 


FIG.  521.  —  Gypsum.     Poland,  Ohio. 

the  more  soluble  sodium  or  magnesium  sulphates  and  chlorides, 
and  therefore  in  the  usual  position  underlying  the  salt,  or  is  near  it 
in  position ;  at  times,  when  there  have  been  several  distinct  periods 
of  concentration,  it  may  be  interbedded  with  salt  and  shales. 
Large  beds  of  rock  gypsum  are  found  in  the  Salina  formations  of 
New  York,  but  here  the  gypsum  beds  are  above  the  salt  and  are 
probably  independent  of  it,  the  concentration  of  the  solution  hav- 
ing been  interrupted  before  the  salt  was  deposited. 

Large  beds  of  gypsum  are  found  in  Nova  Scotia,  Newfoundland, 
Michigan,  and  in  the  borax  lakes  regions  of  California  and  Nevada. 
Gypsum  is  also  formed  near  volcanoes  and  fumaroles ;  small  crys- 
tals of  gypsum  cover  the  walls  of  the  lava  caves  of  Kilauea. 

Commercially  rock  gypsum  is  ground  and  used  as  a  fertilizer. 
The  purer  varieties,  when  heated  at  a  temperature  below  130°  C., 


538  MINERALOGY 

until  one  molecule  of  the  water  of  crystallization  is  driven  off,  form 
a  cement  known  as  "  plaster  of  Paris,"  named  from  the  Mont- 
martre  deposits  near  Paris  where  this  cement  was  first  made.  This 
calcined  product  when  moistened  absorbs  water,  forming  a  network 
of  fibrous  crystals,  and  solidifies  as  a  whole.  When  all  the  water 
of  crystallization  is  driven  off,  it  forms  anhydrite;  the  product 
loses  its  power  to  absorb  water  or  absorbs  it  very  slowly,  and  its 
setting  or  crystallizing  power  is  lost. 

Satin  spar  and  alabaster  are  polished  as  ornamental  stones  and 
for  inexpensive  jewelry. 

EPSOMITE 

Epsomite.  —  Epsom  salts,  MgS04 . 7  H20 ;  MgO  =  16.3,  S03 
=  32.5,  H2O  =  51.2;  Orthorhombic ;  Type,  Digonal  Holoaxial ; 
ft :  b':  c  =  .9902:1:.5709;  100  A  110  =  44°  43';  001*101  = 
29°  58' ;  001  A  Oil  =  29°  43' ;  111  A  111  =  52°  38' ;  Common 
forms,  a  (100),  b  (010),  c  (001),  z  (111),  n  (101) ;  Cleavage, 
b  perfect,  Oil  less  so ;  Brittle ;  Fracture,  conchoidal ;  H.  =  2-2.5  ; 
G.  =  1.68-1.75;  Color  and  streak,  white;  Transparent  to  trans- 
lucent; a  =  1.432;  p  =  1.455;  y  =  1.461;  .  y  -  a  =  .029; 
Optically  (-);  Axial  plane  =  001;  Bxa  =  b;  2E  =  78°  20'. 

B.B.  —  Boils  and  yields  an  infusible  white  alkaline  residue 
which  becomes  flesh-pink  when  treated  with  cobalt  solution  (Mg) . 
Soluble  in  water ;  the  solution  yields  a  white  precipitate  with  barium 
chloride  (BaSO4).  It  has  a  very  bitter  taste. 

General  description.  —  Crystals  are  prismatic  in  habit,  combi- 
nations of  the  sphenoids  and  unit  prism;  they  are  interesting  as 
examples  of  the  holoaxial  type ;  also  in  fine  silky  acicular  crys- 
tals. The  mineral  is  named  from  the  locality  of  Epsom  Springs, 
England,  where  it  was  first  known. 

Magnesium  sulphate  is  easily  soluble  in  water ;  all  springs  and 
percolating  ground  waters  contain  both  magnesium  and  calcium 
sulphates  in  considerable  quantity.  They  cause  the  permanent 
hardness  of  natural  waters.  Where  large  amounts  of  water  are 
evaporating,  as  on  the  face  of  cliffs  or  on  the  surface  of  the  soil 
in  very  dry  seasons,  epsomite  is  left  as  white  crusts.  The  white 
crusts  formed  on  fresh  brick  walls  are  in  part  epsomite.  Such 
residual  crusts  occur  on  the  floors  of  the  caves  of  Tennessee  and 
Kentucky  and  on  many  of  the  alkaline  plains  of  California,  Utah, 
and  Nevada. 


SULPHATES,   CHROMATES,  ETC.  539 

Kieserite,  MgSO4 .  H2O,  is  a  magnesium  sulphate  containing  only 
one  molecule  of  water.  It  is  monoclinic  and  much  less  soluble 
than  epsomite,  but  dissolves  slowly  in  water  and  recrystallizes  as 
epsomite  at  ordinary  temperatures.  Kieserite  separates  from  solu- 
tions above  68°  C.  Kieserite  is  associated  with  carnallite  and  gyp- 
sum at  the  Stassfurt  salt  deposits.  Here  it  has  been  separated  from 
solutions  containing  sodium  and  potassium  salts,  and  under  these 
conditions  a  number  of  double  salts  have  been  formed,  as  blodite, 
Na2Mg(SO4)2  •  4  H2O ;  loweite,  Na2Mg(S04)2 .  2J  H2O ;  picromerite, 
K2Mg(SO4)2.6H2O. 

Isomorphous  with  epsomite  is  the  zinc  sulphate,  goslarite, 
ZnS04 . 7  H20,  derived  from  the  oxidation  of  sphalerite  and 
occurring  on  the  walls  of  old  mine  workings;  also  morenosite, 
NiSO4 . 7  H2O ;  the  nickel  sulphate  is  a  member  of  the  same 
group. 

Commercially  magnesium  sulphate  is  separated  at  the  Stassfurt 
works.  It  is  used  in  medicine  as  a  purgative,  and  as  a  coating  for 
cotton  cloth  in  dyeing. 

MELANTERITE 

Melanterite.  —  Copperas ;  Ferrous  sulphate,  FeS04 .  7  H2O ; 
FeO  =  25.9,  SO3  =  28.8,  H2O  =  45.3;  Monoclinic;  Type, 
Digonal  Equatorial;  a:  b:  c  =  1.1828 :  1 :  1.5427;  p  =  75° 44'  = 
001  A  100;  100,110  =  48°  54';  001 A 101  =  43°  44';  001 A  Oil  = 
56°  13';  111  A  111  =  78°  33';  001A110  =  80°  41';  Common 
forms,  b  (010),  c  (001),  m  (110),  r  (111),  o(011),  v(101);  Cleavage, 
basal  perfect,  m  less  so;  Brittle;  Fracture,  conchoidal;  H.  =  2; 
G.  =  1.89-1.90;  Color,  shades  of  green  to  yellow;  Streak,  white; 
subtransparent  to  translucent ;  a  =  1.471;  p  =  1.478;  y  =  1.485; 
<y  -  a  =  .014;  Optically  (+) ;  Axial  plane  =  010;  BxaAc  =  62° 
28' behind;  2V  =  88°  48'. 

B.B.  —  Fuses  and  blackens,  leaving  a  magnetic  residue  on  coal, 
which  reacts  only  for  iron  with  the  fluxes.  In  the  closed  tube 
yields  water.  Soluble  in  water ;  the  solution  acidified  with  HC1 
yields  a  white  precipitate  with  barium  chloride ;  yields  a  sulphur 
reaction  with  soda  on  silver.  It  has  an  astringent,  metallic  taste. 

General  description.  —  Crystals  are  short,  prismatic  in  habit, 
but  the  salt  in  nature  is  usually  fine  fibrous,  massive,  or  concretion- 
ary. The  yellow  color  of  some  specimens  is  caused  by  oxidation 


540  MINERALOGY 

on  exposure.  Chemically  it  may  contain  some  magnesium,  as 
epsomite  is  very  closely  related. 

Pisanite  is  a  variety  containing  copper. 

Melanterite  is  the  result  of  the  oxidation  of  such  sulphides  as 
pyrite,  marcasite,  pyrrhotite,  or  other  sulphides  containing  iron; 
the  soluble  ferrous  sulphate  is  carried  off  in  solution,  to  be  deposited 
by  evaporation,  as  copperas  if  the  conditions  are  favorable.  All 
mine  waters  in  sulphide  regions  contain  ferrous  sulphate  in  solution, 
which  according  to  the  conditions  may  form  several  iron  sulphates, 
as  coquimbite,  Fe2(SO4)3 .  9  H2O,  the  ferric  sulphate,  or  the  ferrous 
ferric  sulphate  roemerite,  FeFe2(S04)4 . 12  H2O. 

Iron  sulphates  are  not  common  in  nature,  as  owing  to  their 
solubility  they  may  occur  only  under  very  restricted  conditions. 

The  commercial  copperas  is  a  by-product  produced  in  the  pre- 
cipitation of  copper  sulphate  with  scrap  iron.  It  is  used  as  a  disin- 
fectant, in  dyeing,  in  the  manufacture  of  ink  and  pigments. 

CHALCANTHITE 

Chalcanthite. — Blue  vitriol;  Copper  sulphate,  CuS04.5H20; 
CuO  =  31.8  ;  S03  =  32.1  ;  H2O  =  36.1 ;  Triclinic;  Type,  Centro- 
symmetric ;  a :  b  :  c  =  0.5721 :  1 :  .5554 ;  a  =  82°  5' ;  (3  =  107° 
8';  <y  =  102°4r;  100 A 010  =  79° 6';  100 A 110  =  26° 7';  010 A 011  = 
64°  58' ;  Oil  *  Oil  =  56°  59' ;  100  A  Oil  =  69°  50' ;  Common  forms, 
a  (100),  b  (010),  m(110),  M  (110),  p  (111),  Cleavage,  M,  m,  and  p 
imperfect;  Brittle;  Fracture,  conchoidal;  H.  =  2.5;  G.  =  2.12- 
2.3;  Color,  shades  of  blue;  Streak,  white;  Luster,  vitreous; 
Translucent;  a  =  1.514;  p  =  1.537;  y  =  1.543;  y  -  a  =  .029; 
Optically  (  — ) ;  2E  =  93°.  The  normal  to  the  optic  plane  makes 
an  angle  of  53°  30'  with  the  normal  to  110,  with  ifO  an  angle  of 
12°  30',  and  with  the  normal  to  111,  67°. 

B.B.  —  Fuses  easily  on  coal  and  blackens;  reduced  with  soda, 
yields  copper,  and  a  sulphur  reaction  on  silver.  In  the  closed  tube 
yields  water.  Soluble  in  water,  and  has  a  metallic  taste. 

General  description.  —  Crystals  stout,  tabular  parallel  to  111 
with  the  zone  c,  b  striated  parallel  to  their  intersection.  More 
often  massive,  granular,  stalactitic  or  in  crusts. 

Chalcanthite  occurs  in  many  copper  mines,  where  it  is  deposited 
from  the  mine  waters  on  evaporation.  It  is  derived  from  the  oxi- 
dation of  sulphides  in  the  upper  levels  of  the  ore,  deposits  and  is 


SULPHATES,   CHROMATES,   ETC.  541 

carried  down  to  the  lower  zones  in  solution,  where  it  is  usually  re- 
deposited  by  the  action  of  pyrite  as  sulphide ;  but  where  conditions 
favor  the  evaporation  of  the  solution,  it  may  form  the  sulphate. 
The  mine  waters  of  Butte,  Montana,  and  at  Bisbee,  Arizona,  con- 
tain considerable  copper  in  solution  as  the  sulphate,  which  is  re- 
covered by  allowing  it  to  run  slowly  over  scrap  iron. 

Good  specimens  are  obtained  from  the  Copper  Queen  mine,  Bis- 
bee, Arizona;  and  from  Isabella  mine,  Polk  County,  Tennessee. 

Brochantite,  Cu4(OH)6SO4,  is  a  basic  copper  sulphate,  ortho- 
rhombic  in  symmetry,  insoluble  in  water,  and  without  taste ;  other 
tests  like  chalcanthite ;  occurs  in  the  Tintic  district,  Utah  ;  Chaff ee 
County,  Colorado  ;  and  at  several  localities  in  Arizona. 

Owing  to  its  rare  occurrence  chalcanthite  is  unimportant  com- 
mercially; the  artificial  blue  vitriol  is  used  in  electric  batteries, 
as  a  fungicide  in  Bordeaux  mixture,  and  as  a  mordant  in  dyeing. 

ALUNITE 

Alunite.  —  KA12(OH)6(S04)2.3  H20 ;  a  basic  potassium  alumin- 
ium sulphate;  K2O  =  11.4,  A1203  =  37.0,  S03  =  38.6,  H20  =  13.0; 
Hexagonal;  Type,  Dihexagonal  Alternating ;  c  =  1.252;  0001 A 
1011  =  55°  19';  lOliaiOl  =  90°  50';  Cleavage,  basal  distinct; 
Brittle ;  Fracture,  uneven  ;  H.  =  3.5-4 ;  G.  =  2.58-2.75 ;  Color, 
white,  gray,  or  pale  red ;  Streak,  white ;  Luster,  vitreous  to  pearly  ; 
Transparent  to  opaque;  o>  =  1.572;  €  =  1.592;  €  —  o>  =  .020; 
Optically  (+). 

B.B.  —  Infusible,  but  may  decrepitate  when  ignited ;  treated 
with  cobalt  solution  becomes  blue  (Al).  In  the  closed  tube  yields 
water,  and  fused  in  R.  F.  with  soda  and  a  little  coal  dust  yields  a 
sulphur  reaction  on  silver.  Insoluble  in  HC1,  soluble  in  H2SO4. 

General  description.  —  Crystals  are  small  and  rhombohedral 
in  habit,  usually  combinations  of  several  rhombohedrons  of  the 
same  series.  More  often  massive,  granular,  or  of  a  fibrous-like 
structure. 

Alunite  is  very  local  in  its  occurrence,  and  it  has  been  produced 
by  the  action  of  sulphurous  fumes  on  the  feldspars  of  such  rocks  as 
rhyolites,  andesites,  or  trachytes,  or  by  the  decomposition  of  these 
rocks  by  percolating  waters  containing  sulphuric  acid,  as  in  the 
Goldfield  district  of  Nevada ;  here  the  formation  of  alunite  has  a 
direct  connection  with  the  workable  deposits  of  gold.  It  also 


542  MINERALOGY 

occurs  at  Cripple  Creek,  Colorado;  in  Mariposa  County,  Cali- 
fornia ;  near  Morenci,  Arizona. 

Alunogen,  A12(SO4)3 . 18  H20,  hydrous  sulphate  of  aluminium,  is 
soluble  in  water  and  occurs  as  an  efflorescence  on  the  walls  of  coal 
mines.  Formed  by  the  action  of  sulphuric  acid  on  shales.  A  large 
deposit,  fibrous  in  character,  occurs  at  Smoky  Mountain,  North 
Carolina. 

Aluminite  (A10)2SO4 . 9  H20,  a  basic  sulphate  insoluble  in  water, 
is  found  in  concretionary  forms  imbedded  in  clay. 

Kalinite,  KA1(S04)2 . 12H2O,  is  a  natural  potash  alum,  found  as 
an  efflorescence  on  slates  and  on  the  walls  of  caves  of  Tennessee. 

All  these  minerals  where  found  in  sufficient  quantities  are  used  in 
the  manufacture  of  soluble  aluminium  salts  and  alum. 

WOLFRAMITE 

Wolframite.  —  Tungstate  of  iron  and  manganese,  (Fe  .  Mn)  W04 ; 
when  Fe  :  Mn:  :  4  : 1,  FeO  =  18.9,  MnO  =  4.7,  W03  =  76.4 ; 
Monoclinic  ;  Type,  Digonal  Equatorial ;  a  :  b  :  c  =  .8300  :  1 : 
.8678 ;  p  =  89°  21'  =  001 A 100 ;  100  A 110  =  39s  41' ;  001 A 101  = 
45°  56';  001.011  =40°  57';  Common  forms,  a  (100),  m(110), 
t  (102),  y  (102),  o  (111) ;  Twinning  axis  c,  composition  plane,  100; 
Cleavage,  b  perfect ;  Brittle ;  Fracture,  uneven ;  H.  =  5-5.5 ; 
G.  =  7.2-7.5  ;  Color,  dark  brown  to  nearly  black  ;  Streak,  brown- 
ish or  reddish  to  nearly  black;  Luster,  metallic  adamantine  to 
dull ;  Opaque,  rarely  translucent. 

B.B.  —  Fuses  at  three  or  four  to  a  globule  which  in  R.  F.  is 
usually  magnetic.  When  dissolved  in  the  S.  Ph.  bead  and  reduced 
with  tin  on  coal  yields  a  blue  solution  when  dissolved  in  HC1. 
Fused  with  soda  in  0.  F.  yields  green  sodium  manganate. 

General  Description.  —  Crystals  tabular  with  100  prominent, 
or  stout  prismatic,  striated  on  100  parallel  to  the  vertical  axis. 
Also  in  granular  masses. 

Wolframite  is  an  isomorphous  mixture  of  hiibnerite,  MnW04,  the 
tungstate  of  manganese,  and  ferberite,  FeWO4,  the  tungstate  of 
iron.  Hiibnerite  occurs  as  brown,  translucent,  bladed  crystals,' while 
ferberite  is  black  and  opaque. 

The  three  minerals  occur  under  the  same  conditions,  usually  in 
quartz  veins  in  granites,  associated  with  sulphides  ;  here  they  have 
probably  been  precipitated  from  hot  solutions.  They  also  occur 


SULPHATES,   CHROMATES,   ETC. 


543 


in  pegmatites,  associated  with  cassiterite,  as  in  Cornwall,  England ; 
in  the  Black  Hills,  South  Dakota ;  also  in  the  Seward  Peninsula, 
Alaska.  In  all  such  cases  their  origin,  like  that  of  the  cassiterite, 
is  due  to  pneumatolitic  agencies.  A  third  but  less  common  occur- 
rence is  the  replacement  in  limestone,  as  at  Turnbull,  Connecticut. 

Wolframite  and  hubnerite  occur  in  numerous  localities  in  the 
Western  states,  associated  with  gold-bearing  quartz  veins,  but  al- 
ways in  small  amounts.     Wolframite  is  mined  on  a  commercial 
scale  in  Boulder  County,  Colo- 
rado.    Hubnerite  was  first  de- 
scribed  or   obtained  from   the 
Enterprise  mine,  Nevada,  where 
it  is  associated  in  a  vein  with 
apatite,  fluorite,  and  scheelite. 

Commercially  wolframite  is 
the  principal  source  of  the  metal 
tungsten  and  its  salts.  The 
metal  is  added  to  steel  in  the 
form  of  ferrotungsten,  produc- 
ing a  tungsten  steel  which  will 
retain  its  temper  when  working 
at  or  near  a  red  heat ;  from  this 
steel,  lathe  tools,  drills,  hack 
saws,  etc.,  are  manufactured. 
Incandescent  lamp  filaments 
made  of  tungsten  yield  a  very 
white  light  and  reduce  the  cur- 
rent used  to  1J  watts  per  candle 
power,  while  the  carbon  fila- 
ment requires  three  watts.  Sodium  tungstate  is  used  in  fireproof- 
ing  curtains  and  draperies ;  as  a  mordant  in  dyeing.  Calcium  tung- 
state is  the  phosphorescent  salt  with  which  the  screen  used  in 
viewing  the  Rontgen  rays  is  coated. 

Artificial  wolframite  may  be  produced  by  fusing  sodium  tung- 
state and  the  chlorides  of  sodium,  manganese,  and  iron.  When  the 
iron  is  left  out,  hubnerite  is  the  result. 


FIG.  522.  —  Wolframite  Crystal  from 
Zinnwald,  Bohemia. 


SCHEELITE 

Scheelite.  —  Calcium   tungstate,    CaWO4 ;    CaO  =  19.4,   W03 
=  80.6;    Tetragonal;    Type,  Tetragonal  Equatorial ;    c  =  1.5356; 


544 


MINERALOGY 


001  A  101  =56°  55';  111  *  111  =  79°  55';  101  A  Oil  =  72°  40';  Com- 
mon forms,  p(lll),  e(101),  c(001),  h  (313),  s  (311)  ;  Twinning 
plane,  100,  both  contact  and  interpenetrating;  Cleavage,  111 
distinct,  e  interrupted  ;  Brittle  ;  Fracture,  uneven  ;  H.  =  4.5-5  ; 
G.  =  5.9-6.1  ;  Color,  white,  pale  yellowish  white,  pale  yellow  to 
brown;  Streak,  white;  Luster,  adamantine  to  vitreous;  Trans- 
parent to  opaque;  (0  =  1.934;  €  =  1.918;  <o  -  €  =  .016; 
Optically  (-). 

B.B.  —  Fuses  at  5.  When  dissolved  in  S.  Ph.,  the  bead  is 
yellow  in  O.  F.,  blue  in  R.  F.  when  cold  ;  the  same  bead  reduced 
beside  tin  on  coal  shows  tungsten,  page  587.  Soluble  in  HNO3,  or 
HC1,  leaving  a  yellow  powder  (W03),  which  is  soluble  in  ammonia. 

General  description.  —  Crystals  octahedral  in  habit,  combina- 
tions in  which  the  pyramid  of  the  second  order  usually  predomi- 


FIG.  523.  —  Scheelite  and  Fluorite  from  Schwarzenberg,  Saxony. 

nates.  Crystals  from  Turnbull,  Connecticut,  are  combinations 
of  the  first  and  second  order  pyramids  in  which  the  former  pre- 
dominates. 

Scheelite  is  interesting  as  an  example  of  the  tetragonal  equa- 
torial type,  and  pyramids  of  all  three  orders  occur  on  crystals  from 
Schlackenwald  and  Zinnwald,  Bohemia.  It  also  occurs  massive. 

Chemically  copper  may  replace  some  of  the  calcium ;  the  pure 
copper  tungstate  is  the  mineral,  cuprotungstite,  CuWO4,  from  La 
Paz,  Lower  California.  Molybdenum  may  replace  the  tungsten, 
as  in  the  variety,  or  mineral,  powellite  of  western  Idaho. 

Tungstic  acid  (W03)  was  discovered  in  scheelite,  in  1781,  by  the 


SULPHATES,   CHROMATES,   ETC.  545 

chemist  Scheele,  for  whom  it  was  named.  Tungsten  is  Swedish 
for  heavy  stone,  in  reference  to  the  high  specific  gravity  of  all  the 
minerals  containing  it. 

In  occurrence,  association,  and  artificial  production,  scheelite  is 
like  wolframite,  with  which  it  is  very  closely  associated,  often  being 
a  secondary  mineral  derived  from  it. 

Scheelite  occurs  at  Chesterfield,  Massachusetts ;  Carabarus 
County,  North  Carolina ;  Mammoth  district,  Nevada ;  at  Turn- 
bull  and  Huntington,  Connecticut,  and  large  crystals  at  Marlow, 
Beauce  County,  Quebec. 

Stolzite  is  the  lead  tungstate  isomorphous  with  scheelite. 

WULFENITE 

Wulfenite.  —  Molybdate  of  lead,  PbMoO4 ;  PbO  =  60.7, 
Mo03  =  39.3;  Tetragonal _;  Type,  Tetragonal_  Polar ;  c  =  1.5777; 
001  A  101  =57° 37';  111  A  111  =  80° 22' ;  102*102  =  76°  31';  Com- 
mon forms,  a  (111),  u  (102),  c  (001),  m(110),  f  (320) ;  Cleavage, 
111  good,  e  and  s  (113)  less  so;  Brittle;  Fracture,  uneven;  H.  = 
2.75-3 ;  G.  =  6.7-7 ;  Color,  various  shades  of  yellow,  red,  or  green ; 
Streak,  yellowish  white ;  Luster,  adamantine  to  resinous ;  Trans- 
parent to  translucent ;  eor  =  2.402 ;  €r  =  2.304 ;  co  -  €  =  .098  ; 
Optically  (-). 

B.B.  —  Fuses  easily  and  boils.  Reduced  with  soda  on  coal  yields 
lead  buttons  and  a  lead  coat.  In  the  S.  Ph.  bead  it  is  yellowish 
green  in  O.  F.,  clear  green  in  R.  F. ;  this  bead  shows  molybdenum, 
page  586. 

General  description.  —  Crystalline  habit,  tabular  parallel  to 
the  base;  combinations  of  the  base  with  several  pyramids,  as  at 
the  Red  Cloud  mine,  Yuma  County,  Arizona;  or  the  base  with 
short  prisms,  as  at  Radersberg,  Montana.  The  tabular  crystals 
are  at  times  very  thin  and  scaly. 

Wulfenite  is  interesting  as  an  example  of  the  tetragonal  polar 
type.  Crystals  from  New  Mexico  show  this  polar  development  of 
the  pyramids,  while  crystals  from  Phcenixville,  Pennsylvania,  have 
the  upper  and  lower  base  unequally  developed. 

Wulfenite  is  a  secondary  mineral  associated  with  oxidized  lead 
ores.  It  is  found  only  in  small  quantities,  though  in  some  localities 
it  occurs  in  sufficient  amounts  to  constitute  an  ore  of  molybdenum. 


2N 


PART   III 
CHAPTER  I 

DESCRIPTION  OF  THE  INSTRUMENTS,  REAGENTS,  AND 
CHEMICAL  TESTS  USED  IN  THE  BLOWPIPE  TABLE 
FOR  THE  DETERMINATION  OF  THE  MINERAL  SPE- 
CIES 

VERY  little  apparatus  and  few  reagents  are  necessary  for  the 
blowpipe  determination  of   the   common  minerals.     While  elab- 
orate apparatus  and  mechanical  blowpipes 
have  been  devised,  equally  as  good  results 
can  be  obtained  by  the  careful  worker  with 
but  few  and  very  simple  instruments;  the  success 
of  blowpipe  work  will  depend  upon  the  care  and 
skill  of  the  worker  rather  than  upon  the  elaborate- 
ness of  the  instruments  used. 

The  blowpipe  is  by  far  the  most  important  of  all 
the  instruments  required.  It  is  in  constant  use 
and  takes  part  in  almost  every  experiment.  The 
style  of  blowpipe  to  be  recommended  is  that  known 
as  the  Freiberg  model,  Fig.  524.  This  model  pos- 
sesses a  trumpet  mouthpiece,  either  of  hard  rubber 
.  or  horn,  with  which  blowing  may  be  continued  for 

a  long  time  with  very  little,  if  any,  fatigue.  A 
trumpet  mouthpiece  is  not  necessary,  but  it  in- 
volves considerable  muscular  energy  to  keep  the 
lips  closed  around  a  tube  inserted  in  the  mouth ; 
with  the  necessary  air  pressure  on  the  cheeks, 
to  yield  a  strong  and  constant  blast.  Where  both 
kinds  of  mouthpieces  have  been  used  there  will 
never  be  any  doubt  that  the  trumpet  style  will  be 
the  one  chosen.  The  shaft  B  may  vary  in  length 
according  to  the  desire  and  convenience  of  the 
individual,  but  as  manufactured  it  is  about  23 
cm.  long.  Freiberg  blowpipes  are  fitted  with  plati- 
num tips;  this  is  expensive,  but  necessary  where 
much  work  is  to  be  done.  Some  dealers  furnish  a 
spun  tip,  which  is  always  very  thin  at  the  point,  so 
FIG.  524.  546 


INSTRUMENTS  AND   CHEMICAL  TESTS 


547 


thin,  in  fact,  that  it  will  not  hold  its  shape  and  is  very  liable  to 
split  or  crack  with  but  little  use,  and  become  worthless.  The 
tip  A  should  be  thick  at  the  very  end,  with  a  smoothly  drilled 
hole  of  0.5  mm.  diameter.  The  tip  should  always  be  kept  clean 
of  dust  and  when  in  use  yield  a 
well -pointed  symmetrical  flame  at 
right  angles  to  the  shaft,  and  should 
under  no  circumstances  be  used  as  a 
probe  to  stir  or  turn  the  assay. 

Burner. — As  the  fuel  used  in  labo- 
ratories and  under  ordinary  circum- 
stances is  gas,  the  Bunsen  burner 
will  be  used.  It  is  constructed  as 
in  Fig.  525,  stands  13  cm.  high,  with 
an  orifice  a,  near  the  base  of  the  tube, 
to  admit  air,  which  mixes  with  the 
gas  entering  at  b ;  this  mixture  passes 
up  through  the  tube  c  to  be  burned 
at  the  top.  When  the  orifice  a  is  open 
the  flame  should  be  nearly  colorless, 
and  should  not  deposit  soot  upon  any 
object  held  in  the  upper  portion  of  the 
flame.  This  colorless  or  light  blue 
flame  is  used  to  boil  test  tubes,  heat  glass  tubes,  for  fusion  with 
the  fluxes,  or  for  any  experiment  where  a  moderate  heat  only 

is  required.  The  sub- 
stance being  heated 
should  be  held  near 
the  top  of  the  flame, 
which  is  by  far  the  hot- 
test portion,  and  not 
near  the  top  of  the 
burner  tube,  as  the 
beginner  is  always  in- 
clined to  do.  When 
the  orifice  a  is  closed 
and  no  air  is  allowed 
to  enter  and  mix  with 
the  gas  as  it  passes  up, 
the  flame  will  be  yellow  and  yield  a  thick  deposit  of  soot 
upon  any  object  that  may  be  held  in  it;  while  this  flame  is  useless 


FIG.  525.  — The  Bunsen  Burner. 


FIG.  526.  —  The  Blowpipe  Burner. 


548  MINERALOGY 

for  ordinary  purposes,  it  is  the  flame  always  used  with  the 
blowpipe.  It  is  better  to  have  two  burners,  one  the  Bunsen 
burner  as  just  described,  and  another,  a  modified  burner,  as  in 
Fig.  526.  This  burner  is  fitted  with  a  special  cap  a  which  flattens 
the  flame  in  the  direction  of  the  blast.  This  cap  simply  pushes  on 
the  top  of  the  burner  tube  and  is  easily  removed,  when  the  burner 
may  be  used  in  the  ordinary  way.  For  convenience  in  use  and  ease 
in  blowing,  this  modified  burner  should  stand  not  more  than  8  cm. 
high. 

Where  gas  is  not  to  be  had,  as  in  field  work  and  prospecting,  the 
paraffine  candle  will  take  the  place  of  the  blowpipe  burner,  or  a 
kerosene  oil  lamp,  with  a  flat  wick,  will  furnish  more  heat  and  larger 
charges  may  be  used.  For  heating  test  tubes  the  alcohol  lamp  is 
easily  carried  and  yields  a  hot,  colorless  flame,  which  deposits  no 
soot. 

Before  testing  substances  in  the  blowpipe  flame  it  is  necessary 
to  thoroughly  understand  the  difference  between  the  oxidizing  flame, 
always  designated  the  O.  F.,  and  the  reducing  flame,  designated 
the  R.  F.  The  student  must  practice  blowing  these  two  flames 
until  a  pure  flame  is  obtained  in  each  case,  which  can  be  kept  con- 
stant and  can  be  continued  for  some  time  with  ease  and  without 
exertion. 

Structure  of  the  flames.  —  Light  the  Bunsen  burner,  open  the 
orifice  and  admit  air  to  mix  with  the  gas,  have  the  cock  opened 
until  the  flame  stands  about  7  cm.  high.  Immediately 
above  the  tube  of  the  burner  will  be  seen  a  conical-shaped 
area,  a,  Fig.  527.  This  cone  is  formed  by  the  upward 
pressure  of  the  gas.  Here  the  flame  is  hollow,  the  gas  has 
not  yet  ignited,  and  there  is  little  or  no  heat.  If  a  plati- 
num wire  be  pushed  quickly  across  this  cone,  it  will  become 
red-hot  on  either  side  of  this  area  and  in  the  center  will  re- 
main dark  for  some  little  time.  Surrounding  this  area 
like  a  mantle  is  the  inner  blue  cone  b,  Fig.  527,  where  the 
ignited  gas  is  being  decomposed ;  here  it  contains  a  large 
proportion  of  carbon  monoxide,  which  burning  to  carbon 
dioxide  colors  the  flame  blue.  Carbon  monoxide  has  a 
strong  affinity  for  oxygen ;  thus  this  blue  mantle  will  re- 
duce  a  large  number  of  compounds  when  held  in  it.  The 
most  effective  reducing  portion  of  the  blue  cone  is  just 
below  the  tip ;  the  substance  to  be  reduced  is  held  so  as  to  be 
completely  surrounded  by  the  blue  flame. 


INSTRUMENTS  AND   CHEMICAL  TESTS  549 

The  third  area  c,  a  large  purplish  cone  inclosing  the  blue  mantle, 
is  formed  by  the  heated  products  of  combustion  mixed  with  air, 
which  rushes  into  the  flame  from  all  sides.  Area  c  is  oxidizing,  as 
substances  held  in  it  are  raised  to  a  high  temperature  and  while 
heated  come  in  contact  with  the  oxygen  of  the  air.  The  hottest 
portion  is  immediately  outside  the  blue  mantle.  In  heating  test 
tubes  and  other  large  objects  they  should  always  be  held  above  the 
blue  cone  and  never  down  near  the  tube  of  the  burner. 

The  areas  of  the  O.  F.  as  produced  with  the  blowpipe  are  very 
similar  to  those  of  the  Bunsen  burner.  To  produce  the  0.  F., grasp 
the  blowpipe  in  the  right  hand  with  the  first  and  second  fingers 
above,  the  third  and  fourth  fingers  below  the  shaft  of  the  instru- 
ment, with  the  thumb  braced  up  toward  the  mouthpiece.  In  this 
position  a  firm  pressure  may  be  applied  to  the  mouthpiece,  a  very 
material  aid  in  keeping  the  flow  of  air  constant,  and  at  the  same 
time  the  blowpipe  will  not  slip  through  the  hand.  With  the  hand 
in  the  position  described,  place  the  lips  against  the  mouthpiece, 
fill  the  cheeks  with  air  from  the  lungs ;  while  the  air  from  the  cheeks 
is  passing  slowly  through  the  blowpipe  take  another  breath  through 
the  nose.  With  each  breath  taken  keep  the  tension  on  the  cheek 
muscles  as  nearly  constant  as  possible.  With  a  little  practice  the 
constant  draft  through  the  blowpipe  can  be  continued  almost 
indefinitely  with  but  little  exertion. 

Having  practiced  the  breathing  as  described,  place  the  forearm, 
about  halfway  from  the  elbow,  against  the  edge  of  the  desk.  The 
lighted  burner  should  be  placed  to 
suit  this  position  and  with  the  nar- 
row slot  in  the  cap  parallel  to  the 
edge  of  the  desk,  slanting  down 
to  the  left.  Place  the  tip  of  the 
blowpipe  over  the  slot,  covering 
half  of  it,  and  5  mm/above  the 
cap.  Blow  a  strong  constant  draft 

through  the  blowpipe  parallel  to  FIG.  528.— The  O.  F. 

the  slot.  The  flame  will  be  di- 
rected down  and  to  the  left,  by  the  draft,  as  in  Fig.  528.  It  should 
be  about  3  cm.  long,  ^harp-pointed,  with  a  well-defined  inner  blue 
cone  a  corresponding  to  the  blue  mantle  of  the  Bunsen  burner 
flame.  It  should  burn  steadily  without  sputtering  or  hissing,  and 
should  contain  no  yellow  stripes.  If  it  is  irregular,  forked,  or  hisses, 
the  tip  is  probably  imperfect,  contains  particles  of  dust,  the  orifice 


550  MINERALOGY 

is  rough,  or  the  draft  has  been  directed  against  the  cap  of  the 
burner.  All  of  these  must  be  looked  to,  until  all  irregularities 
of  flame  are  corrected.  The  length  of  the  flame  may  be  regulated 
by  the  position  of  the  platinum  tip.  If  a  short  flame  is  required, 
the  tip  is  pushed  a  little  farther  over  the  slot,  and  when  a  long  flame 
is  needed  the  tip  is  withdrawn,  when  the  flame  will  shorten  or 
lengthen  as  the  case  may  be. 

The  O.  F.  is  used  to  oxidize  substances ;  for  this  use  the  object  is 
held  beyond  the  tip  of  the  blue  cone.  It  must  be  remembered  that 
the  blue  cone  is  formed  by  burning  carbon  monoxide,  which  is  reduc- 
ing, therefore  the  blue  cone  must  not  come  in  contact  with  the 
substance  being  oxidized.  The  0.  F.  is  used  for  testing  the  fusi- 
bility of  minerals,  testing  for  flame  colorations,  and,  as  it  is  the 
hottest  flame,  in  general  where  a  high  temperature  is  required. 

To  test  the  temperature  of  the  O.  F.  take  a  piece  of  platinum 
wire  0.1  mm.  in  diameter,  bend  5  mm.  of  the  end  at  right  angles  ; 
holding  the  wire  in  the  forceps,  place  it  in  the  flame  just  so  that  the 
extremity  touches  the  tip  of  the  blue  cone,  with  the  bent  portion 
parallel  to  the  draft  and  pointing  in  the  direction  of  the  blowpipe. 
If  the  conditions  are  right,  the  wire  will  be  seen  to  shorten  and  a 
small  globule  of  fused  platinum  form  on  the  end.  Platinum  fuses 
at  a  temperature  of  1755°  C.  This  temperature  must  have  been 
reached  in  the  small  flame  or  the  globule  could  not  have  formed. 
It  will  also  be  seen  that  it  is  not  the  largest  flame,  but  a  small  well- 
pointed  flame  which  fuses  the  wire  the  more  quickly. 

The  reducing  flame,  R.  F.  —  It  is  only  necessary  to  turn  the  gas 
up  somewhat  and  withdraw  the  blowpipe  tip  until  the  point  is 
just  over  the  edge  of  the  burner  and  somewhat  higher  than  when 
blowing  the  0.  F.  The  entire  flame  will  be  deflected  by  the  draft 
to  the  left  and  a  little  below  the  horizontal. 

The  R.  F.  is  composed  of  burning  hydrocarbons  and  carbon 
monoxide  and  colored  with  particles  of  glowing  carbon,  all  of  which 
are  reducing  agents.  The  R.  F.  will  reduce  many  oxides  to  a  lower 
state  of  oxidation.  If  red  oxide  of  iron,  Fe203,  be  held  in  it  for  only  a 
short  time  it  will  be  reduced  to  the  protoxide,  FeO,  thus  Fe203  -f- 
CO  =  2  FeO  +  C02 ;  one  third  of  the  oxygen  will  be  taken  up  by 
the  R.  F.  If  the  FeO  be  held  in  a  pure  O.  F.  it  will  pass  back  to 
Fe203.  In  all  such  cases  a  portion  of  the  oxygen  is  at  the  com- 
mand of  the  operator ;  it  can  be  taken  away  or  added  to  the  sub- 
stance according  to  the  flame  used. 

Another  class  of  oxides  when  treated  in  the  R.  F.  is  reduced  di- 


INSTRUMENTS  AND   CHEMICAL  TESTS  551 

rectly  to  metal,  thus  PbO  +  CO  =  Pb  +  CO2,  which  again  may 
be  oxidized  by  the  O.  F.  The  substance  to  be  reduced  should  be 
completely  covered  by  the  flame.  In  a  flickering,  unsteady  flame, 
where  the  assay  is  alternately  covered  by  the  R.  F.  and  then 
exposed  to  the  oxygen  of  the  air,  there  is  reduction,  then  oxidation, 
and  in  this  way  the  operation  may  be  prolonged  indefinitely,  in 
fact  good  results  never  obtained. 

Coal  and  its  use.  —  Charcoal  is  used  as  a  support  for  substances 
to  be  tested,  either  in  the  0.  F.  or  R.  F.,  and  when  used  with  the 
R.  F.  materially  aids  the  reduction ;  as  a  reducing  agent  it  is  mixed 
directly  with  the  assay.  The  best  coal  is  that  from  light  woods,  as 
bass  and  willow ;  such  coal  contains  a  small  amount  of  ash  and  is 
a  better  non-conductor.  It  can  be  held  in  the  fingers  while  in  use 
for  a  long  time  without  discomfort.  It  is  sawn  in  lengths  of  12  cm. 
with  a  square  cross  section  2  cm.  wide ;  coals  of  this  shape  are 
economical,  as  all  four  sides  can  be  used.  They  should  be  dry, 
free  of  cracks,  burn  without  snapping  or  cracking,  and  yield  a  light 
flocculent  ash,  which  is  easily  blown  away.  A  single  piece  of  coal 
will  last  for  a  long  time,  if  after  each  experiment  the  surface  is 
either  filed  or  scraped  clean  with  the  spatula.  In  treating  an  assay 
on  coal  a  cavity  is  made  with  the  steel  coal  borer,  near  the  end  of 
the  coal ;  the  depth  of  the  cavity  will  depend  upon  the  character  of 
the  experiment.  If  a  reduction  is  required,  the  hole  is  made  rather 
deep,  3  to  5  mm.,  with  steep  walls.  If  an  oxidizing  reaction,  the 
cavity  should  be  shallow,  only  sufficient  to  support  the  assay.  In 
heating  the  assay,  the  coal  is  held  between  the  first  finger  and  thumb, 
with  its  long  axis  parallel  to  the  blast,  slightly  tilted  toward  the 
flame,  as  in  Fig.  529.  In  this  position  the  coal  protects  the  hand, 


FIG.  529.  —  Position  of  Assay  on  the  Coal. 

while  the  draft  from  the  blowpipe  will  sweep  any  volatile  compounds 
over  the  outer  end  of  the  coal,  where  they  may  collect  as  coats. 


552  MINERALOGY 

Substances  are  either  heated  alone  on  coal  or  mixed  with  a  flux 
to  aid  the  reaction.  When  heated  alone  the  following  phenomena 
may  occur:  (1)  It  volatilizes,  disappears  a*s  a  vapor.  Volatility 
must  not  be  confounded  with  (2)  decrepitation,  where  the  mineral 
is  thrown  off  the  coal  by  crackling  due  to  the  evolution  of  gases  in 
cavities  causing  the  mineral  to  explode.  (3)  A  coat  may  form  on  the 
cooler  portion  of  the  coal.  Metallic  vapors  driven  from  the  assay 
come  in  contact  with  the  oxygen  of  the  air,  are  oxidized,  and  settle 
on  the  coal,  forming  coats.  (4)  Some  oxides  and  compounds  are 
reduced  to  metal  when  heated  along  on  coal,  as  Cu,  Pb,  Sn,  Ag,  Au, 
Pt  all  yield  metallic  globules,  which  again  upon  further  heating 
may  volatilize  and  yield  a  coat,  according  to  the  metal  reduced. 
(5)  Magnetism,  in  the  case  of  compounds  of  iron,  the  assay  majr 
become  magnetic ;  nickel  and  cobalt  when  reduced  to  metal  will 
also  be  magnetic.  (6)  Fusion,  the  fragment  becomes  rounded  on 
the  edges  or  if  easily  fused  forms  a  spherical  globule.  (7)  Odors, 
as  of  a  burning  match,  very  characteristic  of  sulphur  dioxide  ; 
arsenic,  selenium,  and  tellurium  also  yield  odors.  (8)  Flame  color- 
ation, the  vapors  of  some  metals  when  burning  color  the  flame,  as 
antimony,  zinc,  copper,  or  lead. 

Illustration.  —  In  a  shallow  cavity  on  coal  place  some  oxide  of 
lead,  then  holding  the  coal  as  directed,  blow  a  short,  hot  R.  F.,  cov- 
ering the  assay  with  the  flame.  It  will  be  seen  to  fuse  first,  then 
little  globules  of  metallic  lead  will  appear,  which  grow  in  size  until 
all  the  oxide  is  reduced  to  metal.  Change  the  flame  to  the  0.  F., 
holding  the  globule  just  beyond  the  tip  of  the  blue  cone.  The  me- 
tallic lead  will  boil,  the  vapors  of  lead  coloring  the  flame  an  azure- 
blue.  If  the  0.  F.  is  continued,  the  globule  will  volatilize  entirely ; 
the  hot  metallic  vapor  will  combine  with  the  oxygen  of  the  air  and 
settle  on  the  coal  beyond  the  assay,  forming  a  yellow  oxide  of  lead 
coat. 

Roasting  is  a  metallurgical  term,  used  when  either  sulphur,  ar- 
senic, antimony,  or  other  volatile  compounds  are  either  burned  or 
driven  off  an  ore  by  heat  and  oxidation.  In  blowpipe  work  roast- 
ing is  in  some  cases  a  necessary  operation  preliminary  to  reduction 
on  coal  or  testing  with  the  fluxes.  The  substance  to  be  roasted  is 
ground  to  a  fine  powder,  spread  out  in  a  thin  layer,  in  a  broad  shal- 
low cavity  on  coal,  then  heated  very  gently  with  the  0.  F.  The 
assay  should  be  carefully  watched  and  not  allowed  to  fuse,  as  fusion 
will  prevent  the  air  from  coming  in  contact  with  each  particle.  If 
the  assay  has  been  fused,  it  should  be  powdered  again,  returned  to 


INSTRUMENTS  AND   CHEMICAL  TESTS  553 

the  coal,  and  the  roasting  continued.  The  assay  is  stirred  from  time 
to  time  or  turned  over  .with  the  spatula  until  all  parts  are  brought  in 
contact  with  the  flame  and  air.  If  sulphates  and  arsenates  are 
present,  the  O.  F.  and  R.  F.  should  be  used  alternately.  When 
white  fumes  no  longer  escape  or  the  odor  of  sulphur  dioxide  is  no 
longer  perceptible,  the  roasting  is  completed ;  the  residue  remain- 
ing, generally  speaking,  is  an  oxide,  and  is  in  condition  for  use 
in  subsequent  bead  or  reduction  tests  with  the  fluxes. 

Illustration.  —  Spread  finely  ground  pyrite,  FeS2,  in  a  thin  layer 
over  a  shallow  cavity  on  coal,  heat  gently  in  the  O.  F.,  but  not  hot 
enough  to  fuse  it.  Bluish  fumes  of  SO2,  identified  by  the  odor,  will 
arise  as  the  result  of  oxidation;  the  assay  will  blacken  from  the 
formation  of  the  magnetic  oxide  of  iron,  Fe3O4,  which  may  be 
proven  with  the  magnet.  If  the  roasting  is  completed  and  the 
oxidation  continued,  the  final  product  will  be  the  red  oxide  of  iron, 
Fe203,  which  is  not  magnetic. 

Platinum  wire,  .4  mm.  in  diameter,  is  used  to  support  fine  pow- 
der in  testing  for  flame  colorations,  for  fusing  silicates  with  soda 
(Na2C03),  but  especially  as  a  support  for  the  borax  and 
salt  of  phosphorus  beads,  in  testing  for  the  bead  colora- 
tions, so  characteristic  of  many  oxides.  For  this  purpose 
the  wire  is  held  in  the  platinum  wire  holder,  Fig.  530, 
the  free  end  is  bent  in  a  loop  as  nearly  circular  as  pos- 
sible 4  mm.  in  diameter.  To  make  the  borax  bead,  heat 
the  loop  of  wire  in  the  0.  F. ;  while  hot  touch  it  to  the 
powdered  borax,  or  to  a  grain  of  borax  of  suitable  size, 
then  fuse  it  in  the  0.  F.  until  all  bubbles  have  dis- 
appeared, and  the  bead  is  clear  and  colorless  when  cold. 
Sufficient  borax  should  be  used  to  fill  the  loop  and  form 
a  spherical  bead  when  cold.  Oxides  are  dissolved  in 
the  borax  bead  by  touching  the  hot  bead  to  the  oxide, 
when  some  of  it  will  stick  to  it.  It  is  then  heated  once 
more  in  the  0.  F.  until  all  the  little  particles  seen  floating 
in  the  fused  borax  have  disappeared  and  the  bead  is  FIG~53o 
clear  and  transparent.  If  oxide  of  manganese  is  used, 
the  bead  when  cold  will  be  colored  violet-red.  In  viewing  a 
bead  it  should  be  held  up  to  the  light  of  a  window,  or  with  a 
white  background ;  never  hold  it  between  the  eye  and  the 
yellow  flame  of  the  burner.  If  the  bead  is  dark  and  opaque, 
an  excess  of  oxide  has  been  dissolved,  and  the  color  will  appear 
only  after  it  has  been  flattened  with  a  gentle  pressure  with 


554  MINERALOGY 

the  hammer  while  it  is  still  plastic ;  or  better  still,  make  a  fresh 
bead  and  dissolve  less  oxide  in  it.  It  is  always  better  to  use  a  very 
small  amount  of  material  at  a  time  and  work  up  to  the  color  by 
several  additions,  examining  the  bead  after  each  addition,  than  to 
at  once  saturate  the  bead.  After  an  examination  of  the  color  pro- 
duced in  the  O.  F.  the  same  bead  is  now  held  in  the  R.  F.,  so  that  it  is 
completely  covered,  on  all  sides,  by  the  flame,  and  in  this  way 
protected  from  the  oxygen  of  the  air.  The  manganese  sesquioxide, 
Mn203,  which  colored  it  violet-red  in  the  O.  F.,  will  be  slowly  re- 
duced to  manganese  protoxide,  MnO,  which  has  no  perceptible 
coloring  effect  in  borax.  The  bead  when  cold  and  completely 
reduced  will  be  clear  and  colorless.  Highly  charged  beads  may 
assume  a  light  violet  color  due  to  oxidation  while  cooling.  They 
should  be  cooled  quickly  by  pressing  them  while  still  hot  on 
the  anvil  with  the  hammer.  This  same  bead  may  be  again  oxi- 
dized, when  it  will  become  violet-red.  For  practice  in  handling 
the  flames  the  student  should  alternately  oxidize  and  reduce  it 
several  times. 

Salt  of  phosphorus,  NaNH4HP04 . 4  H2O,  is  used  in  the  bead 
tests  in  the  same  way  as  borax ;  the  two  are  known  as  the  glass 
fluxes.  Upon  first  heating  the  salt  boils  violently,  due  to  the  large 
quantity  of  water  it  contains ;  for  this  reason  the  bead  must  be 
built  up  by  installments,  heating  after  each  addition  until  boiling 
ceases  and  a  clear  colorless  glass  remains.  During  the  fusion  the 
composition  has  been  changed  by  the  loss  of  water  and  ammonia  ; 
sodium,  metaphosphate  remains,  NaP03;  this  is  the  S.  Ph.  bead. 
Its  use  is  the  same  as  the  borax  bead. 

Platinum  forceps  are  used  to  hold  fragments  of  minerals  in  the 
O.  F.  either  to  test  their  fusibility  or  for  a  flame  coloration.  The 
most  convenient  model,  Fig.  531,  is  made  of  steel  nickel  plated ;  one 
end  is  the  ordinary  steel  forceps  used  in  picking  up  hot  beads,  etc.  ; 

the  other  end  is  self- 
holding  and  platinum- 
tipped.   Minerals  with 
FIG.  531.  luster  which   fuse 

easily,  or  those  which 

reduce  to  metal,  should  not  be  held  in  the  platinum,  as  they  will 
alloy  with  it  and  the  forceps  will  be  ruined. 

Illustration.  —  Heat  a  slender  fragment  of  strontionite  (SrC03) 
in  a  small  well-pointed  O.  F.,  holding  the  fragment  of  mineral  in 
such  a  manner  that  it  projects  out  beyond  the  platinum  tips  and 


INSTRUMENTS   AND  CHEMICAL   TESTS  555 

may  be  held  at  the  tip  of  the  blue  cone  where  it  will  be  heated  very 
hot  without  actually  heating  the  forceps  at  all.  When  sufficiently 
hot,  it  will  color  the  flame  intensely  red.  In  the  testing  of  minerals 
for  fusibility  a  slender  fragment,  not  larger  than  1  mm.  in  thickness 
at  the  point,  should  be  selected ;  if  the  mineral  fuses  with  difficulty, 
very  fine-pointed  pieces  should  be  tested  before  it  is  decided  that 
the  mineral  is  infusible.  Select  a  suitable  piece  of  almandite 
garnet,  hold  it  as  in  testing  for  flame  coloration,  at  the  tip  of  the 
blue  cone,  care  being  taken  that  it  projects  beyond  the  forceps. 
Almandite  fuses  at  3 ;  its  edges  will  become  rounded  and  at  the 
highest  temperature  the  entire  end  of  the  fragment  will  be  globular. 
Select  a  piece  of  orthoclase  of  the  standard  size  (fusibility  5) ;  after 
heating  in  the  same  way  it  will  be  found  that  the  edges  are  rounded 
only;  fine  needle-like  pieces  will  fuse  to  a  globule  on  the  end. 

Fusibility  is  determined  by  comparison  with  a  mineral  selected  as 
a  standard.  The  scale  of  fusibility  was  arranged  by  Von  Kobell  and 
modified  by  Penfield.  Specimens  of  the  standard  minerals  should 
be  tested. 

Fusibility  1,  Stibnite,  large  fragments  fuse  in  the  yellow  gas 
flame. 

Fusibility  2,  Chalcopyrite,  small  fragments  fuse  to  a  globule  in 
the  yellow  flame. 

Fusibility  3,  Almandite,  coarse  fragments  become  globular  in 
the  0.  F. 

Fusibility  4,  Actinolite,  coarse  edges  are  rounded  in  the  O.  F. 

Fusibility  5,  Orthoclase,  needle-like  fragments  become  globular 
in  O.  F. 

Fusibility  6,  Bronzite,  needle-like  fragments  become  rounded 
on  the  point. 

While  heating  a  mineral  for  its  fusibility,  it  should  be  carefully 
watched  and  the  following  noted  :  whether  it  intumesces,  that  is, 
swells  and  bubbles  when  it  fuses ;  whether  it  swells  or  curls  with- 
out fusing — exfoliates;  whether  it  becomes  enameled  or  is  glassy 
and  clear  after  fusion;  or  whether  it  fuses  to  a  blebby,  vesicular 
glass.  All  these  conditions  are  quite  important  in  the  determina- 
tion of  minerals. 

Hammer  and  anvil.  — Any  small  hammer  will  serve,  as  it  is  used 
only  to  break  small  pieces  of  minerals  and  to  flatten  malleable 
buttons. 

A  block  of  hardened  steel  3  cm.  square  serves  as  an  anvil ;  both 
hammer  and  anvil  should  be  well  polished  and  kept  free  of  rust. 


556  MINERALOGY 

Agate  mortar  and  pestle.  —  Used  to  grind  minerals  to  a  fine 
powder,  and  in  the  washing  of  assays  for  malleable  buttons.  A 
fragment  of  the  mineral  to  be  ground  is  wrapped  in  paper  and 
broken  down  to  a  coarse  powder  with  the  hammer  on  the  anvil.  A 
small  amount  is  then  placed  in  the  mortar  and  ground.  In  testing 
for  solubility,  gelatinization,  and  in  fusion  with  the  fluxes  all 
minerals  must  be  very  finely  ground. 

Magnifying  glass.  —  Used  to  examine  small  crystals,  to  search 
washed  slags  for  malleable  buttons,  and  to  examine  fragments  of 
minerals  after  heating  to  determine  their  fusibility. 

Magnet.  —  A  small  bar  magnet  shaped  like  a  cold-chisel,  used 
in  testing  the  magnetism  of  minerals  and  slags. 

Files  triangular  and  flat.  —  Used  for  testing  the  hardness  of 
minerals  and  cleaning  coals. 

GLASSWARE 

Dropping  Bottle.  —  Used  to  drop   cobalt  solution  or  to  drop 

water  in  mixing  charges  for  reduction  on  coal.  The  ordinary  med- 
icine dropper,  Fig.  532,  is  convenient, 
or  one  can  be  quickly  made  by  fitting 
a  small  bottle  with  a  perforated  cork 
through  which  a  glass  tube  extends 
to  near  the  bottom  of  the  bottle,  which 
is  cut  off  2  cm.  above  the  cork.  In 
use  the  cork  is  grasped  between  the 
thumb  and  second  finger,  while  the 
first  finger  is  placed  over  the  open 
end  of  the  glass  tube  when  the  cork 

FIG.  532.— Dropper.  an<^  tube  ^s  withdrawn  from  the  bot- 

tle;  by  releasing  the  pressure  of  the 

finger  on  the  top    of   the  tube,  the    liquid    contained  will   fall 

slowly  out,  a  drop  at  a  time. 

Test  tubes.  —  Used  for  chemical  tests  in  the  wet  analysis,  for 

boiling  acids,  etc.    -A  convenient  size  15  mm.  in  diameter  by  15  cm. 

long. 

Filter  funnels.  —  Glass  funnels  5  cm.  in  diameter  and  cut  filter 
papers  7  cm.  in  diameter.  In  use  the  papers  are  folded  twice, 
forming  a  quadrant,  one  side  of  which  is  opened,  and  the  cone  thus 
formed  is  fitted  in  the  funnel  and  dampened  with  water,  when  it  is 
ready  for  use.  The  solution  to  be  filtered  is  carefully  poured  into 
the  paper,  never  filling  it  more  than  two  thirds  full.  After  the 


INSTRUMENTS  AND   CHEMICAL  TESTS.  557 

solution  has  drained  off,  the  solids  are  washed  by  dropping  distilled 
water  from  the  wash  bottle  around  the  edge  of  the  paper ;  after  it 
drains,  repeat  several  times,  when  the  solids  may  be  considered  prac- 
tically free  of  salts  carried  in  solution,  at  least  free  enough  for 
qualitative  tests. 

Closed  tubes  or  matrasses  are  made  from  hard  glass  tubing  6  mm. 
inside  diameter.  The  tube  is  cut  in  15  cm.  lengths ;  these  are  heated 
in  the  middle  in  the  Bunsen  burner  flame  until  the  glass  softens, 
constantly  turning  the  tube ;  draw  apart  quickly  until  the  two  ends 
part.  Reheat  the  end  that  has  been  drawn  out,  while  soft  cut  off 
the  extremity  with  an  old  pair  of  scissors,  then  fuse  in  the  flame 
until  rounded.  They  should  be  cooled  slowly  or  annealed  to  pre- 
vent cracking  when  they  are  reheated  while  in  use. 

The  closed  tube  is  used  to  heat  substances  out  of  contact  with  the 
oxygen  of  the  air,  therefore  there  is  little  or  no  oxidation.  Sub- 
stances to  be  heated  ,^_____^_^^^ 

ments  or  coarse  powder,  FIG  533  _  Matrags  Holder 

and  simply  dropped  to 

the  bottom.     The  tube,  held  in  the  matrass  holder,  Fig.  533,  is 

heated  with  the  O.  F.  gently  at  first,  finally  increasing  the  heat 

until  the  walls  of  the  tube  are  fused.     During  the  heating  the 

following  phenomena  are  looked  for. 

a.  Water.  —  Some  minerals  contain  water  of  crystallization  or 
water  of  constitution ;  the  first  is  driven  off  at  a  comparatively  low 
temperature,  the  latter  at  a  much  higher  temperature.  This 
water  will  collect  in  little  drops  or  as  a  mistlike  coat  on  the  cold 
walls  of  the  upper  end  of  the  tube.  It  is  needless  to  say  that  care 
must  always  be  taken  that  the  open  end  is  always  cold 
enough  to  condense  the  water.  Heat  a  small  piece  of  calamine 
(Zn2SiO4.H20). 

6.  Sublimates  are  solids  condensed  on  the  cold  walls  of  the  tube, 
formed  of  vapors  driven  from  the  mineral  being  heated.  They 
may  be  white  or  colored.  Heat  some  arsenopyrite  (FeSAs), 
gently  at  first,  when  a  bright  red  sublimate  of  sulphide  of  arsenic 
will  form,  becoming  brownish  red  as  it  collects.  On  continued 
heating  crystals  of  metallic  arsenic  will  form  a  little  below  the 
sulphide,  finally  forming  a  complete  band  or  metallic  mirror  of 
arsenic. 

c.  Odors  caused  by  escaping  gases  and  acid  fumes. 

d.  Charring.  —  The   substance   blackens   and  usually  emits  a 


558  MINERALOGY 

bituminous  odor.  If  the  black  residue  burns  or  glows  in  the  air 
when  heated,  the  presence  of  carbon  or  organic  matter  may  be 
assumed. 

e.  Change  of  color.  —  Characteristic  of  some  oxides. 

Open  tube.  —  Pieces  of  hard  glass  tube  15  cm.  long  and  7  mm. 
inside  diameter  are  used  to  heat  coarsely  powdered  minerals  in  a 
current  of  air,  oxidizing  the  volatile  compounds  as  they  are  driven 
off  by  the  heat.  Substances  to  be  tested  are  placed  about  3  cm. 
from  the  end.  The  tube  is  held  in  the  matrass  holder  ;  the  O.  F.  is 
directed  on  the  glass  directly  under  the  coarse  powder,  at  the  same 
time  the  tube  is  held  diagonally  across  the  flame  and  45°  from  the 
perpendicular.  The  object  is  to  obtain  a  draft  of  air  to  furnish  the 
oxygen  and  to  also  carry  the  vapors  up  through  the  tube,  where 
they  may  condense  as  sublimates  or  pass  out  as  gases.  Heat  is 
applied  gently  at  first,  increasing  the  temperature  to  full  redness. 

a.  The  escaping  gas  smells  like  a  burning  sulphur  match,  due  to 
sulphur  dioxide.  Arsenic  and  selenium  also  yield  characteristic 
odors. 

Illustration.  —  Heat  some  coarsely  powdered  pyrite  in  the  open 
tube.  Here  a  sublimate  of  sulphur  is  not  obtained,  as  in  the  closed 
tube,  but  the  sulphur  combines  with  the  oxygen  of  the  air  passing 
through  the  tube,  forming  SO2,  which  escapes  at  the  upper  end, 
yielding  the  odor. 

6.  Sublimates.  —  They  may  be  white  or  colored  as  in  the  closed 
tube. 

Illustration.  —  Heat  a  small  fragment  of  arsenic  in  an  open  tube. 
The  sublimate  formed  is  not  a  metallic  mirror  as  in  the  closed 
tube,  but  a  white  sublimate  of  trioxide  of  arsenic  (A2O3)  which  is 
composed  of  octahedral  crystals.  Examine  with  the  magnifying 
glass. 

Watch  glasses  to  hold  powdered  mineral,  soda,  borax,  etc.,  and 
one  or  more  porcelain  evaporating  dishes  will  be  required  to  con- 
centrate solutions  and  to  gelatinize  powdered  minerals.  If  the 
under  sides  of  these  porcelain  dishes  are  unglazed,  they  will  serve  as 
streak  tablets  also. 

Blue  Glass.  —  A  darkly  colored  piece  of  cobalt  glass  5  cm.  square 
used  for  the  absorption  of  sodium  light  in  testing  for  potassium. 
Bottles  of  clear  glass  with  plane  parallel  sides,  and  filled  with 
solutions  of  potassium  permanganates  or  chrome  alum  are  much 
better  than  the  blue  glass  but  less  convenient  for  use  in  the 
potassium  test,  see  page  563. 


INSTRUMENTS  AND   CHEMICAL  TESTS  559 

REAGENTS 

Solid  reagents  are  kept  in  small  salt-mouth  bottles  from  which 
they  are  easily  removed  in  small  quantities,  with  a  spatula,  as 
required  for  use.  To  insure  a  clean  and  pure  reagent  the  excess 
should  never  be  returned  to  the  bottle. 

Borax  glass,  fused  sodium  biborate,  Na2B4O7,  in  granules,  the 
size  of  rice  grains,  and  also  finely  powdered  borax  for  use  as  a  flux. 

Salt  of  Phosphorus.  —  Sodium  ammonium  hydrogen  phosphate, 
NaNH4HPO4 ,  4  H2O.  For  S.  Ph.  beads. 

Soda,  dry  sodium  carbonate,  Na2CO3,  used  as  a  flux  in  the  decom- 
position of  silicates.  Five  parts  of  soda  are  mixed  with  one  part 
of  finely  powdered  mineral  and  a  drop  or  two  of  water  added  to 
form  a  paste  ;  this  charge  is  fused  on  coal  or  platinum  wire  until  it 
stops  effervescing,  when  the  charge  is  dissolved  in  dilute  acid. 

Potassium  bisulphate.  —  Potassium  hydrogen  sulphate,  KHSO4, 
is  used  as  a  component  of  special  fluxes.  Upon  heating  potassium 
bisulphate  loses  water,  passing  over  to  potassium  pyrosulphate, 
K2S207 ;  with  continued  heating  SOs  is  driven  off  as  white  fumes. 
The  residue  is  normal  potassium  sulphate,  K2S04. 

Potassium  nitrate,  KN03,  is  used  in  small  quantities  as  an  oxidiz- 
ing agent. 

Copper  oxide,  CuO,  finely  powdered,  is  used  in  the  dry  test 
for  chlorine. 

Test  lead,  both  granular  and  sheet,  free  of  silver,  is  used  in  the 
cupellation  test  for  silver. 

Tin,  granular  and  sheet,  is  used  in  the  reduction  of  salts  in  solu- 
tion, also  as  a  reducing  agent  when  fused  with  the  borax,  or  S.  Ph. 
bead  on  coal. 

Silver,  a  bright  silver  surface,  as  a  coin,  is  used  in  the  test  for 
sulphur. 

Bone  ash.  Used  in  making  the  cupels  in  the  separation  of  silver 
and  gold  from  the  lead  button. 

Turner's  flux.  —  Made  by  mixing  one  part  of  powdered  fluor- 
spar with  four  parts  of  potassium  bisulphate.  Used  in  the  decom- 
position of  borates  in  testing  for  the  boric  acid  flame. 

Lithium  flux.  —  Made  by  mixing  one  part  of  powdered  fluor- 
spar with  one  and  one  half  parts  of  potassium  bisulphate.  Used 
in  the  decomposition  of  silicates  in  testing  for  a  lithium  flame. 

Potassium  flux.  —  Made  by  mixing  three  parts  of  powdered 
calcium  carbonate  and  one  of  ammonium  chloride.  Used  in 


560  MINERALOGY 

the  decomposition  of  silicates  in  testing  for  the  potassium  flame. 
The  flux  should  be  tested  for  the  potassium  flame  to  insure  its 
purity. 

Von  Kobell's  flux.  —  Made  by  mixing  equal  parts  of  potassium 
iodide  and  flowers  of  sulphur.  Used  in  .testing  for  the  lead  and 
bismuth  iodide  coats  on  coal. 

Blue  litmus  paper.  Used  in  testing  the  acidity  of  fumes  and  of 
the  water  yielded  in  the  closed  tube. 

Turmeric  paper.  Used  in  the  alkaline  test  and  also  for  the  detec- 
tion of  boric  acid  and  zirconium. 

REAGENTS  USED  IN  SOLUTION 

Wet  reagents  are  made  up  in  the  strengths  indicated  in  each 
instance.  They  should  be  kept  in  bottles  with  tightly  fitting 
ground  glass  stoppers.  The  water  used  is  distilled. 

Hydrochloric  acid,  used  in  two  strengths.  Concentrated  as 
obtained  from  the  supply  houses  is  spc.  1.19  to  1.20,  containing 
about  39  per  cent.  HC1.  The  dilute  acid  is  made  by  adding  an 
equal  volume  of  water  to  the  concentrated ;  this  yields  a  solution  of 
HC1  a  little  stronger  than  five  times  normal,  5  n.  . 

Sulphuric  acid,  H2SO4,  the  concentrated  as  obtained  is  spc. 
1.84  and  is  36  times  normal.  The  dilute  is  made  by  pouring  slowly 
one  volume  of  concentrated  acid  into  six  volumes  of  water ;  the 
diluted  acid  will  be  approximately  five  times  normal,  5  n. 

Nitric  acid.  —  The  concentrated  acid  is  spc.  1.42,  and  contains 
69  per  cent.  HN03,  equal  to  16  times  normal.  The  dilute  acid  is 
made  by  mixing  5  volumes  of  acid  with  11  volumes  of  water;  the 
diluted  acid  will  be  approximately  5  n. 

Acetic  acid,  CH3COOH,  used  in  some  wet  tests,  as  in  the  tests 
for  chromates. 

Hydrogen  sulphide,  H2S,  is  generated  by  treating  ferrous  sul- 
phide with  sulphuric  acid,  FeS  +  H2SO4  =  FeSO4  +  H2S.  A 
bottle  may  be  used  as  the  generator ;  the  acid  is  poured  in  the 
thistle  tube  or  funnel,  the  lower  end  of  which  extends  to  the  bottom 
of  the  bottle.  The  H2S  gas  generator  is  led  off  through  the  bent 
tube.  It  is  either  used  as  a  gas  or  absorbed  in  water.  The  water 
solution  is  kept  in  a  bottle,  thus  avoiding  the  use  of  the  generator 
in  each  test. 

Sodium  hydroxide,  NaOH,  200  gm.  of  the  sticks  are  dissolved  in 
water  and  diluted  to  a  liter.  The  solution  will  be  5  n.  One  cc.  of 


INSTRUMENTS  AND   CHEMICAL  TESTS  561 

this  solution  will  neutralize  1  cc.  of  the  dilute  acids.  It  is  used  to 
precipitate  hydroxides. 

Sodium  phosphate,  disodium  phosphate,  Na2HPO4 .  12  H2O. 
Dissolve  171  gm.  in  water  and  dilute  to  a  liter.  This  solution  is 
used  in  the  test  for  magnesium. 

Ammonia,  ammonium  hydroxide,  NH4OH.  Strong  ammonia 
is  spc.  .90,  it  contains  29  per  cent.  NH3.  One  part  of  the  strong 
solution  diluted  with  3  parts  of  water  will  yield  a  solution  nearly 
5  n.  1  cc.  of  this  solution  will  neutralize  1  cc.  of  the  dilute  acids. 
Ammonia  is  used  in  many  wet  tests  and  to  precipitate  hydroxides. 

Ammonium  sulphide,  (NH4)2S,  made  by  saturating  the  dilute 
ammonium  solution  with  H2S.  It  is  used  to  precipitate  metallic 
sulphides  and  in  some  cases  hydroxides. 

Ammonium  carbonate,  (NH4)2CO3.  —  To  make  a5n.  solution, 
dissolve  200  gms.  of  commercial  ammonium  carbonate  in  350  cc. 
of  the  dilute  ammonia,  and  dilute  to  a  liter.  It  is  used  to  precipi- 
tate metallic  carbonates. 

Ammonium  oxalate,  (NH4)2G2O4 .  2  H2O.  —  To  make  a  normal 
solution  dissolve  80  gm.  in  water  and  dilute  to  one  liter.  Used  in 
testing  for  calcium. 

Ammonium  molybdate,  (NH4)2MoO4.  —  Dissolve  60  gm.  of 
molybdic  trioxide,  MoO3,  in  440  cc.  of  water  and  60  cc.  of  strong 
ammonia.  This  solution  is  slowly  poured  with  constant  stirring  into 
250  cc.  concentrated  nitric  acid  previously  diluted  with  250  cc.  of 
water.  Let  the  solution  stand  for  24  hours,  when  it  is  either 
filtered  or  decanted.  On  further  standing  the  molybdic  acid 
gradually  separates,  forming  a  light  yellow  crust  on  the  sides  and 
bottom  of  the  bottle,  thus  losing  its  strength  on  standing.  This 
reagent  is  used  to  precipitate  phosphoric  acid. 

Barium  hydroxide,  Ba(OH)2 .  8  H2O.  —  A  J  n.  solution  is  used; 
dissolve  31  gm.  in  one  liter  of  water.  Used  in  the  detection  of 
carbon  dioxide. 

Barium  chloride,  BaCl2 .  2  H20.  —  To  make  a  normal  solution 
dissolve  122  gm.  in  a  liter  of  water.  The  solution  is  used  in  the 
tests  for  sulphates. 

Lead  acetate,  Pb(CH3COO)2 . 3  H20.  —  To  make  a  normal 
solution  dissolve  189  gm.  in  a  liter  of  water.  It  is  used  in  the 
detection  of  -chromates. 

Silver  nitrate,  AgN03.  —  To  make  J  normal  solution  34  gm. 
in  a  liter  of  water.  It  is  used  in  the  test  for  chlorine. 

Ferrous  sulphate,  FeS04 . 7  H2O.  —  To  make  a  normal  solu- 
2o 


562  MINERALOGY 

tion  dissolve  139  gm.  in  a  liter  of  water.     It  is  used  in  the  test  for 
nitrates. 

Cobalt  nitrate,  Co(NO3)2.  — 10  gm.  are  dissolved  in  100  cc.  of 
water.  A  supply  is  kept  in  a  dropping  bottle  for  use.  Various 
compounds  after  ignition  in  the  0.  F.  with  cobalt  solution  yield 
characteristic  colors,  as  alumina  and  zinc  oxide. 


IDENTIFICATION  TESTS  OF  THE  ELEMENTS 

Included  here  are  the  blowpipe  tests  and  the  most  important 
chemical  reactions  used  in  the  identification  of  minerals.  The 
order  followed  is  to  a  large  extent  that  of  MendeleefFs  periodic 
system.  The  common  elements  and  therefore  the  most  important 
are  placed  first  in  each  group. 


THE  ALKALIES 

Sodium,  Na ;  Potassium,  K ;  Lithium,  Li ;  and  the  rare  elements 
Rubidium,  Rb  ;  Caesium,  Cs. 

Owing  to  the  great  solubility  of  all  salts  of  the  alkali  metals  there 
is  no  simple  direct  test  to  prove  their  presence  in  a  mineral  in 
the  wet  way.  From  a  blowpipe  standpoint  they  all  agree  in  yield- 
ing distinguishing  flame  colorations.  When  present  only  in  small 
amounts,  as  is  generally  the  case  of  rubidium  and  caesium,  the 
spectroscope  is  necessary  for  their  identification  with  certainty. 
Salts  of  the  alkali  metals,  except  borates,  phosphates,  silicates, 
and  some  salts  of  rare  acids,  when  strongly  ignited  in  0.  F.  yield 
an  alkaline  reaction  with  turmeric  paper. 

Example.  —  Ignite  some  powdered  sodium  chloride,  NaCl, 
in  the  O.  F.  on  platinum  wire,  moisten  the  wire  and  touch  it  to  the 
powder,  when  enough  will  stick  to  it  for  the  test.  Place  on  a  clean 
watch  glass  a  small  square,  1  cm.,  of  turmeric  paper.  When  the 
ignited  salt  has  cooled  it  is  placed  on  the  paper  and  a  single  drop  of 
water  from  a  dropper  is  dropped  on  the  ignited  powder.  The  water 
dissolves  the  alkaline  salt,  and  on  being  absorbed  by  the  paper 
colors  it  reddish  brown.  In  some  cases  it  may  be  necessary  to 
allow  the  fragment  to  rest  on  the  paper  for  a  few  minutes,  when  the 
alkaline  reaction  will  appear  directly  under  the  fragment,  where 
it  is  in  contact  with  the  paper.  In  salts  that  are  not  alkaline  before 
ignition  the  reaction  is  due  to  a  decomposition  of  the  salt  by  the 


INSTRUMENTS   AND   CHEMICAL  TESTS  563 

heat  of  the  O.  F. ;  the  acid  radicle  is  partially  or  completely  volatil- 
ized. 

Potassium,  K.     Atomic  weight,  39.10.     Fusing  point,  62.5°  C. 

a.  Compounds  of  potassium,  except  silicates,  phosphates,  and 
borates,  when  heated  on  wire  or  in  the  forceps  yield  a  light,  violet 
flame.     When  sodium  or  lithium  is  present,  the  potassium  flame  is 
masked.     In  such  cases  the  flame  is  viewed  through  the  blue  cobalt 
glass,  which  if  dark  enough  absorbs  the  sodium  and  lithium  flames 
but  allows  the  violet  rays  of  potassium  to  pass  through.     In  place 
of  the  blue  glass  a  clear  bottle,  with  flat  sides,  filled  with  a  solu- 
tion of  potassium  permanganate,  or  better  still,  a  solution  of  chrome 
alum,  will  serve  the  same  purpose.     The  required  depth  of  color 
is  obtained  by  experimenting  with  solutions  of  different  strengths. 
After  a  time  the  permanganate,   by  decomposition,   deposits  a 
brown  film  on  the  walls  of  the  bottle ;  from  this  fault  a  solution  of 
chrome  alum  is  free.     Viewed  through  all  these  media  the  potas- 
sium flame  is  violet-red. 

Illustration.  —  Heat  some  sylvite,  KC1,  on  platinum  wire  in 
0.  F.  and  observe  the  violet  flame,  also  through  the  blue  glass. 

b.  In  case  of  silicates,  borates,  and  phosphates,  the  finely  ground 
mineral  is  mixed  to  a  stiff  paste  with  water  and  four  parts  of 
potassium  flux.     It  is  then  fused  in  the  blue  mantle  of  the  Bunsen 
burner  flame ;  the  violet-red  of  potassium  will  appear  through  the 
blue  glass. 

Illustration.  —  Powder  some  orthoclase,  KAlSisOs,  in  the  agate 
mortar ;  add  four  parts  of  potassium  flux  and  water  to  a  stiff  paste. 
Hold  the  mixture  on  platinum  wire  in  the  Bunsen  burner  flame  about 
halfway  up  and  on  the  side ;  observe  the  potassium  flame  through 
the  blue  glass,  when  it  will  appear  violet-red. 

c.  Platinum  chloride  test.  —  If  to  a   slightly   acid  solution  of 
potassium  salts  hydrochloro-platinic  acid,  H2PtCle,  is  added,  the 
solution  evaporated  nearly  to  dryness,  then  diluted  with  alcohol, 
the  potassium  will  separate  as  yellow  octahedral  crystals  of  potas- 
sium platinic  chloride,  K2PtCl6.     The  corresponding  sodium  salt 
is  soluble  in  alcohol.  -  Ammonium,  lithium,  rubidium,  and  caesium 
yield  similar  precipitates  insoluble  in  alcohol. 

Illustration.  —  Dissolve  a  little  sylvite  in  water,  addx  a  drop  of 
HC1,  and  precipitate  with  K2PtCl6  and  alcohol. 

Insoluble  silicates  must  be  fused  with  soda.  Fuse  1  part  of  finely 
powdered  orthoclase  with  5  parts  of  soda,  either  make  several  beads 
on  wire,  or  fuse  on  platinum  foil.  The  fusion  is  boiled  in  water, 


564  MINERALOGY 

acidified  with  HC1,  evaporated  to  dryness  on  the  bath  in  a  small 
porcelain  dish ;  boil  the  residue  with  2  cc.  of  water,  cool,  dilute 
with  2  cc.  alcohol  and  filter  through  a  small  filter ;  to  the  filtrate 
one  drop  of  dilute  HC1  is  added  and  the  potassium  is  precipi- 
tated as  K2PtCl6. 

Sodium,  Na.  —  Atomic  weight  23.      Fusing  point,  95.6°  C. 

a.  Sodium  compounds  when  heated  in  the  O.  F.  yield  an  intense 
yellow  flame.  This  test  is  so  delicate  that  great  care  and  judgment 
must  be  exercised  in  its  use,  before  deciding  that  sodium  is  present 
in  sufficient  amount  to  be  considered  as  a  constituent  of  the  min- 
eral. The  flame  must  therefore  be  both  strong  and  persistent. 

Illustration.  —  Heat  some  powdered  halite,  NaCl,  on  platinum 
wire. 

Lithium,  Li.     Atomic  weight,  6.93.     Fusing  point,  186°  C. 

a.  Most  lithium  compounds  when  heated  in  the  O.  F.  either  on 
wire  or  in  the  forceps  yield  a  very  bright  crimson  flame.  Stron- 
tium is  another  metal  that,  yields  a  crimson  flame,  very  much  like 
that  of  lithium,  and  care  must  be  taken  not  to  confound  the  two. 
Sodium  generally  occurs  with  lithium  ;  after  heating  for  some  time 
the  yellow  flame  of  sodium  may  mask  the  lithium  flame.  If  the 
fused  powder  or  fragment  is  momentarily  removed  from  the  flame, 
then  brought  in  contact  with  it  again  slowly,  the  pure  crimson 
flame  of  lithium  will  appear,  first  unmixed  with  yellow ;  as  lithium 
salts  are  more  volatile  than  sodium  salts  they  color  the  flame  first. 

Illustration.  —  Heat  a  small  fragment  of  lepidolite  in  the  for- 
ceps, in  O.  F.,  observe  the  crimson  flame  of  lithium,  and  finally  also 
the  yellow  flame  of  sodium. 

6.  Some  minerals,  especially  silicates,  do  not  readily  yield  a 
lithium  flame  when  heated  alone;  before  deciding -that  a  mineral 
does  not  contain  lithium,  it  should  be  mixed  with  4  parts  of  lithium 
flux ;  the  mixture  is  fused  on  wire  in  the  Bunsen  burner  flame. 

Illustration.  —  Powdered  spodumene,  LiAl(SiO3)2,  is  mixed  to 
a  stiff  paste  with  water  and  4  parts  of  lithium  flux  and  fused  on 
wire  in  the  Bunsen  burner  flame.  After  the  flux  has  had  time  to 
decompose  the  silicate  the  crimson  flame  of  lithium  will  appear. 

Rubidium,  Rb.     Atomic  weight,  85.45.     Fusing  point,  28.5°  C. 

Caesium,  Cs.     Atomic  weight,  132.1.     Fusing  point,  26.37°  C. 

Both  very  rare  metals ;  they  occur  in  small  quantities  in  lepidolite, 
and  some  sphalerites.  They  may  be  separated  from  other  metals 
and  silicates,  as  is  potassium ;  the  precipitate  is  tested  with  the 
spectroscope. 


INSTRUMENTS   AND   CHEMICAL  TESTS  565 

Ammonium,  NH4. 

Ammonia,  NH3,  is  a  gas  at  ordinary  temperatures ;  when  dis- 
solved in  water  it  forms  ammonium  hydroxide,  NH4OH,  commonly 
called  ammonia.  It  is  driven  from  the  water  solution  by  heat 
as  NHs,  which  possesses  a  very  marked  odor,  reacts  as  a  base  in 
forming  salts,  which  in  their  chemical  behavior  are  very  similar 
to  the  salts  of  potassium,  except  its  salts  are  volatile,  and  for  that 
reason  ammonium  is  termed  the  volatile  alkali. 

a.  If  a  compound  containing  ammonium  is  boiled  in  a  test  tube 
with  sodium  hydroxide,  ammonia  will  be  liberated  and  carried  off 
in  the  steam,  when  it  is  detected  by  the  odor ;  or  if  a  glass  rod 
moistened  with  HC1  is  held  over  the  tube,  a  white  cloud  of  am- 
monium chloride,  NH4C1,  will  appear. 

GROUP  II:  THE  ALKALI  EARTHS 

Barium,  strontium,  and  calcium  are  precipitated  from  alkaline 
solutions  as  carbonates,  phosphates,  oxalates,  or  borates.  They 
are  alike  also  in  yielding  before  the  blowpipe  flame  colorations. 
Magnesium,  generally  placed  in  this  group,  yields  no  flame  color- 
ation, and  its  salts  are  more  soluble.  They  all  agree  in  that,  if 
their  salts,  except  the  silicates,  borates,  and  phosphates,  are  ignited, 
they  yield  an  alkaline  reaction  with  turmeric  paper. 

Barium,  Ba,.     Atomic  weight,  137.37.     Fusing  point,  850°  C. 

a.  Flame.  —  If  a  mineral  containing  barium,  except  silicates,  is 
heated  at  the  tip  of  blue  cone  in  the  0.  F.,  it  will  yield  a  yellowish 
green  flame.     Borates  and  phosphates  also  yield  green  flames  that 
must  not  be  mistaken  for  barium.     Silicates  must  be  tested  as  in  &. 

b.  Wet  test.  —  Barium  salts  in  solution  yield  a  white  precipitate 
with  sulphuric  acid.     This  precipitate  when  filtered  out  and  dried 
will  yield  a  yellowish  green  flame. 

Silicates  and  insoluble  compounds  are  fused  with  four  parts  of 
soda,  boiled  in  water  and  filtered.  The  residue  will  contain  the 
barium  as  barium  carbonate.  This  is  dissolved  in  a  few  drops  of 
dilute  HC1.  The  solution  allowed  to  run  through  the  filter  is  col- 
lected in  a  test  tube.  In  this  solution  H2SO4  will  precipitate  the 
barium  as  before. 

Illustration.  —  Heat  a  small  fragment  of  witherite,  BaCO3,  in 
O.  F.  After  continued  heating  observe  the  yellowish  green  flame. 
Place  the  ignited  fragment  on  turmeric  paper ;  moisten  with  a  drop 
of  water.  It  will  react  alkaline.  Dissolve  a  little  powdered  mineral 


566  MINERALOGY 

in  dilute  HC1 ;  add  a  drop  of  dilute  H2SO4 ;  a  white  precipitate  of 
BaSO4  will  appear.  Filter  and  test  this  precipitate  on  wire  for  the 
barium  flame. 

Strontium,  Sr.     Atomic  weight,  87.6.     Fusing  point,  900°  C. 

a.  Flame.  —  Strontium  minerals  when  heated  in  the  O.  F.  yield 
a  bright  crimson  flame.     This  will  be  more  marked  if  after  the  first 
heating  the  fragment  is  moistened  with  HC1.     Care  must  be  taken 
not  to  confuse  the  strontium  flame  with  the  lithium  flame,  or  in 
case  HC1  has  been  used,  with  the  calcium  flame,  which  is  yellowish 
red.     Silicates  are  fused  with  soda  as  in  the  case  of  barium. 

b.  Wet  test.  —  Strontium  in  solution  yields  a  white  precipitate 
of  strontium  sulphate,   SrSO4.     Insoluble  compounds  are  fused 
with  soda  and  treated  as  under  barium,  §  b.     The  precipitate  of 
sulphates  may  in  some  cases  contain  both  strontium  and  barium. 
In  testing  it  on  wire  the  strontium  flame  will  appear  first.     After 
moistening  with  HC1  strontium  will  disappear  first  and  the  green 
of  barium  after  continued  heating. 

Illustration.  —  Heat  a  fragment  of  strontianite,  SrCO3,  in  the 
forceps  in  the  O.  F.,  and  observe  the  bright  crimson  flame.  Place 
the  ignited  fragment  on  turmeric  paper,  after  moistening;  it  will 
react  alkaline.  Dissolve  a  small  fragment  in  2  cc.  of  dilute  HC1  ; 
add  a  drop  of  H2SO4,  when  a  white  precipitate  of  strontium  sul- 
phate, SrS04,  will  appear ;  filter  and  test  it  for  the  flame. 

Calcium,  Ca.     Atomic  weight,  40.07.     Fusing  point,  780°  C. 

a.  Calcium  salts,  especially  after  moistening  with  HC1,  yield 
before  the  blowpipe  a  yellowish  red   flame.     When  the  mineral 
fails  to  yield  the  calcium  flame,  calcium  should  be  separated  in  the 
wet  way,  as  in  6. 

b.  Wet  test.  —  Calcium  sulphate  is  more  soluble  than  either 
barium  or  strontium  sulphates  and  is  not  precipitated  from  dilute 
solutions.     This  condition  may  be  used  to  make  a  qualitative  sep- 
aration of  calcium  from  barium  and  strontium.     The  solution  of 
the  mineral  is  obtained   by  fusion  with  soda,  as  under  barium; 
the  solution  is  diluted  and  dilute  H2S04  is  added  until  a  white 
precipitate  no  longer  forms ;  boil  and  filter ;  the  calcium  will  now 
be  found  in  the  filtrate,  from  which  it  is  separated  by  evaporation 
to  very  small  volume  and  precipitated  with  concentrated  sulphuric 
acid.     The  white  precipitate  of  calcium  sulphate,  CaS04,  is  tested 
for  the  calcium  flame,  or  concentrated  solutions  before  precipitation 
may  be  tested  directly  by  moistening  &  platinum  wire  with  it  and 
holding  it  in  the  Bunsen  burner  flame. 


INSTRUMENTS  AND   CHEMICAL  TESTS  567 

Illustration.  —  Heat  a  small  fragment  of  calcite,  CaCOs,  in  the 
O.  F. ;  at  first  there  will  be  no  flame  coloration,  but  as  the  fragment 
becomes  incandescent  a  very  delicately  colored  flame  will  appear. 
After  cooling,  moisten  the  same  fragment  with  HC1  and  heat 
again ;  the  yellowish  red  flame  of  calcium  will  at  once  appear. 
Place  the  ignited  fragment  on  turmeric  paper  and  moisten  with  a 
drop  of  water ;  the  paper  will  show  the  alkaline  reaction. 

Radium,  Ra.     Atomic  weight,  226.4. 

Radium  is  not  contained  in  any  mineral  in  amounts  sufficiently 
large  to  yield  blowpipe  tests.  In  its  chemical  properties  it  is  like 
barium,  and  may  be  concentrated  and  separated  with  barium,  from 
which  it  is  separated  by  fractional  crystallization.  It  differs  from 
barium  in  yielding  a  crimson  flame. 

Magnesium,  Mg.    Atomic  weight,  24.32.    Fusing  point,  632.6°  C. 

a.  Cobalt  solution  affords  the  only  blowpipe  test  for  magnesium. 
A  large  number  of  light-colored  minerals  containing  magnesium 
become  pink  or  flesh-colored  after  ignition  with  cobalt  solution. 
This  test  at  best  is  not  very  satisfactory,  and  especially  as  it  cannot 
be  applied  to  minerals  dark  in  color,  OF  those  which  become  dark 
upon  ignition. 

6.  Alkaline  test.  —  Some  minerals  containing  magnesium  when 
strongly  ignited  and  placed  on  turmeric  paper  will  yield  the  alkaline 
reaction.  In  most  cases  the  reaction  is  not  very  marked ;  the  frag- 
ment must  be  allowed  to  rest  on  the  paper  for  several  minutes,  and 
even  then  the  alkaline  reaction  may  be  found  only  directly  under 
the  fragment. 

c.  Wet  test.  —  Magnesium  is  a  component  of  a  large  number  of 
silicates  and  other  difficultly  soluble  minerals,  in  which  it  is  asso- 
ciated with  iron,  aluminium,  calcium,  etc.,  and  from  which  the 
magnesium  must  be  separated  before  the  wet  test  can  be  applied. 

The  insoluble  mineral  is  finely  powdered  and  fused  either  on  wire 
or  platinum  foil  with  four  parts  of  soda ;  the  fusion  is  dissolved  in 
dilute  HC1,  and  evaporated  in  a  small  porcelain  dish  to  dryness  on 
the  bath.  The  dry  residue  is  moistened  with  HC1,  then  dilute  with 
5  cc.  of  water ;  stir  and  let  stand  on  the  bath  for  a  minute  or  so. 
The  residue  remaining  insoluble  will  be  silica,  and  is  filtered  out ; 
the  filtrate  is  collected  in  a  test  tube,  boiled  with  3  or  4  drops  of 
nitric  acid  to  oxidize  the  iron  to  ferric  iron ;  3  drops  of  concen- 
trated HC1  are  now  added,  the  solution  shaken  and  neutralized 
with  ammonia,  when  iron  and  aluminium  are  precipitated  as  hy- 
droxides ;  after  filtering,  the  filtrate,  containing  the  magnesium 


568  MINERALOGY 

and  other  alkali  earths,  is  collected  in  a  test  tube ;  to  this  solution  a 
few  drops  of  ammonia  are  added,  then  ammonium  oxalate;  boil 
and  let  the  precipitated  calcium,  barium,  or  strontium  oxalates 
settle,  when  the  clear  solution  is  tested  with  another  drop  of  am- 
monium oxalate  to  insure  complete  precipitation.  The  filtrate 
from  the  calcium  oxalate  should  be  concentrated,  either  on  the  bath 
or  by  gently  boiling  in  a  test  tube  to  2  cc.,  when  if  it  is  not  per- 
fectly clear  it  is  filtered  through  a  small  filter.  3  cc.  of  strong 
ammonia  is  now  added,  then  20  drops  of  sodium  phosphate,  shak- 
ing the  solution  between  each  drop ;  let  stand  in  the  cold.  A 
white  crystalline  precipitate,  ammonium  magnesium  phosphate, 
NH4MgPO4 .  6  H2O,  indicates  the  presence  of  magnesium. 

Illustration.  — Heat  a  fragment  of  brucite,  Mg(OH)2,  in  O.  F. ; 
after  cooling  moisten  with  a  drop  of  cobalt  solution  and  ignite 
again  in  the  0.  F.  strongly  for  some  time ;  after  cooling  observe  that 
it  has  turned  a  delicate  pink  or  flesh  color.  Moisten  the  ignited 
fragment  on  turmeric  paper  and  observe  that  the  alkaline  reaction 
is  rather  weak.  Dissolve  some  of  the  mineral  in  HC1  and  precipi- 
tate magnesium  as  directed  in  §  c  above. 

GROUP  III 

Metals  precipitated  as  hydroxides  by  ammonia  and  by  hydrogen 
sulphide  in  alkaline  solutions. 

I.     Common  elements,  aluminium  and  chromium. 

II.  Rare  elements,  beryllium,  thorium,  zirconium,  yttrium, 
cerium,  lanthanum,  didymium,  titanium,  tantalum,  columbium. 

Aluminium,  Al.     Atomic  weight,  27.1.     Fusing  point,  657°  C. 

a.  Cobalt  solution.  —  Light-colored  minerals  are  ignited  in  the 
O.  F.,  after  cooling  are  moistened  with  cobalt  solution,  and  again 
strongly  ignited  in  the  O.  F.,  when  if  they  contain  much  aluminium 
the  ignited  residue  will  be  bright  blue  on  cooling.  Silica  and  min- 
erals which  fuse  will  yield  a  blue  color  which  is  not  due  to  alumin- 
ium, but  the  glassy  slag  is  colored  blue  with  oxide  of  cobalt.  Very 
hard  minerals  as  topaz  must  be  finely  ground  before  ignition. 

Illustration.  —  Heat  a  small  splinter  of  cyanite,  Al2SiO5.  In 
O.  F.  when  cool  moisten  with  a  drop  of  cobalt  solution  and  ignite 
again  in  a  hot  O.  F.,  holding  the  fragment  just  without  the  tip  of 
the  blue  cone ;  after  cooling,  the  blue  color  due  to  the  presence  of 
aluminium  will  appear. 

6.  Wet  test.  —  Insoluble  compounds  and  silicates  are  treated  as 


INSTRUMENTS  AND   CHEMICAL  TESTS  569 

under  magnesia,  §  c.  The  hydroxides  precipitated  with  ammonia 
containing  the  aluminium  are  scraped  from  the  paper  and  boiled 
in  a  test  tube  with  sodium  hydrate,  the  aluminium  hydroxide  dis- 
solves, and  the  insoluble  hydroxides,  generally  iron,  are  filtered  off 
and  washed.  The  filtrate  is  acidified  with  HC1  and  the  aluminium 
is  reprecipitated  with  ammonia,  filtered,  dried,  ignited,  and  tested 
for  the  blue  color  with  cobalt  solution.  . 

Chromium,  Cr.     Atomic  weight,  52.1.     Fusing  point,  1515°  C. 

a.  Bead  test.  —  When  oxide  of  chromium  is  dissolved  in  the 
borax  bead  in  O.  F.  it  will  color  the  bead  yellow  while  hot,  becoming 
greenish  yellow  on  cooling.  If  more  oxide  is  dissolved  in  the  bead, 
it  becomes  red  while  hot,  yellowish  green  while  cooling,  and  finally 
green  when  cold. 

In  the  S.  Ph.  bead,  when  in  sufficient  quantity,  chromium 
yields  a  fine  emerald  green  in  both  flames.  Vanadium  yields  a 
green  bead  very  similar  to  chromium,  except  in  S.  Ph.,  in  0.  F.  The 
vanadium  bead  is  yellow  when  cold.  As  there  are  several  oxides 
which  yield  green  beads,  in  case  of  doubt  the  wet  test  for  chromium 
must  be  applied. 

Illustration.  —  Use  Cr2O3,  and  a  very  pure  O.  F.  to  obtain  the 
yellow  described  above,  otherwise  the  bead  will  be  green  on  cool- 
ing. 

6.  Wet  test.  —  The  very  finely  ground  mineral  is  fused  on  wire 
with  four  parts  of  soda  and  two  parts  of  borax ;  while  the  fused 
bead  is  still  hot  it  is  touched  to  a  small  grain  of  potassium  nitrate 
and  fused  once  more  in  the  0.  F.  If  much  chromium  is  present, 
the  cold  bead  will  be  yellow,  due  to  the  sodium  chromate,  Na2Cr04, 
formed  in  the  oxidation  with  KN03.  The  fusion  is  dissolved  in 
2  cc.  of  water  and  acidified  with  acetic  acid ;  if  not  now  clear  it  is 
filtered.  To  the  clear  solution  3  drops  of  lead  acetate  are  added, 
when,  if  chromium  is  present,  a  bright  yellow  precipitate  of  lead 
chromate,  PbCrO4,  will  form ;  this  may  be  filtered  off  and  tested  for 
the  green  bead. 

Illustration.  —  Fuse  powdered  chromite,  FeO,  Cr2O3,  and  proceed 
as  directed  above. 

Titanium,  Ti.  -  Atomic  weight,  48.1.     Fusing  point,  3000°  C. 

a.  Bead  test.  —  The  distinctive  bead  reaction  for  titanium  is  the 
violet  color,  in  R.  F.  and  S.  Ph.  when  cold,  especially  marked 
when  reduced  with  tin  on  coal.  When  this  bead  reaction  is  ob- 
scured by  the  presence  of  other  oxides,  the  wet  test  must  be  applied. 

Illustration.  —  Use  very  finely  ground  rutile,  TiO2,  and  heat  the 


570  MINERALOGY 

bead  in  the  0.  F.  until  all  the  particles  have  dissolved,  then  in  the 
R.  F.  for  considerable  time,  when  it  will  be  a  light  violet  when  cold. 
In  the  treatment  with  tin,  the  bead  is  removed  from  the  wire  and 
placed  in  a  shallow  cavity  beside  a  piece  of  tin  of  about  half  its 
size,  then  fused  in  the  R.  F.  for  some  time ;  the  tin  and  S.  Ph.  beads 
will  fuse,  but  remain  distinct,  simply  sticking  to  each  other  and 
rolling  around  in  contact  during  the  fusion.  The  fused  tin  oxi- 
dizes at  the  expense  of  the  oxygen  combined  with  the  titanium, 
which  is  reduced  to  Ti203,  coloring  the  bead  violet  when  cold. 

b.  Wet  test.  —  Insoluble  substances  are  finely  powdered,  made 
into  a  stiff  paste  with  water  and  6  parts  soda  and  a  little  borax, 
and  thoroughly  fused  either  on  coal  or  platinum  foil.     The  fusion  is 
dissolved  in  2  cc.  of  concentrated  HC1.     Titanium  is  now  present 
in  solution  as  TiO2 ;  if  much  iron  is  present,  the  solution  will  be 
yellow  from  the  ferric  chloride.   Granulated  tin  is  now  added,  which 
dissolves  in  the  HC1,  forming  stannous  chloride,  SnCl2,  and  liberates 
hydrogen.     As  the  hydrogen  reduces  the  ferric  chloride  to  ferrous 
chloride  the  yellow  color  due  to  iron  disappears,  then  the  TiO2  is 
reduced  to  Ti2O3,  and  the  solution  assumes  a  violet  color,  especially 
on  standing. 

c.  Hydrogen  peroxide  test.  —  If  the  amount  of  titanium  present 
is  less  than  3  per  cent.,  the  above  tests  are  not  sufficiently  delicate. 
The  substance  is  fused  as  directed  above,  but  is  dissolved  in  3  cc. 
dilute  H2S04  by  boiling  in  a  test  tube ;  when  dissolved,  dilute  with 
water  to  10  cc.  and  add  2  cc.  of  hydrogen  peroxide  solution,  when 
if  titanium  is  present  the  solution  will  assume  a  golden  yellow  and  if 
much  is  present  an  orange  color. 

Illustration.  —  Use  powdered  rutile  and  proceed  with  the  tests 
as  directed. 

Columbium  (Niobium)  Cb(Nb).  Atomic  weight,  93.5.  Fusing 
point,  1950°  C. 

a.  Wet  test.  —  Columbium  compounds  are  very  insoluble  in 
acids,  and  must  be  thoroughly  fused  with  borax  or  potassium 
bisulphate  on  wire  or  foil ;  the  fusion  is  then  dissolved  in  con- 
centrated HC1  as  in  the  case  of  titanium,  when  granular  zinc  is 
added  and  the  solution  is  reduced  in  the  same  way.  If  columbium 
is  present,  it  will  become  blue.  In  the  reduction  if  titanium  is 
present  also,  the  solution  will  be  violet  first,  as  titaniuni  is  the  first 
to  reduce ;  then  the  columbium  will  be  reduced,  and  the  solution  will 
be  finally  blue.  Tungsten  is  another  metal  which  yields  blue  solu- 
tions after  reduction  with  tin ;  for  distinguishing  tests  see  §  6,  p.  587. 


INSTRUMENTS   AND  CHEMICAL  TESTS  571 

Illustration.  —  For  above  test  use  columbite  (Fe,  Mn)Cb2O6. 

Tantalum,  Ta.  —  Atomic  weight,  181.5.     Fusing  point,  2250°  C. 

There  is  no  blowpipe  test. 

a.  Wet  test.  —  Tantalum  is  always  associated  with  columbium, 
from  which  it  is  separated  with  difficulty.  It  does  not,  however, 
yield  the  blue  solution  on  reduction  with  tin.  Columbium  and 
tantalum  may  be  extracted  from  minerals  and  separated  from  each 
other  by  the  following  method.  The  finely  ground  mineral  is 
fused  with  5  parts  of  acid  potassium  fluoride,  the  fusion  is  pul- 
verized and  extracted  in  a  platinum  dish  with  boiling  water,  con- 
taining a  little  hydrofluoric  acid,  evaporated  nearly  to  dryness,  and 
dissolved  in  the  least  possible  quantity  of  boiling  water ;  on  cooling, 
tantalum  will  separate  as  needle-like  crystals  of  potassium  tantalum 
fluoride,  K2TaF7,  which  are  brownish  when  dry.  Columbium  will 
separate  as  potassium  columbium  fluoride,  K2CbF7,  from  the  solu- 
tion on  concentration  as  tablets. 

Zirconium,  Zr.     Atomic  weight,  90.6.     Fusing  point,  1500°  C. 

Blowpipe  tests,  none. 

a.  Wet  test.  —  The  mineral  is  fused  with  4  parts  soda,  the  fusion 
dissolved  in  a  few  drops  of  concentrated  HC1,  diluted  with  three 
volumes  of  water.  A  piece  of  turmeric  paper  is  moistened  with  this 
diluted  solution,  and  then  gently  dried,  when  if  zirconium  is  present, 
it  will  turn  the  paper  reddish  brown  to  orange-red,  the  depth  of 
color  depending  upon  the  amount  of  zirconium  present.  Titanium 
if  present  must  be  reduced  with  tin,  or  Ti  also  will  yield  a  reddish 
brown  color.  Molybdates  and  boric  acid  also  yield,  under  similar 
conditions,  a  brown  color  with  turmeric  paper,  and  must  be  distin- 
guished by  their  special  tests  from  zirconium. 

Illustration.  —  Use  powdered  zircon,  ZrSiO4. 

Beryllium  (Glucinum),  Be.  Atomic  weight,  9.1.  Fusing  point, 
960°  C. 

Blowpipe  tests,  none. 

a.  Wet  test.  —  Beryllium  is  separated  from  silicates  in  the  same 
way  as  aluminium,  §  b.  After  treating  the  hydroxides  with  sodium 
hydroxide  and  filtering,  beryllium  will  be  found  in  the  filtrate  with 
the  aluminium  and  is  separated  from  it  by  diluting  the  solution  with 
water  and  boiling,  when  beryllium  is  reprecipitated  as  a  white  floc- 
culent  hydroxide,  Be(OH)2 ;  the  aluminium  stays  in  solution. 

Illustration.  —  Beryl,  Be3Al2(SiO3)6,  maybe  used  for  this  separa- 
tion. 

Cerium,  Ce.     Atomic  weight,  140.25.     Fusing  point,  623°  C. 


572  MINERALOGY 

Associated  with  cerium  chemically  is  a  group  of  elements,  dis- 
tinguished and  separated  from  each  other  with  difficulty ;  they  are 
known  as  the 

Rare  Earths 

Their  hydroxides  are  precipitated  by  either  ammonia  or  sodium 
hydroxide,  which  separates  them  from  the  alkali  earths.     From  the 
method  of  separation,  the  solubility  of  their  oxalates,  they  are 
divided  into  two  groups  : 
Group    I.     Gadolinite    earths    or  Group  II.     Cerite  earths 

Yttrium  earths 
Yttrium,        Y.,       Atomic  Lanthanum,       La.,       Atomic 

weight 89.  weight 139. 

Scandium,       Sc.,    Atomic  Cerium,  Ce.,  Atomic  weight  .     140.5 

weight 44.1     Praseodymium,    Pr.,    Atomic 

Samarium,    Sm.,    Atomic  weight 140.6 

weight 150.4     Neodymium,        Nd.,    Atomic 

Gadolinium,  Gd.,   Atomic  weight 144.3 

weight 157.3     Thorium,  Th.,  Atomic  weight .     232.4 

Terbium,      Tr.,  '  Atomic  Some  doubtful  elements  may 

weight 159.2         be  added  to  this  group 

Erbium,        Er.,       Atomic 

weight     .     .     .     .     .     .     167.7 

Thulium,     Tm.,     Atomic 

weight 168.9 

Ytterbium,    Yb.,    Atomic 

weight 172. 

Dysprosium,  Dy.,  Atomic 

weight 162.5 

They  may  be  extracted  from  a  mineral  as  follows.  If  the  mineral 
is  insoluble  in  acids,  it  is  mixed  to  a  stiff  paste  with  strong  sulphuric 
acid,  the  paste  is  heated  carefully  until  the  SO3  fumes  are  driven 
off,  and  then  baked  hard  and  dry ;  the  dry  mass  is  pulverized  and 
leached  with  boiling  water  until  all  soluble  salts  are  extracted. 

The  free  acid  is  neutralized  with  ammonia.  To  the  hot  solution 
a  few  cc.  of  ammonium  acetate  and  a  large  excess  of  a  boiling 
saturated  solution  of  ammonium  oxalate  are  added ;  let  cool  and 
stand  overnight.  The  crystalline  precipitate  is  composed  of  the 
oxalates  of  the  rare  earths  with  the  exception  of  thorium,  which 
is  still  in  solution ;  these  are  filtered,  dried,  and  ignited  to  the  ox- 
ides. From  the  filtrate  thorium  is  precipitated  as  hydroxide  with 
sodium  hydrate.  The  ignited  oxides  are  dissolved  in  the  least 
possible  quantity  of  HC1,  then  saturate  this  solution  with  potas- 
sium sulphate,  let  stand,  filter,  and  wash  with  a  strong  solution  of 


INSTRUMENTS  AND   CHEMICAL  TESTS  573 

potassium  sulphate.  The  precipitate  contains  the  cerium  group 
as  sulphates,  the  nitrate  contains  the  yttrium  group.  The  sul- 
phates of  the  cerium  group  may  be  dissolved  in  boiling  water, 
to  which  a  little  HC1  has  been  added,  and  reprecipitated  as  hy- 
droxides, with  sodium  hydroxide.  From  the  filtrate  containing 
the  yttrium  group,  their  hydroxides  may  also  be  precipitated  with 
sodium  hydroxide. 

GROUP   IV 

Metals  which  are  precipitated  with  hydrogen  sulphide  from  alka- 
line solutions  as  sulphides,  but  not  from  acid  solutions.  Common 
elements  are  zinc,  manganese,  cobalt,  nickel,  and  iron;  rare  ele- 
ments are  uranium,  vanadium,  indium,  thalium,  gallium. 

Zinc,  Zn.     Atomic  weight,  65.37.     Fusing  point,  419°  C. 

a.  Coat.     If  zinc  minerals  are  mixed  in  a  stiff  paste  with  water, 
and  4  parts  soda  and  a  little  coal  dust,  placed  in  a  shallow  cavity 
on  coal  and  heated  in  a  hot  reducing  flame,  the  zinc  is  reduced 
(zinc  volatilizes  at  the  temperature  of  reduction  so  that  zinc  but- 
tons are  not  found  in  the  assay  upon  washing  in  the  mortar), 
volatilizes,  and  settles  as  a  zinc  oxide  coat  near  the  assay,  which  is 
straw  yellow  while  hot,  becoming  white  when  cold.     Zinc  minerals 
are  apt  to  contain  cadmium ;  in  such  cases  the  coat  will  be  slightly 
yellow  when  cold.     At  times  it  is  difficult  to  decide  whether  a  slight 
coat  has  been  obtained  or  whether  it  is  only  an  ash,  produced  by  the 
burning  coal.     The  coat,  however  slight,  is  always  accompanied  by 
a  bluish  border,  where  the  white  coat  thins  out  over  the  coal ;  and 
then,  too,  the  ash  is  easily  blown  off  the  coal  with  a  moderate 
breath. 

b.  Cobalt  solution  test.  —  The  zinc  coat  is  now  moistened  with 
a  drop  of  cobalt  solution,  and  heated  in  O.  F. ;  the  assay  is  also 
heated  in  the  blue  cone  to  volatilize  a  little  more  zinc  if  possible ; 
after  cooling,  if  the  coat  is  zinc  oxide,  it  will  be  a  grass-green,  at 
least  in  spots.     If  the  assay  contains  silica  or  alumina,  it  will  be- 
come blue  with  cobalt  solution.     This  blue  color  of  the  assay  is, 
therefore,  no  indication  of  zinc.     Cobalt  solution  serves  to  dis- 
tinguish the  zinc  and  tin  coats,  which  are  very  much  alike  in  all 
other  respects,  save  the  zinc  coat  is  grass-green  and  the  tin  coat  is 
blue  with  cobalt  solution. 

Illustration.  —  Grind  some  smithsonite,  ZnCOs,  mix  to  a  stiff 
paste  with  4  parts  soda,  1  part  powdered  borax,  and  a  little  coal 
dust  and  water.  Heat  gently  at  first  in  a  shallow  cavity  on  coal 


574  MINERALOGY 

until  all  water  is  driven  off,  then  in  a  hot  R.  F.,  holding  the  coal 
close  up  to  the  burner  and  using  the  blue  portion  of  an  O.  F.,  which 
is  reducing.  Note  the  coat  is  quite  yellow  while  hot  and  is  sur- 
rounded by  the  bluish  border  which  serves  to  distinguish  a  coat 
of  this  color  and  position  from  an  ash.  After  cooling,  the  coat  will 
be  white.  Moisten  with  cobalt  solution  and  heat  carefully  in  an 
O.  F.  so  as  not  to  blow  the  coat  off ;  let  cool,  when  the  coat  will  be 
grass-green,  at  least  in  spots.  If  the  green  does  not  appear,  the 
assay  is  heated  in  the  R.  F.  to  drive  off  a  little  zinc ;  let  cool  and  the 
green  color  will  appear. 

Manganese,  Mn.    Atomic  weight,  54.93.    Fusing  point,  1245°  C. 

a.  Bead  reactions.  —  Oxides  of  manganese  when  dissolved  in 
either  the  borax  or  S.  Ph.  bead  yield  in  O.  F.  an  amethyst  or  violet- 
red   color,    which    becomes    colorless    in   R.  F.      Small   charges 
should  be  used  at  first,  as  very  little  manganese  will  yield  an  opaque 
bead  reducing  in  R.  F.  with  difficulty.     The  color  is  much  lighter 
in  S.  Ph.  than  in  borax,  and  as  some  silicates  are  not  decomposed  in 
S.  Ph.,  borax  will  therefore  yield  the  better  results. 

b.  Soda  and  niter  test.  —  Any  compound  of  manganese  when 
fused  with  soda  on  platinum  wire  in  O.  F.  and  again  fused  with  a 
small  grain  of  niter  in  the  O.  F.,  if  manganese  is  present  the  bead 
will  be  green,  bluish-green,  or  a  dark  opaque  blue  according  to  the 
amount  of  manganese  present.     This   is   a  very   delicate'  test; 
amounts  as  low  as  .1  per  cent,  can  be  easily  detected.     The  color 
is  due  to  the  formation  of  sodium  manganate,  Na2Mn04. 

Illustration.  —  Use  any  compound  of  manganese,  as  pyrolusite, 
MnO2. 

Cobalt,  Co.     Atomic  weight,  58.97.     Fusing  point,  1530°  C. 

a.  Bead  test.  —  Compounds  of  cobalt  when  dissolved  in  borax 
or  S.  Ph.  yield  in  both  flames  a  dark  smalt-blue,  a  very  delicate  test. 
If  copper  or  nickel  are  present  they  may  conceal  the  cobalt  when  in 
small  quantities ;  in  this  case  the  borax  bead  is  taken  from  the  wire 
and  reduced  beside  tin  on  coal,  when  copper  and  nickel  are  reduced 
to  metal  and  absorbed  by  the  tin,  when  if  cobalt  is  present  the 
borax  will  be  blue.  See  also  under  nickel. 

Illustration.  —  Use  oxide  of  cobalt.  Sulphides,  arsenides,  and 
antimonides  must  be  roasted  before  dissolving  in  the  bead. 

Nickel,  Ni.     Atomic  weight,  58.68.     Fusing  point,  1484°  C. 

a.  Bead  reaction.  —  Oxides  of  nickel  color  the  borax  bead  in 
O.  F.  a  violet,  which  on  cooling  becomes  brownish,  or  if  highly 
charged  a  reddish  brown ;  in  R.  F.  the  bead  is  gray,  due  to  metallic 


INSTRUMENTS  AND   CHEMICAL  TESTS  575 

nickel.  The  S.  Ph.  bead  is  reddish  yellow  while  hot,  yellow  on 
cooling  in  both  flames.  Small  quantities  of  cobalt  and  copper  will 
interfere  with  the  bead  reactions  of  nickel.  Cobalt  and  nickel  are 
closely  associated  in  minerals,  generally  in  sulphides  and  arsenides ; 
in  this  case  the  mineral  is  fused  in  the  R.  F.  to  a  round  globule  and 
a  large  proportion  of  the  arsenic  and  sulphur  is  roasted  off.  A  piece 
of  borax  double  the  size  of  the  globule  is  placed  beside  it  on  coal 
and  treated  with  the  0.  F.  a  short  time,  when  the  color  of  the  borax 
is  observed.  If  the  globule  contains  iron,  nickel,  cobalt,  and 
copper,  they  will  be  oxidized  not  all  at  once,  but  one  at  a  time  in  the 
order  mentioned,  each  in  turn  imparting  its  characteristic  color  to 
fresh  charges  of  borax.  The  amount  of  each  metal  present  can 
be  roughly  judged  by  the  number  of  fresh  charges  of  borax  it  takes 
to  absorb  it.  If  it  takes  only  two  or  three  bead$  to  remove  the 
cobalt,  it  is  present  as  a  minor  component  of  the  arsenide  and  after 
the  blue  of  cobalt  has  disappeared,  nickel  will  oxidize  and  color 
the  borax  reddish  brown. 

Illustration.  —  Use  a  fragment  of  niccolite,  NiAs,  which  always 
contains  some  cobalt,  about  the  size  of  a  rice  grain  and  fuse  it  on 
coal  in  a  hot  R.  F.  to  a  round  globule ;  place  some  borax  beside  it 
and  treat- with  the  0.  F.  The  two  globules  will  roll  around  under 
the  flame  in  contact,  but  will  remain  quite  distinct;  any  cobalt  in 
the  arsenide  will  be  oxidized  by  the  O.  F.  and  be  absorbed  by  the 
borax.  Carefully  pick  the  arsenide  globule  out  of  the  borax,  with 
the  steel  end  of  the  forceps,  and  place  to  one  side  for  future  treat- 
ment ;  while  still  hot  pull  the  borax  out  with  the  forceps  in  a  thread ; 
it  will  be  blue  from  cobalt,  or  if  opaque,  some  of  it  is  dissolved  with 
fresh  borax  on  wire  and  the  color  observed.  Return  the  arsenide 
globule  to  the  coal  after  cleaning  the  cavity  of  all  old  borax  and 
repeat  the  treatment  with  a  fresh  supply.  After  several  treatments 
all  cobalt  will  have  been  oxidized  and  the  borax  will  become  red- 
dish brown  from  nickel. 

Iron,  Fe.     Atomic  weight,  55.84.     Fusing  point,  1600°  C. 

a.  Bead  reaction.  —  Oxides  of  iron  when  dissolved  in  the  borax 
bead  in  0.  F.  yield,  while  hot,  a  light  yellow  to  dark  red  color 
according  to  the  amount  present ;  this  becomes  colorless,  or  if  well 
charged,  yellow  when  cold.  In  R.  F.  the  bead  is  green,  and  if  well 
charged  a  dirty  yellowish  green  on  cooling ;  if  reduced  beside  tin 
on  coal,  this  bead  is  a  clear  bottle  green.  The  moderately  charged 
S.  Ph.  bead  in  0.  F.  is  yellowish  red  while  hot,  becoming  yellow 
and  finally  colorless  when  cold.  In  R.  F.  the  well-charged  bead 


576  MINERALOGY 

is  red  while  hot,  yellow  while  cooling,  finally  smoky-brown  when 
cold.  There  are  many  oxides  which  interfere  with  the  bead  reac- 
tion for  iron,  and  there  are  other  elements  which  yield  nearly  the 
same  colors.  The  table  of  bead  reactions  on  page  594  must  be 
consulted.  Bead  reactions  in  combination  with  the  magnetic 
test  below  will  serve  to  identify  the  presence  of  iron. 

6.  Magnetism.  —  Compounds  of  iron  when  ground  finely  and 
treated  on  "coal  in  R.  F.  become  magnetic ;  this  is  especially  so  if 
fused  with  soda  on  coal,  the  fusion  washed  in  the  mortar,  when 
the  black  residue  will  be  magnetic.  It  must  be  remembered  that 
cobalt  and  nickel  become  magnetic  also;  however,  these,  if  present, 
can  easily  be  detected  by  their  bead  reactions.  In  testing  the 
residue  after  reduction  it  should  be  cold  and  in  fine  powder  ;  spread 
out  on  a  white  paper,  the  magnet  is  passed  carefully  over  it ;  if 
magnetic,  particles  should  move  to  the  magnet.  Particles  may 
stick  to  the  magnet  from  moisture  or  other  causes  and  still  not 
be  magnetic. 

Illustration.  —  For  the  above  tests  use  hematite,  Fe203. 

Uranium,  U.     Atomic  weight,  238.5. 

a.  Bead  reaction.  —  The  colors  of  uranium  oxides  in  borax  are 
like  iron;  in  S.  Ph.  uranium  oxide  is  yellow  while  hot,  yellowish 
green  on  cooling  in  0.  F. ;  in  R.  F.  the  bead  is  a  fine  green  when 
cold.  Chromium,  vanadium,  and  molybdenum  also  give  green 
beads  in  R.  F. 

6.  Wet  test.  —  Dissolve  the  powdered  mineral  in  HC1,  or  if 
insoluble  first  fuse  with  soda,  nearly  neutralize  the  free  acid  with 
ammonia,  then  add  a  solution  of  sodium  carbonate  until  a  precipitate 
ceases  to  form,  then  half  as  much  more,  let  stand.  Uranium  at 
first  precipitated  is  redissolved  in  the  excess  of  sodium  carbonate 
and  will  be  found  on  filtering  in  the  filtrate.  Acidify  the  filtrate 
with  HC1,  boil  to  expel  CO2,  add  an  excess  of  ammonia,  when 
yellow  ammonium  urinate,  (NH4)2U207,  will  precipitate.  This  is 
filtered  and  tested  for  the  bead  reaction  as  above,  or  dissolved  in 
1  cc.  of  dilute  H2SO4,  a  small  scrap  of  zinc  added,  which  reduces 
the  uranium,  yielding  at  first  blue,  turning  finally  to  green.  If 
the  green  solution  is  poured  off  the  zinc,  in  another  test  tube,  and 
hydrogen  peroxide  added,  then  sodium  carbonate  in  excess,  the 
solution  will  assume  a  cherry-red  color ;  this  is  a  very  delicate  test 
for  uranium. 

Illustration.  —  Use  powdered  uraninite. 

Vanadium,  V.  —  Atomic  weight,  51.      Fusing  point,  1680°  C. 


INSTRUMENTS   AND   CHEMICAL  TESTS  577 

a.  Bead  reaction.  —  When  dissolved  in  borax,  oxides  of  vana- 
dium are  yellow  to  yellowish  green  in  O.   F.  while  hot,  nearly 
colorless  when  cold.     In  R.  F.  a  dirty  green  when  hot  and  a  fine 
green  when  cold.     The  S.  Ph.  bead  is  yellow  in  O.  F.,  fine  green 
in  R.  F.  when  cold. 

b.  Wet  test.  —  The  well-roasted  mineral  is  fused  with  4  parts 
soda  and  2  of  niter,  the  fusion  is  dissolved  in  boiling  water  and 
any  insoluble  residue  filtered  off ;  the  soluble  alkali  vanadates  will 
be  found  in  the  filtrate ;  this  is  acidified  with  acetic  acid,  and  lead 
acetate  added,  which  will  precipitate  the  vanadium  as  lead  vana- 
date,  Pb3  (¥64)2,  yellow  at   first,  but  soon  turning  white ;   this 
precipitate  may  be  tested  in  the  S.  Ph.  bead  as  above.     Molyb- 
denum and  chromium  will  also  yield  yellow  precipitates  with  lead 
acetate,  and   green   beads   with   S.  Ph.     To   separate   vanadium 
from  these,  the  solution  of  alkali  vanadate  from  the  soda  fusion 
is  not  acidified  with  acetic  acid,  but  solid  ammonium  chloride  is 
added  to  almost  saturation,  when  upon  standing  for  some  time 
ammonium  metavanadate,  NH4VO3,  will  separate  as  a  slightly 
yellowish  crystalline  precipitate ;  filter  and  wash  with  ammonium 
chloride  solution.     If  this  precipitate  is  heated  in  a  porcelain 
crucible,  the  ammonia  is  driven  off,  leaving  a  red  residue,  pentoxide 
of  vanadium,  V2O5.     If  it  is  wished  to  further  test  this  residue, 
it  may  be  dissolved  in  HC1  and  reduced  with  zinc ;  the  solution, 
yellow  at  first,  quickly  becomes  green  and  finally  blue ;  the  blue 
poured  off  the  zinc,  and  hydrogen  peroxide  added,  becomes  red. 

Illustration.  —  Use  oxide  of  vanadium. 

Thallium,  Tl.     Atomic  weight,  204.     Fusing  point,  301.7°  C. 

Indium,  In.     Atomic  weight,  114.8.    Fusing  point,  155.3°  C. 

Gallium,  Ga.     Atomic  weight,  69.9.    Fusing  point,  30.15°  C. 

These  three  very  rare  elements  are  associated  in  some  sphalerites. 
Of  the  three  thallium  alone  occurs  in  large  enough  quantities  to 
characterize  two  rare  minerals  as  thallium  minerals,  crookesite 
and  lorandite. 

They  are  best  detected  with  the  spectroscope. 

Thallium  yields  a  bright  green  flame.  Its  salts  are  volatile, 
and  when  in  R.  F.  on  coal  it  yields  a  white  oxide  coat.  If  mixed 
with  von  Kobell's  flux,  it  yields  a  lemon-yellow  coat  very  similar 
to  lead ;  the  bright  green  flame  will,  however,  distinguish  it  frpm 
lead. 


2p 


578  MINERALOGY 

GROUP   V 

Metals  which  are  precipitated  from  acid  solution  by  hydrogen 
sulphide  as  sulphides,  and  the  sulphides  of  which  are  insoluble  in 
ammonium  sulphide. 

Common  metals:  silver,  mercury,  lead,  bismuth,  copper,  and 
cadmium. 

Rare  metals:   palladium,  rhodium,  osmium,  ruthenium. 

SUver,  Ag.     Atomic  weight,  107.87.     Fusing  point,  955°  C. 

a.  Reduction.  —  Compounds  containing  silver  when  fused  on 
coal  with  4  parts  soda  in  R.  F.  yield  metallic  silver.  If  the  silver 
button  does  not  appear  bright,  it  may  be  heated  on  coal  in  the 
0.  F.  beside  borax ;  the  base  metals  will  oxidize  first ;  their  oxides 
are  dissolved  by  the  borax,  leaving  the  silver  button  bright. 

When  the  amount  of  silver  in  a  mineral  is  small,  considerable 
of  the  powdered  mineral  should  be  mixed  with  2  volumes  of  test 
lead  and  one  volume  of  powdered  borax ;  this  charge  is  placed  in  a 
deep  cavity  in  the  coal  and  heated  with  the  R.  F.,  gently  at  first, 
until  the  lead  is  fused ;  the  reduction  is  continued  for  a  couple  of 
minutes,  shaking  the  coal  now  and  then  while  under  the  flame  to 
collect  the  lead  in  one  globule.  All  silver  and  other  reduced 
metals  the  charge  may  have  contained  will  be  collected  in  the 
lead  globule.  The  O.  F.  is  now  used  to  refine  the  lead,  by  oxidiz- 
ing arsenic,  antimony,  or  any  easily  oxidized  elements  the  lead 
may  contain.  When  the  lead  begins  to  boil  freely,  the  assay  is 
allowed  to  cool,  the  lead  is  picked  out  and  cleaned  of  slag  by 
hammering  on  the  anvil.  A  cupel  is  now  prepared  by  cutting  a 
cavity  6  mm.  deep  and  12  mm.  in  diameter  near  the  end  of  a  firm 
piece  of  coal ;  this  is  packed  full  of  dry  bone  ash  and  pressed  down 
with  the  large  end  of  the  agate  pestle,  giving  a  twist  to  the  pestle 
at  the  last  which  will  leave  the  concave  surface  smooth  and  hard. 
The  loose  bone  ash  is  blown  off  and  the  cupel  is  ignited  in  the 
O.  F.  to  drive  off  all  moisture.  The  clean  lead  button  is  placed 
on  the  cupel  and  fused  with  the  R.  F. ;  after  the  lead  is  fused  and 
the  cupel  is  hot,  a  well-pointed  O.  F.  is  directed  on  the  lead. 
The  flame  should  be  no  hotter  than  is  necessary  to  keep  the  lead 
fused  and  the  bone  ash  directly  underneath  it  well  heated  or  it 
will  not  absorb  the  oxide  of  lead  as  it  is  formed.  It  is  better  to 
roll  the  lead  by  tilting  the  cupel  to  fresh  portions  of  the  surface 
from  time  to  time,  but  the  lead  should  never  be  allowed  to  freeze 
or  solidify,  once  the  oxidation  has  commenced.  The  experiment 


INSTRUMENTS   AND   CHEMICAL  TESTS  579 

must  be  carried  to  the  end  without  interruption.  In  cupellation 
the  lead  is  oxidized,  the  oxide  is  both  volatilized  and  absorbed  by 
the  bone  ash.  As  the  percentage  of  lead  decreases,  the  globule 
becomes  more  spherical  and  a  thin  iridescent  film  of  oxide  of  lead 
is  constantly  moving  over  its  surface;  as  the  last  of  the  lead  is 
driven  off,  this  film  disappears,  the  button  brightens,  and  a  dis- 
tinct change  in  color  is  noticed.  This  change  must  be  carefully 
watched  and  oxidation  stopped,  as  silver  is  volatile  also,  and  con- 
siderable of  it  may  be  lost  if  the  heating  is  continued  after  all  the 
lead  has  been  oxidized.  The  silver  button  will  be  white  unless 
it  contains  considerable  gold,  when  it  will  be  yellowish.  It  will 
also  contain  the  metals  of  the  platinum  group  if  the  ore  contained 
any.  If  the  ore  contained  no  silver,  the  lead  will  disappear  en- 
tirely in  the  bone  ash,  but  the  spot  where  it  disappeared  should 
always  be  examined  with  a  lens. 

6.  Wet  test.  —  The  powdered  mineral  is  dissolved  in  nitric 
acid,  the  solution  cooled  and  filtered;  a  drop  of  HC1  is  added, 
when  if  silver  is  present  a  white  precipitate  of  silver  chloride, 
AgCl,  is  formed,  becoming  dark  on  exposure  to  light.  Lead  and 
mercurous  mercury  will  also  yield  white  chlorides.  Lead  chloride 
is  soluble  in  hot  water  ;  if  the  solution  containing  the  precipitate  is 
boiled,  lead  chloride  will  redissolve ;  or  it  is  filtered  and  washed  with 
boiling  water,  when  if  the  precipitate  dissolves,  it  is  lead ;  when 
insoluble,  ammonia  is  poured  over  it  on  the  filter ;  if  it  blackens  it 
is  mercury ;  if  there  is  no  effect  it  is  silver. 

Illustration.  —  Any  ore  containing  silver  may  be  used  for  these 
tests. 

Mercury,  Hg.     Atomic  weight,  200.6.     Fusing  point,  -  38.85°  C. 

a.  The  substance  is  mixed  in  powder  with  3  parts  dry  soda, 
placed  in  a  closed  tube,  with  a  little  pure  dry  soda  placed  on  top 
of  the  charge.  The  tube  is  now  heated  in  the  burner  flame  or 
O.  F.  The  mercury  is  reduced  and  volatilized,  forming  a  gray 
sublimate  on  the  cold  walls  of  the  tube.  This  sublimate  is  com- 
posed of  minute  globules  of  mercury.  They  may  be  rubbed  to- 
gether, poured  out  in  a  watch  glass,  and  examined  with  a  lens. 

Illustration.  • —  Use  cinnabar,  HgS. 

Lead,  Pb.     Atomic  weight,  206.9.     Fusing  point,  327°  C. 

a.  Reduction  test.  —  Lead  is  reduced  to  a  malleable  gray  metal 
with  soda  and  borax  in  R.  F.  on  coal.  The  powdered  mineral  is 
mixed  to  a  stiff  paste  with  water,  4  parts  soda,  1  part  borax,  and 
some  coal  dust ;  this  charge  is  fused  in  R.  F.  on  coal ;  the  resulting 


580  MINERALOGY 

fusion  is  cut  away  from  the  coal  and  ground  in  the  mortar  under 
water.  If  pressure  is  used  in  the  grinding  the  lead  buttons  will 
be  flattened  out  in  thin  scales,  when  if  a  gentle  stream  of  water  is 
allowed  to  run  in  the  mortar,  the  coal  and  slag,  being  much  lighter 
than  the  lead  buttons,  will  be  carried  away.  When  very  little 
lead  is  present,  the  end  of  the  pestle  must  be  examined  for  metallic 
streaks ;  also  the  coal, '  for  the  yellow  oxide  coat  (see  below) . 
The  button  of  lead  may  be  further  tested  by  dissolving  it  in  nitric 
acid  and  adding  a  drop  of  H2SO4,  when  if  it  is  lead  a  white  precipi- 
tate of  lead  sulphate  will  form. 

6.  Yellow  lead  coat.  —  Minerals  and  compounds  containing 
lead  are  roasted  in  the  O.  F.  for  a  time ;  then  reduced  with  the  R.  F. 
lead  is  reduced  to  metal,  and  if  the  heat  is  continued,  volatilizes, 
forming  a  yellow  oxide  of  lead  coat.  The  yellow  coat  may,  when 
the  mineral  contains  much  sulphur,  be  masked  by  a  white  sul- 
phate coat ;  if  the  white  coat  is  heated  in  the  O.  F.  until  it  glows, 
the  white  coat  will  be  reduced  and  on  cooling  will  appear  yellow. 

c.  Bismuth  yields  a  yellow  oxide  coat  which  at  times  is  dis- 
tinguished from  the  lead  coat  with  difficulty.  In  this  case  the 
powdered  mineral  is  mixed  with  an  equal  volume  of  von  KobelFs 
flux  and  treated  with  a  moderately  hot  R.  F.  on  coal,  when  if 
lead  is  present,  a  lemon-yellow  iodide  of  lead  coat  will  form,  at  a 
distance  from  the  assay.  Under  similar  conditions  bismuth  will 
yield  a  brick-red  coat. 

Illustration.  —  For  the  above  tests  use  galena,  PbS. 

Bismuth,  Bi.     Atomic  weight,  208.     Fusing  point,  269°  C. 

a.  Coat.  —  Bismuth  compounds,  when  heated  in  a  moderately 
hot  R.  F.  with  soda  and  borax  as  in  the  case  of  lead,  yield  metallic 
globules,  which,  however,  are  not  malleable.     If  the  flame  is  con- 
tinued, the  metal  volatilizes,  forming  a  yellow  oxide  coat. 

b.  Von  Kobell's  test.  —  The  finely  powdered  mineral  is  mixed 
with  2  parts  of  von  KobelPs  flux  and  treated  with  a  moderately 
hot  R.  F.  on  coal ;  if  bismuth  is  present,  a  brick-red  iodide  of  bis- 
muth coat  will  form  at  a  considerable  distance  from  the  assay, 
inside  of  which  there  may  also  form  a  yellow  oxide  coat.     This 
brick-red  coat  is  a  very  characteristic  test  for  bismuth  and  serves 
to  distinguish  it  from  lead. 

Illustration.  —  Use  bismuth  oxide. 

Copper,  Cu.     Atomic  weight,  63.57.     Fusing  point,  1084°  C. 
a.  Flame  test.  —  A  very  delicate  test  for  copper  is  to  roast  the 
mineral  in  0.  F.  on  coal ;  the  charge  is  allowed  to  cool  and  is  then 


INSTRUMENTS   AND   CHEMICAL  TESTS  581 

moistened  with  a  drop  of  HC1 ;  heated  once  more  in  a  small  R.  F., 
a  bright  azure-blue  copper  chloride  flame  will  appear,  which  on 
long  heating,  if  much  copper  is  present,  will  give  way  to  the  green 
flame  of  copper  oxide.  The  azure-blue  flame  with  care  will  detect 
.1  per  cent,  of  copper. 

b.  Bead    test.  —  Oxide    of    copper    when    dissolved    in    borax 
yields  in  O.  F.  a  green  bead  while  hot,  becoming  blue  when  cold. 
In  R.  F.,  if  it  contains  much  oxide,  the  bead  is  opaque  red  when 
cold,  due  to  cuprous  oxide,  Cu2O ;  if  the  reduction  is  continued, 
the  cuprous  oxide  is  reduced  to  metallic  copper  and  the  borax  is 
colorless,  especially  if  reduced  on  coal.     In  S.  Ph.  the  colors  are 
the  same  as  in  borax. 

c.  Reduction    test.  —  Copper    is    reduced    from    its    oxidized 
compounds  with  soda  and  borax  in  R.  F.  on  coal,  yielding  red 
buttons  and  scales  of  malleable  metal  on  washing  the  charge  in 
the  mortar.     Sulphides  and  ores  containing  arsenic  and  antimony 
should  be  roasted  before  mixing  with  soda. 

Illustration.  —  Powder  some  chalcopyrite,  spread  it  out  in  a 
thin  layer  on  coal,  and  heat  it  with  an  O.  F.,  but  do  not  fuse  it ; 
turn  it  over  and  continue  the  roasting  until  the  odor  of  S02  has 
disappeared.  Allow  it  to  cool  and  moisten  with  a  drop  of  HC1, 
heat  now  in  the  R.  F. ;  a  bright  azure-blue  flame  proves  the  presence 
of  copper. 

Mix  the  roasted  mineral  with  4  parts  soda,  1  part  borax,  and 
some  coal  dust  to  a  stiff  paste  with  a  little  water.  The  charge  is 
reduced  on  coal,  and  is  finally  heated  in  the  blue  cone  of  the  O.F. 
until  thoroughly  fused,  when  it  is  cut  away  from  the  coal  and 
ground  in  the  mortar  under  water.  The  copper  buttons  or  scales, 
being  malleable,  will  not  grind  down,  but  the  slag  and  coal  will  be 
broken  up  and  washed  away,  leaving  the  red  scales  of  metallic 
copper. 

Cadmium,  Cd.     Atomic  weight,  112.4.     Fusing  point,  321.7°  C. 

a.  Coat.  —  Cadmium  compounds  when  heated  with  soda  and 
borax  in  R.  F.  are  reduced ;  on  continued  heating,  the  metal  is 
volatilized,  yielding  an  orange-yellow  oxide  of  cadmium  coat.  Cad- 
mium when  associated  with  lead  may  be  difficult  to  detect  as  a 
coat ;  in  such  cases  the  coat  is  scraped  from  the  coal  and  treated 
in  a  closed  tube  with  the  O.  F. ;  care  is  taken  not  to  heat  the  tube 
hot  enough  to  drive  off  lead  or  zinc ;  a  yellow  ring  of  cadmium 
oxide  will  form  just  above  the  assay.  If  much  is  present,  it  will 
form  a  metallic  mirror. 


582  MINERALOGY 

Platinum,  Ft.     Atomic  weight,  195.      Fusing  point,   1755°  C. 

Platinum  is  reduced  from  an  ore  and  cupeled  as  described 
under  silver,  p.  578.  If  much  platinum  is  present,  the  lead  button 
will  solidify  before  all  the  lead  is  driven  off.  The  remaining  lead 
may  be  removed  by  treatment  in  O.  F.  beside  borax  on  coal ;  the 
lead  is  oxidized  and  absorbed  by  the  borax.  The  remaining 
button  will  contain  the  silver,  gold,  and  the  metals  of  the  platinum 
group;  this  is  dissolved  in  nitro-hydrochloric  acid,  solid  ammo- 
nium chloride  is  added,  and  the  solution  evaporated  nearly  to 
dryness ;  it  is  now  diluted  with  alcohol  and  the  yellow  ammonium 
platinic  chloride,  NH4PtCl6,  is  filtered  out,  washed  with  alcohol, 
carefully  ignited,  when  the  platinum  will  remain  as  sponge  plati- 
'num,  with  other  metals  of  the  platinum  group.  If  there  was  any 
gold  in  the  button,  it  will  be  found  in  the  filtrate  from  the  ammo- 
nium platinic  chloride. 


Other  Metals  of  the  Platinum  Group 

Ruthenium,  Ru.     Atomic  weight,  101.7     Fusing  point,  1950°  C. 

Rhodium,  Rh.     Atomic  weight,  102.9.     Fusing  point,  2000°  C. 

Palladium,  Pd.     Atomic  weight,  106.7.     Fusing  point,  1586°  C. 

Osmium,  Os.     Atomic  weight,  190.9.     Fusing  point,  2500°  C. 

Iridium,  Ir.     Atomic  weight,  193.1.     Fusing  point,  1950°  C. 

All  the  above  five  metals  are  rare.  They  are  associated  in 
small  quantities  with  some  native  platinum.  Their  separation  is 
difficult  and  there  are  no  blowpipe  or  simple  wet  tests  by  means 
of  which  small  quantities  may  be  conveniently  detected  in  minerals. 

GROUP  VI 

Metals,  the  sulphides  of  which  are  insoluble  in  dilute  acids, 
but  soluble  in  alkali  sulphides. 

Common  elements:  gold,  platinum,  tin,  antimony,  arsenic. 

Rare  elements:  germanium,  iridium,  molybdenum,  tungsten, 
tellurium,  selenium. 

Gold,  Au.     Atomic  weight,  197.2.     Fusing  point,  1065°  C. 

a.  Gold  is  generally  present  as  metallic  gold,  but  in  such  fine 
particles  as  not  to  be  discernible  by  the  eye  or  hand  lens.  It  is 
necessary  to  collect,  or  concentrate,  the  gold  from  a  pound  or 
more  of  ore.  The  ore  is  ground  to  pass  an  80-mesh  sieve  ;  after  all 
the  sample  has  been  sifted,  the  sieve  is  examined  for  particles 


INSTRUMENTS  AND  CHEMICAL  TESTS  583 

of  gold  which  have  been  flattened  in  the  grinding ;  if  any  are  found, 
they  are  added  to  the  ground  sample.  The  sample  is  placed  in  a 
tin  or  iron  pan  2  in.  deep  and  8  in.  across;  2  cc.  of  mercury  is 
added,  the  pan  is  filled  with  water,  taken  in  the  hands,  and  shaken 
with  a  circular  motion ;  by  this  motion  the  sample  is  stratified  and 
the  materials  will  be  arranged  according  to  their  specific  gravity, 
with  the  heavier  particles  in  the  bottom,  where  the  gold  will  come 
in  contact  with  the  mercury  and  form  an  amalgam.  With  care 
the  light  materials  may  be  allowed  to  flow  over  the  side  of  the 
pan,  especially  if  a  stream  of  water  is  running  through  it  at  the 
same  time  it  is  shaken  with  a  circular  motion,  throwing  the  light 
particles  to  the  top.  If  the  heavy  concentrates  contain  pyrite, 
they  should  be  dried,  and  roasted  to  drive  off  the  sulphur,  then 
ground  and  repanned  with  the  same  mercury.  Finally  all  heavy 
material  together  with  the  mercury  is  washed  in  the  mortar  and 
ground  under  water,  also  allowing  a  stream  to  flow  through  it, 
which  is  so  regulated  as  to  carry  the  light  materials,  as  they  are 
ground,  over  the  edge,  leaving  in  a  very  short  time  the  clean 
bright  mercury  which  -contains  the  gold.  The  mercury  is  poured 
on  a  thick  piece  of  chamois  or  buckskin,  folded  up,  and  most  of 
the  mercury  is  squeezed  through ;  if  the  chamois  is  thick  and 
tight,  only  mercury  will  pass  through,  the  gold  amalgam  remain- 
ing. This  is  transferred  to  a  porcelain  crucible,  heated  gently  at 
first,  and  finally  to  redness,  to  volatilize  the  mercury ;  or  if  there 
is  only  a  small  quantity  of  amalgam,  the  mercury  may  be  driven 
off  with  the  O.  F.  on  coal.  In  either  case  the  residue  contains 
gold  and  silver,  which  is  collected  on  a  small  piece  of  sheet  lead : 
and  to  insure  enough  silver  for  parting,  pure  silver  to  twice  the 
amount  of  the  residue  is  folded  with  it  in  the  lead  and  fused  in 
the  O.  F. ;  then  cupeled  as  in  the  case  of  silver,  p.  578.  The 
button  of  gold  and  silver  if  large  is  flattened  on  the  anvil,  and 
parted;  that  is,  the  gold  is  separated  from  the  silver.  After 
flattening  the  button  it  is  heated  with  2  cc.  of  dilute  nitric  acid  in 
a  porcelain  crucible ;  heating  serves  to  start  the  reaction  and  then  it 
is  allowed  to  proceed  slowly.  The  nitric  acid  dissolves  the  silver, 
leaving  the  gold  as  a  black-looking  powder  or  a  brownish,  spongy 
mass.  The  nitric  acid  is  decanted.  The  gold  is  treated  in  the 
same  way  with  strong  nitric  acid,  decanted,  and  washed  with 
water.  If  the  sample  was  weighed  in  the  beginning  and  the 
operation  carried  through  with  care,  the  gold  residue  may  be 
weighed  and  will  represent  a  fair  assay  of  the  ore.  When  only  a 


584  MINERALOGY 

qualitative  test  for  gold  is  required,  the  residue  from  the  mercury 
is  tested  in  the  wet  way  as  below ;  cupellation  and  parting  are  not 
necessary. 

Ores  and  minerals  containing  considerable  gold  may  be  treated 
in  the  same  manner  as  the  silver  assay,  p.  578. 

b.  Wet  test.  —  The  powdered  mineral,  or  residue  of  gold  from 
the  cupel,  is  dissolved  in  nitro-hydrochloric  acid,  diluted  and 
filtered,  the  solution  evaporated  nearly  to  dryness  to  drive  off 
free  acid,  diluted  with  water,  and  a  solution  of  ferrous  sulphate 
added,  when  the  gold  will  be  reduced  to  the  metallic  state  and 
precipitated  in  the  form  of  a  brown  powder,  but  if  the  quantity 
is  very  small,  the  solution  will  be  colored  bluish  or  purple  by  the 
fine  particles  of  suspended  gold. 

Tin,  Sn.  Atomic  weight,  .119.  Fusing  point,  232°  C. 
a.  Reduction  test.  —  The  powdered  mineral,  if  a  sulphide,  is 
first  roasted,  then  mixed  with  soda,  borax,  and  coal  dust,  reduced 
and  washed  as  described  under  lead,  p.  579.  The  tin  globules 
are  white  and  malleable.  They  may  be  mistaken  for  silver,  from 
which  they  are  distinguished  by  yielding  insoluble  white  meta- 
stannic  acid  on  heating  in  a  test  tube  with  HNO3 ;  also  see  coat 
below. 

On  heating  in  the  R.  F.  the  charge  is  well  fused,  but  the  flame 
should  not  be  continued  long,  as  tin  after  reduction  is  easily  vola- 
tilized, when  no  buttons  will  be  found  in  the  charge,  upon  wash- 
ing in  the  mortar. 

6.  Tin  is  easily  volatile,  forming  an  oxide  coat,  very  near  the 
assay,  yellow  while  hot  and  white  on  cooling.  In  making  the 
reduction  test  the  coal  should  always  be  examined  for  a  coat. 
Tin  and  zinc  coats  are  much  alike  in  their  position  on  the  coal, 
color,  and  both  are  volatilized  with  difficulty.  If  the  tin  coat  is 
moistened  with  cobalt  solution  and  heated  with  the  0.  F.,  it 
will  become  blue  or  bluish  green  when  cold.  A  zinc  coat  will 
become  grass-green. 

Illustration.  —  Use  cassiterite,  Sn02.  If  the  reduction  test  a  is 
heated  for  a  longer  time,  the  tin  coat  of  6  will  appear. 

Antimony,  Sb.     Atomic  weight,  120.2.     Fusing  point,  630°  C. 
a.  Coat.  —  Compounds  of  antimony  when  heated  on  coal  in 
the  R.  F.  are  reduced  to  metal;   in  some  cases  it  is  necessary  to 
roast  the  charge  in  0.  F.  before  reduction,  but  in  all  cases  the 
metal  after  reduction  is  volatilized,  yielding  a  white  oxide  coat, 
which  settles  at  a  considerable  distance  from  the  assay. 


INSTRUMENTS   AND   CHEMICAL  TESTS  585 

On  the  outer  edge,  where  the  coat  is  thin,  it  appears  bluish,  due 
to  the  black  coal  showing  through  the  thin  white  film.  The  anti- 
mony coat  is  volatile  and  may  be  driven  from  the  coal  with  either 
flame,  but  with  more  difficulty  than  the  arsenic  coat,  which  it 
very  closely  resembles.  If  a  well-pointed  0.  F.  about  an  inch 
long  is  blown  and  the  coat  brought  up  quickly  to  the  middle  of 
the  flame  and  in  a  slanting  position,  the  coat  will  volatilize  and 
at  the  same  time  color  the  flame  a  yellowish  green.  This  flame 
coloration  serves  to  distinguish  antimony  from  arsenic. 

Illustration.  —  Use  stibnite,  Sb2Ss. 

Arsenic,  As.     Atomic  weight,  75.     Fusing  point,  450°  C. 

a.  Compounds  of  arsenic  when  heated  in  the  O.   F.  on  coal 
oxidize  ;  when  heated  in  R.  F.  volatilize  yielding  a  white  oxide  coat, 
As203,  which  settles  on  the  coal  at  a  distance  from  the  assay.     It  is 
very  similar  to  the  white  coat  yielded  by  antimony,  but  when  treated 
with  the  0.  F.  the  flame- is  not  colored  green.     Arsenical  vapors 
as  they  rise  from  the  assay,  especially  after  heating  in  R.  F.,  yield 
a  garlic-like  odor,  very  characteristic.     In  case  the  mineral  con- 
tains sulphur,  it  is  mixed  with  several  volumes  of  soda  to  retain 
the  sulphur,  when  the  arsenic  odor  is  easily  detected. 

b.  Arsenic  mirror.  —  Fusible  and  volatile  compounds  of  arsenic 
are  mixed  with  coal  dust ;  infusible  compounds  must  be  mixed 
with    soda    and   coal 

dust.      The    mixture,  d 

in  either  case,  is  placed 

in    the   bottom    of    a 

narrow    closed    tube, 

also  a  small  fragment 

of  coal   is   placed    on 

top  of  the  charge,  the 

O.  F.  is  now  directed 

upon  this  fragment  of  FIG.  534. 

coal  first  until  it  glows, 

then  the  assay  is  heated.     Arsenic  is  reduced  and  condenses  as 

a  metallic  mirror  on  the  cold  walls  of  the  tube.     If  the  amount 

of  arsenic  is  small,  the  closed  tube  should  be  drawn  out  at  the 

bottom  as  represented  in  Fig.  534,  in  which  d  is  the  mirror. 

The  mirror  may  be  further  tested  to  distinguish  it  from  anti- 
mony, in  which  the  bottom  of  the  tube  is  broken  off ;  the  mirror 
is  now  heated  in  the  Bunsen  flame;  the  arsenical  odor  may  be 
detected  by  quickly  smelling  the  fumes  as  they  escape  from  the 


586  MINERALOGY 

open  end  of  the  tube.  If  the  fumes  are  allowed  to  escape  in  the 
Bunsen  burner  flame  they  will  color  it  violet ;  antimony  will  color 
it  yellowish  green. 

Compounds  of  arsenic  when  mixed  with  soda  and  potassium 
cyanide  will  yield  the  mirror  in  the  closed  tube. 

c.  Wet  test.  —  All  compounds  of  arsenic  may  be  tested  as 
follows.  The  substance  is  finely  ground  and  mixed  with  4  parts 
soda  and  3  of  niter,  and  fused  on  platinum  foil  or  wire.  The 
fusion  is  boiled  in  water  to  dissolve  the  sodium  arsenate,  Na3AsO4, 
and  filtered ;  the  filtrate  acidified  with  HC1,  then  a  solution  of 
magnesium  sulphate,  and  finally  strong  ammonia  in  excess  is 
added,  the  solution  is  shaken  and  let  stand.  Arsenic  will  separate 
as  ammonium  magnesium  arsenate,  NH4MgAs04 . 6  H20,  which 
is  filtered  off,  washed  with  a  solution  of  ammonia,  dried,  and  tested 
for  the  mirror  as  in  b. 

Illustration.  —  Use  arsenopyrite,  FeSAs. 

Germanium,  Ge.     Atomic  weight,  72.5.     Fusing  point,  900°  C. 

Germanium  is  a  very  rare  element  found  in  only  three  minerals, 
argyrodite,  euxinite,  and  canfieldite.  In  the  R.  F.  it  is  reduced, 
then  volatilizes,  forming  an  oxide  coat,  GeO2,  white  near  the  assay 
and  yellowish  at  a  distance  from  it.  It  also  has  a  peculiar  glazed 
appearance.  It,  however,  yields  no  odor  or  flame  coloration. 
Germanium  when  treated  as  in  arsenic,  §  6,  yields  a  mirror. 

Molybdenum,  Mo.     Atomic  weight,  96. 

a.  Bead  test.  —  Molybdic  oxide  when  dissolved  in  the  borax 
bead  is  colorless  or  nearly  so  in  O.  F.      In  R.  F.  it  is  brown  to 
black.     In  S.  Ph.,  0.  F.,  the  bead  is  colorless  or  nearly  so;   in 
R.  F.  a  fine  green. 

b.  Coat.  —  Some  compounds  of  molybdenum  yield  on  coal  an 
oxide,  MoO3,  coat,  yellowish  while  hot,  white  on  cooling.     If  the 
R.  F.  is  brushed  over  this  white  coat,  it  is  partially  reduced, 
yielding  a  very  deep  blue  color.     On  heating  the  white  coat  in 
O.  F.  most  of  it  is  volatilized ;  the  coal,  however,  reduces  some  of 
it  to  binoxide,  which  is  non-volatile  and  remains  on  the  coal  as  a 
copper-red  film. 

c.  Flame.  —  Some  compounds  of  molybdenum  when  heated  in 
O.  F.  yield  a  green  flame. 

d.  Wet  test.  —  If  compounds   of  molybdenum   are  fused   on 
platinum  wire  with  3  parts  of  soda  and  3  parts  niter,  several  of 
these  beads  are  dissolved  in  a  test  tube  with  boiling  water.     The 
clear  solution  is  decanted,  acidified  with   HC1,  a  small  piece  of 


INSTRUMENTS   AND   CHEMICAL   TESTS  587 

copper  foil  added  and  heated  slightly,  when  the  molybdenum  is 
reduced,  coloring  the  solution  blue.  Tungsten  is  reduced  but 
slightly  by  copper,  but  will  yield  a  blue  solution  with  zinc. 

Illustration.  —  Use  molybdenite,  MoS2. 

Tungsten  (wolframum)  W.  Atomic  weight,  184.  Fusing 
point,  2800°  C. 

a.  Bead  test.  —  Oxide  of  tungsten  when  dissolved  in  the  S.  Ph. 
bead  in  O.  F.  is  colorless  or  nearly  so ;  in  R.  F.  the  moderately 
charged  bead  is  dirty  green  while  hot  and  when  thoroughly  cold 
is  blue. 

6.  Wet  test.  — •  Compounds  containing  tungsten  are  dissolved 
in  the  S.  Ph.  bead,  removed  from  the  wire,  and  reduced  beside 
tin  on  coal.  After  reduction,  it  is  powdered  and  dissolved 
in  1  cc.  dilute  HC1,  when  the  solution  becomes  blue,  or  it  may  be 
necessary  to  heat  the  solution  with  powdered  tin. 

Illustration.  — Use  wolframite  (Fe.Mn)W04. 

Tellurium,  Te.     Atomic  weight,  127.6.     Fusing  point,  446°  C. 

a.  Coat.  —  Tellurides  when  heated  in  R.  F.  on  coal  yield  a 
white  coat  of  Te02,  much  like  the  antimony  coat,  which  also 
imparts  a  pale  greenish  color  to  the  flame. 

6.  Wet  test.  —  Powdered  tellurides  when  heated  in  a  test  tube 
with  2  or  3  cc.  of  concentrated  H2S04  will  color  the  acid  reddish 
violet. 

c.  The  substance  is  fused  in  the  closed  tube  with  3  parts  soda 
and  coal  dust ;  after  fusion  the  tube  is  cooled  and  water  added ;  if 
tellurium  is  present  the  solution  will  be  reddish  violet. 

Selenium,  Se.     Atomic  weight,  79.2.     Fusing  point,  217°  C. 

a.  Odor.  —  Compounds  of  selenium  when  heated  in  the  R.  F. 
on  coal  yield  a  very  disagreeable  but  characteristic  odor,  which  is 
a  very  delicate  test  for  the  element,  even  in  small  quantities. 

6.  Coat  and  flame.  —  If  there  is  much  Se  present  when  treated 
as.in  a,  a  white  oxide  coat  will  form,  at  times  bordered  with  red. 
The  coat  when  treated  with  the  R.  F.  volatilizes,  coloring  the  flame 
an  intense  azure-blue. 

Non-metallic  acid  elements. 

Sulphur,  S.     Atomic  weight,  32.07.     Fusing  point,  115°  C. 

a.  Odor.  —  Sulphides  when  roasted  on  coal  in  the  O.  F.  yield 
sulphur  dioxide,  SO2,  which  is  detected  by  the  characteristic  odor, 
as  of  a  burning  sulphur  match. 

b.  Soda  test.  —  Sulphur  in  any  form  may  be  detected  by  fusing 
the  powdered  mineral  in  R.  F.  with  4  parts  soda  and  coal  dust. 


588  MINERALOGY 

Sodium  sulphide,  Na^S,  will  be  formed,  soluble  in  water.  The 
fusion  is  cut  away  from  the  coal  and  placed  on  a  bright  silver 
surface,  a  coin  will  answer,  and  moistened  with  a  couple  of  drops 
of  water.  The  dissolved  sodium  sulphide  attacks  the  silver,  form- 
ing a  brown  or  black  stain  of  silver  sulphide. 

c.  Wet  test  for  sulphates.  —  The  substance  if  insoluble  in  acid 
is  fused  on  coal  in  O.  F.  with  4  parts  of  soda,  dissolved  in  boiling 
water  and  filtered ;  to  the  nitrate,  a  few  drops  of  barium  chloride 
are  added,  when,  if  sulphates  are  present,  a  white  precipitate  of 
barium  sulphate,  BaS04,  will  form. 

Illustration.  —  For  tests  in  a  and  6,  use  pyrite,  FeS2 ;  for  c,  use 
gypsum,  CaS04 . 2  H20. 

Chlorine,  Cl.     Atomic  weight,  35.45.     Fusing  point,  —  102°  C. 

a.  Flame  test.  —  Copper  oxide  is  dissolved  in  the  S.  Ph.  bead. 
The  hot  bead  is  touched  to  the  powdered  substance  to  be  tested 
for  chlorine ;  it  is  now  held  just  within  the  blue  cone  of  the  O.  F. ; 
if  chlorine  is  present,  an  azure-blue  copper  chloride  flame  will 
appear. 

6.  Wet  test.  —  The  substance  to  be  tested  is  dissolved  in  nitric 
acid,  if  insoluble  it  is  fused  with  soda  and  boiled  in  water ;  to  the 
clear  solution  a  drop  or  two  of  a  silver  nitrate  solution  is  added, 
when  if  chlorine  is  present,  a  white  precipitate  of  silver  chloride 
will  form,  which  on  exposure  to  light  becomes  violet  and  finally 
black.  Bromine  and  iodine  yield  the  above  reaction  also,  except 
the  precipitate  is  yellowish. 

c.  Separation  of  chlorine,  bromine,  and  iodine.  —  The  silver 
precipitate  is  collected  on  a  filter,  and  washed  with  dilute  ammonia  ; 
silver  chloride  and  silver  bromide  are  dissolved,  leaving  the  silver 
iodide  on  the  filter,  which  is  tested  according  to  §  d.  The  filtrate 
is  made  acid  with  nitric  acid ;  the  bromide  and  chloride  of  silver 
filtered  and  tested  for  bromine  as  under  bromine,  §  c. 

Illustration.  —  Use  halite,  NaCl. 

Bromine,  Br.     Atomic  weight,  79.92.     Fusing  point,  -  7.3°  C. 

a.  Flame  test.  —  Compounds  of  bromine  when  treated  as  in 
chlorine,  §  a,  with  copper  oxide  also  yield  a  blue  flame. 

b.  Bromides  are  precipitated  with  silver  nitrate  as  silver  bro- 
mide, AgBr,  a  slightly  yellowish  precipitate  soluble  in  ammonia. 

c.  If  silver  bromide  is  mixed  with  bismuth  sulphide  and  heated 
gently  in  the  closed  tube,  it  will  yield  a  sublimate  of  yellow  bismuth 
bromide  directly  above  the  assay.     Silver  chloride  under  the  same 
conditions  will  yield  a  white  sublimate  of  bismuth  chloride. 


INSTRUMENTS  AND   CHEMICAL  TESTS  589 

d.  The  substance  is  mixed  with  potassium  bisulphate  and  fused 
in  a  closed  tube,  when  bromine  is  liberated  as  reddish  yellow  vapors, 
best  seen  by  looking  down  in  the  top  of  the  tube. 

Illustration.  —  Use  potassium  bromide. 

Iodine,  I.     Atomic  weight,  126.92.     Fusing  point,  114.2°  C. 

a.  Flame  test.  —  Iodides  when  heated  with  copper  oxide  as  in 
case  of  chlorine  yield  a  green  flame. 

b.  Iodides  in  solution  yield  a  yellow  precipitate  of  silver  iodide, 
with  silver  nitrate,  nearly  insoluble  in  ammonia,  and  which  does 
not  darken  on  exposure  to  light. 

c.  If  the  silver  iodide  from  6  is  collected  in  the  bottom  of  a  test 
tube,  a  few  drops  of  dilute  sulphuric  acid,  and  a  fragment  of  zinc 
added,  the  silver  will  be  reduced  to  metallic  silver,  and  zinc  iodide 
will  go  in  solution.     The  solution  is  decanted,  a  few  drops  of  starch 
paste  (made  by  boiling  starch  in  water)  and  fuming  nitric  acid 
added.     Iodine  will  be  liberated,  coloring  the  starch  solution  a  deep 
blue. 

d.  Compounds  containing  iodine  when  mixed  with  bismuth  sul- 
phide and  heated  in  the  closed  tube  yield  a  brick-red  sublimate  of 
bismuth  iodide. 

e.  If  substances  containing  iodine  are  mixed  with  potassium 
bisulphate  and  heated  in  the  closed  tube,  iodine  will  be  liberated 
as  violet  vapors,  best  seen  by  looking  down  in  the  top  of  the  tube. 

Illustration.  —  Use  potassium  iodide. 

Fluorine,  F.     Atomic  weight,  19.     Fusing  point,  —  223°  C. 

a.  Closed  tube  test.  —  The  ground  mineral  is  mixed  with  two  parts 
of  potassium  bisulphate  and  heated  in  a  closed  tube,  fluorine  is  lib- 
erated and  attacks  the  walls  of  the  tube,  forming  a  ring,  like  frosted 
glass  in  appearance,  due  to  a  deposit  of  silica,  Si02.  The  hydro- 
fluoric acid  liberated  by  the  fusion  forms  silicon  tetrafluoride, 
SiF4,  with  the  silica  contained  in  the  glass ;  this  in  turn  is  decom- 
posed with  water ;  thus  3  SiF4  +  2  H20  =  2  H2SiF6  +  SiO2.  After 
the  frosted  ring  appears,  the  bottom  of  the  tube  is  broken  off,  and 
the  tube  gently  dipped  in  water,  then  dried  carefully ;  if  the  ring 
returns  and  is  non-volatile,  it  is  due  to  fluorine  and  is  not  a  subli- 
mate of  sulphates.  The  fusion  with  potassium  bisulphate  is  not 
applicable  to  silicates. 

6.  Silicates  are  tested  as  follows:  several  S.  Ph.  beads  are  made 
on  wire  and  powdered;  4  parts  of  this  powder  are  mixed  with 
one  part  of  mineral  and  fused  in  the  closed  tube  as  directed  in  a 
above. 


590  MINERALOGY 

Illustration.  —  Use  fluorite,  CaF2,  for  a,  and  for  b  use  topaz. 
Phosphorus,  P.     Atomic  weight,  31.04.     Fusing  point,  44.2°  C. 

a.  Flame  coloration.  —  Phosphates  when  heated  in  fine  powder 
on  wire,  then  moistened  with  concentrated  sulphuric  acid,  and 
heated  gently  until  most  of  the  white  fumes  of  SO3  are  driven  off, 
then  held  at  the  tip  of  the  blue  cone  of  an  O.  F.,  yield  a  yellowish 
green  flame.     The  flame  appears  only  momentarily  and  may  be 
easily  overlooked.     The  charge  must  be  moistened  with  acid  again 
and  hea.ted,  when  the  flame  will  reappear.     Sulphuric  acid  liberates 
the  phosphoric  acid,  which  volatilizes,  coloring  the  flame. 

b.  Wet  test.  —  The  powdered  mineral  is  dissolved  in  nitric  acid  ; 
if  not  soluble,  it  is  first  fused  with  4  parts  soda,  then  dissolved. 
10  cc.  of  ammonium  molybdate  solution  are  heated  in  another  test 
tube  to  boiling,  then  the  nitric  acid  solution  of  the  mineral  is  poured 
into  the  molybdate,  and  well  shaken.    If  phosphoric  acid  is  present, 
a   canary-yellow   precipitate   of   ammonium    phosphomolybdate, 
(NH4)3PO4,  12  MoO3 .  n  (H20),  will  form.     This  yellow  precipitate 
is  very  soluble  in  alkalies.     Arsenic  will  yield  a  similar  yellow  pre- 
cipitate, but  only  upon  heating  the  solution  above  80°  C. 

Illustration.  —  Use  apatite,  Ca4CaF(P04)3. 

Nitrogen,  N.     Atomic  weight,  14.04.     Fusing  point,  —  210°  C. 

a.  Closed  tube  test.  —  The  powdered  mineral  is  fused  in  the  closed 
tube  with  2  parts  of  potassium  bisulphate,  when  nitrogen  will  be 
liberated  as  red  fumes,  N02;  best  observed  by  looking  down  in  the 
open  end  of  the  tube. 

b.  Wet  test.  —  The  substance  is  dissolved  in  sulphuric  acid  (one 
of  water  to  one  of  acid)  and  allowed  to  cool.     Holding  the  tube  in  a 
slanting  position,  a  concentrated  solution  of  ferrous  sulphate  is 
poured  in,  so  as  to  form  a  layer  above  the  solution  to  be  tested,  and 
not  mix  with  it.     If  nitrates  are  present,  a  brown  ring  will  form 
where  the  two  solutions  are  slightly  mixed  on  contact. 

Illustration.  —  Use  niter,  KN03. 

Carbon,  C.     Atomic  weight,  12.     Infusible. 

a.  Carbonates.  —  3  or  4  cc.  of  dilute  hydrochloric  or  nitric 
acid  are  heated  in  a  test  tube  and  several  small  fragments  of  the 
mineral  to  be  tested  are  dropped  in  the  hot  acid ;  if  carbonates  are 
present,  they  will  be  decomposed  with  effervescence.  Minerals 
are  apt  to  contain  a  small  amount  of  carbonates  as  impurities ;  in 
such  cases  the  effervescence  continues  for  a  short  time  only ;  where 
the  mineral  is  a  pure  carbonate,  effervescence  should  continue  until 
the  mineral  has  completely  dissolved.  Carbon  dioxide,  CO2,  is 


DESCRIPTION  OF  THE  INSTRUMENTS  591 

colorless  and  odorless,  which  distinguishes  it  from  other  gases  which 
may  be  yielded  in  dissolving  a  mineral  in  acids.  A  direct  test 
may  be  applied  by  holding  a  glass  rod  down  in  the  test  tube  during 
the  effervescence,  on  the  end  of  which  a  drop  of  lime  water  is 
suspended.  The  drop  will  soon  appear  milky  from  the  formation 
of  calcium  carbonate. 

6.  Organic  carbon.  —  Carbon  in  organic  matter  is  detected  by 
heating  the  substance  in  a  closed  tube,  when  it  will  blacken,  and 
generally  yields  an  empyreumatic  odor,  also  sometimes  an  oily 
distillate. 

c.  Free  carbon,  as  coal  or  graphite,  when  heated  on  platinum  foil, 
glows,  or  burns,  leaving  an  ash  which  is  light  in  color.  Diamond  must 
be  powdered  and  heated  to  a  high  temperature  before  it  burns. 

Illustration.  —  For  a  use  calcite,  CaCOs,  and  for  b  any  organic 
acid,  as  oxalic,  C2H2O4,  will  do. 

Silicon,  Si.     Atomic  weight,  28.4.     Fusing  point,  1200°  C. 

a.  Silica,  SiO2,  when  heated  in  the  S.  Ph.  bead,  dissolves  only 
very  slowly ;  it  yields  therefore  a  translucent  bead.     If  a  fragment 
of  a  silicate  is  heated  in  an  S.  Ph.  bead,  the  bases  or  other  oxides  go 
into  solution,  leaving  the  silica  insoluble,  in  more  or  less  of  the  same 
shape  as  the  fragment  used.     This  is  known  as  the  silica  skeleton. 

Illustration.  —  Use  a  small  fragment  of  orthoclase,  KAlSi3O8. 

b.  Gelatmization.  —  Numerous  silicates  when  in  fine  powder  are 
decomposed  by  hydrochloric  or  nitric  acid,  and  on  evaporating 
the  solution  nearly  to  dryness  the  silicic  acid  will  separate  and 
appear  as  a  jelly.     Best  observed  by  stirring  with  a  platinum  wire ; 
upon  further  evaporation  to  dryness  the  silicic  acid  is  dehydrated. 
If  the  dry  residue  is  now  moistened  with  a  few  drops  of  dilute  HC1, 
other  elements  are  dissolved  as  chlorides,  leaving  the  silica  as  a 
white  insoluble  residue,  which  is  filtered  out,  washed,  and  tested  as 
in  a. 

Illustration.  —  Use  calamine,  Zn2SiO4 . 2  H20. 

c.  All  silicates,  when  fused  with  5  parts  soda  and  dissolved  in 
dilute  hydrochloric  acid  and  evaporated  as  in  6,  yield  gelatinous 
silica ;  on  evaporation  to  dryness  the  silica  is  separated  and  tested 
as  in  a.     This  is  the  method  used  in  qualitative  and  quantitative 
analysis  to  decompose  silicates  and  separate  silica  from  the  bases. 
All  bases  will  be  in  the  filtrate  from  the  silica  and  may  be  detected 
by  appropriate  tests. 

Illustration.  —  Fuse  powdered  orthoclase  with  soda  and  separate 
silica  as  described. 


592  MINERALOGY 

d.  Many  silicates  when  treated  with  acids  decompose,  leaving 
a  residue  of  flocculent  silica,  which  on  evaporation  does  not  gelat- 
inize. 

Boron,  B.     Atomic  weight,  11.     Infusible. 

a.  Flame  coloration.  —  Some  borates  when  heated  alone  in  the 
forceps  yield  a  green  flame  ;  all  borates  when  heated  with  Turner's 
flux  yield  a  green  flame. 

6.  The  green  flame  may  be  due  to  copper  or  phosphoric  acid. 
When  in  doubt,  the  mineral  is  fused  with  soda  and  dissolved  in 
dilute  HC1.  A  piece  of  turmeric  paper  is  moistened  in  this  solution, 
and  carefully  dried,  when  if  boric  acid  is  present  the  paper  will 
turn  reddish  brown. 

Illustration.  —  Grind  some  tourmaline  in  the  mortar,  mix  with 
it  3  parts  of  Turner's  flux  and  a  drop  of  water.  With  a  platinum 
wire  fuse  some  of  this  mixture  in  the  side  of  the  Bunsen  burner 
flame.  Just  after  it  begins  to  fuse,  the  bright  green  flame  of  boron 
will  appear.  Mix  some  of  the  powdered  tourmaline  with  4  parts 
soda  and  fuse  several  beads  of  the  mixture  on  wire.  These  beads  are 
dissolved  in  1  cc.  of  strong  HC1 ;  the  solution  is  then  diluted  with 
4  volumes  of  water.  A  piece  of  turmeric  paper  is  moistened  with 
the  solution,  placed  on  a  clean  watch  glass,  and  dried  on  the  water 
bath.  When  dry  it  will  be  reddish  brown. 


INSTRUMENTS  AND   CHEMICAL  TESTS 
TABLE  L— COATS   ON   COAL 


593 


COLOR 


REMARKS 


SUBSTANCE 


White  .  . 

White  .  . 

White  .  . 

White  .  . 

White  .  . 

White 


Yellow, 
while  hot; 
white,  cold 

Yellow, 
while  hot; 
white,  cold 

Yellow, 
'while  hot ; 
white,  cold 

Yellow 


Yellow 


Brown         to 
yellow  .     . 

Red     ... 


At  considerable  distance  from  assay  ; 
easily  volatile  in  O.  F. ;  in  R.  F. 
yields  a  garlic-like  odor 

At  considerable  distance  from  assay ; 
volatile  in  R.  F.,  tinging  the  flame 
yellowish  green 

At  considerable  distance  from  assay ; 
volatile  in  R.  F.,  tinging  the  flame 
an  intense  green 

Volatile  in  either  flame,  coloring  the 
flame  green 

Silvery  luster,  very  volatile ;  the 
outer  edge  may  be  reddish;  im- 
parts an  azure  blue  color  to  the 
flame 

Due  to  chlorides,  sulphates  of  lead 
bismuth,  zinc,  mercury,  etc.  They 
are  identified  by  other  tests 

Volatile  in  R.  F.,  quickly  becoming 
blue ;  in  O.F.  volatile,  leaving  a 
copperlike  film  on  the  coal 

Very  near  the  assay,  not  volatile  in 
O.  F.,  but  slowly  in  R.  F. ;  treated 
with  cobalt  solution,  blue-green 

Very  near  the  assay,  not  volatile  in 
O.  F.,  but  slowly  in  R.  F. ;  treated 
with  cobalt  solution  becomes  green 

May  have  an  outer  edge  of  white, 
volatile ;  the  substance  treated  with 
von  Kobell's  flux  yields  a  lemon- 
yellow  coat 

May  have  an  outer  edge  of  white, 
volatile;  the  substance  treated 
with  von  Kobell's  flux  yields  a 
brick-red  coat 

Yellow  border  and  iridescent  where 
thin;  volatile 

Generally  preceded  by  a  yellow  or 
white  coat 


Arsenious  oxide, 
As2O3 

Antimonious  oxide, 

Sb2S3 

Thallium  oxide, 
T12O  rare, 

Tellurous  oxide, 
Te02 

Selenious  oxide, 
SeO2 


Molybdenum 
oxide,  MoO3 

Tin  oxide, 
SnO2 

Zinc  oxide, 
ZnO 

Lead  oxide, 
PbO 


Bismuth  oxide, 
Bi203 


Cadmium  oxide. 
CdO 

Silver  with  lead 
or  bismuth 


tri- 


594 


MINERALOGY 


TABLE  II.  — COLOR  REACTIONS  OF  OXIDES  IN  THE 
BORAX  AND  S.Ph.  BEADS 


BORAX 

ELEMENT 

S.Ph. 

O.  F. 

R.  F. 

O.  F. 

R.  F. 

Hot,  red  to  yel- 

Bottle-green 

Iron 

Yellow       to 

Smoky- 

low  ;         cold, 

colorless 

brown 

yellow  to  col- 

orless 

Hot,        yellow  ; 

Brown 

Molyb- 

Yellowish 

Fine  green 

cold,      nearly 

denum 

green 

colorless 

to     color- 

less 

Hot,  orange-red 

Bottle-green 

Uranium 

Greenish- 

Fine  green 

to         yellow  ; 

yellow    to 

cold,      yellow 

colorless 

to  colorless 

Hot,        yellow  ; 

Fine  green 

Vanadium 

Yellow       to 

Green 

cold,      yellow 

colorless 

to  colorless 

Hot,        yellow  ; 

Fine  green 

Chromium 

Green 

Green 

cold,     yellow- 

ish green 

Hot,    pale    yel- 

Yellow      to 

Titanium 

Colorless 

Hot,  yellow; 

low  ;         cold, 

brown 

cold, 

colorless 

violet 

Hot,    pale    yel- 

Yellow      to 

Tungsten 

Yellow       to 

Hot,       dirty 

low  ;         cold, 

brown 

colorless 

blue;  cold, 

colorless 

blue 

Hot,        orange- 

Colorless 

Cerium 

Hot,  yellow  ; 

Colorless 

yellow  ;    cold, 

cold, 

yellow 

colorless 

Hot,          green  ; 

Colorless   to 

Copper 

Blue 

Colorless    to 

cold,  blue 

opaque 

opaque 

red 

red 

Hot,         violet  ; 

Gray 

Nickel 

Yellow 

Yellow 

cold,    reddish 

brown 

Hot,         violet  ; 

Colorless 

VTanganese 

Violet 

Colorless 

cold,    reddish 

violet 

Blue 

Blue 

Cobalt 

Blue 

Blue 

CHAPTER  II 

TABLE  FOR  THE  DETERMINATION  OF  THE  MORE  COM- 
MON MINERALS,  BY  THE  USE  OF  THEIR  PHYSICAL 
PROPERTIES 

THIS  table  is  arranged  in  two  parts.  Part  I  includes  those 
species  which  have  a  metallic  luster  and  usually  yield  a  streak 
dark  in  color,  and  the  mineral  specimens  are  opaque,  even  on  their 
thin  edges.  Those  with  a  bright  luster  are  metallic  and  those  with 
a  dull  luster  are  considered  as  sub-metallic. 

Part  II  includes  all  those  species  which  yield  a  streak,  light  in 
color  or  colorless  ;  they  are  termed  non-metallic. 

As  the  color  of  metallic  minerals  is  quite  characteristic  and  more 
or  less  constant,  Part  I  is  further  divided  into  divisions ;  minerals 
of  nearly  the  same  color  and  the  species  are  arranged  in  each  of 
these  divisions  according  to  their  hardness. 

In  Part  II,  the  non-metallic  minerals  (where  the  color  of  the 
specimens  may  vary  widely),  the  divisions  are  made  according  to 
the  color  of  their  streaks,  and  those  yielding  streaks  of  nearly  the 
same  color  are  placed  in  the  same  division;  and  here  again  the 
species  are  arranged  in  the  divisions  according  to  their  hardness. 

NOTE.  —  All  material  used  for  the  tests  should  be  homogeneous  and  when 
crystalline  the  crystals  should  be  used ;  remembering  always  that  a  mineral  natu- 
rally soft  may  appear  harder  than  it  really  is  from  impurities,  as  sand  or  other  hard 
minerals.  Hard  minerals  may  often  appear  soft  on  the  surface  from  chemical  change 
or  weathering.  Impurities  often  effect  the  color  of  the  streak  of  non-metallic 
minerals,  yielding  streaks  darker  than  normal. 

The  abbreviations  used  are,  H.  =  hardness,  C.  =  color,  G.  =  specific  gravity, 
xls  =  crystals ;  cleavage  parallel  to  a  crystal  form  is  indicated  by  the  letter  repre- 
senting that  form,  as  cleavage  m  =  cleavage  parallel  to  the  unit  prism ;  cleavage 
T  =  parallel  to  the  rhombohedron,  etc. 

The  common  colors  are  placed  first ;  as,  white,  yellowish,  or  green  would  indicate 
that  the  mineral  is  usually  white,  but  yellowish  specimens  occur  which  are  more 
common  than  the  green.  The  same  principle  applies  to  the  order  of  other  descrip- 
tive terms  used. 

595 


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MINERALOGY 


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1 

CHAPTER   III 

TABLE    FOR    THE    DETERMINATION    OF    THE    PRINCIPAL 
ROCK-FORMING  MINERALS  IN  SECTIONS 

I.  The  mineral  is  opaque. 
Magnetite.     Isometric. 

In  crystalline  outlines,  rounded  grains,  or  dustlike. 

By  reflected  light,  bluish  black  in  color ;  p.  373. 

Chromite :    Isometric. 

In  crystalline  outline,  or  rounded  grains. 

By  reflected  light  brownish  and  at  times  the  very  thin  edges 

may  appear  brownish ;  p.  376. 
Ilmenite.     Hexagonal. 
Tabular,  thin  plates,  or  irregular  in  outline. 
By  reflected  light  brownish  and  on  very  thin  edges  brown; 

p.  346. 

Hematite.     Hexagonal. 
In  thin  plates,  scales,  and  grains. 

The  thin  edges  and  scales  dark  red  by  transmitted  light ;  p.  343. 
Graphite.     Hexagonal. 
In  thin  flakes,  foliated  masses,  and  grains. 
By  reflected  light,  metallic  luster  or  dull  black ;  p.  284. 
Pyrite.     Isometric. 

Crystalline  outlines,  irregular  masses,  or  rounded  grains. 
By  reflected  light  yellow,  metallic  luster;  p.  313. 
Pyrrhotite.     Hexagonal. 
Irregular  grains  and  masses. 
By  reflected  light  bronze-yellow  with  a  metallic  luster ;  p.  308. 

II.  Transmits  light  isotropically. 

A .    Index  of  refraction  below  that  of  Canada  balsam. 
Fluorite.     Isometric:  n=  1.43. 

In  irregular  masses  or  rounded  grains,  rarely  showing  crys- 
talline outlines,  filling  cavities  or  veins;     Colorless,  violet 
or  purple;  Cleavage  cracks  well  developed;  Relief  marked, 
from  the  low  refraction;  p.  331. 
Opal:  Amorphous:    n=  1.44. 
2R  609 


610'  MINERALOGY 

In  irregular  masses,  grains,  and  filling  veins,  often  forming  the 
cementing  material  in  sandstones  and  shales;  Colorless  or 
nearly  so;  In  sections  may  be  mistaken  for  leucite  or  so- 
dalite;  p.  369. 

Glass.     Amorphous;  n  =  1.49;  Variable. 

As  the  residuum  of  crystallization,  irregular  in  outline,  filling 
the  spaces  between  the  crystals ;  Colorless,  gray,  or  smoky. 

Analcite.     Isometric;  n  =  1.48. 

Rounded  crystalline  outlines,  cloudy  grains,  or  filling  cavities 
and  'veins ;  Cleavage  cubic  but  not  well  developed ;  Color- 
less ;  Usually  a  secondary  mineral  after  leucite,  sodalite,  or 
nepheline ;  p.  485. 

Sodalite  group.     Isometric;   n  =  1.48-1.50. 

In  six-  or  eight-sided  crystalline  outlines  or  rounded  grains  ; 
Cleavage'  not  well  developed,  often  irregularly  cracked  and 
fractured;  forming  the  ground  mass  between  crystals  of 
other  species ;  Usually  colorless  but  often  yellow  or  blue, 
especially  haiiynite  and  noselite ;  Often  filled  with  dustlike 
inclusions,  arranged  in  zones  or  collected  at  the  center  or 
margin;  p.  436. 

Leucite.     Isometric;  n  =  1.50. 

In  six-  or  eight-sided  crystalline  outlines,  or  rounded  grains ; 
Cleavage  not  well  developed,  often  irregularly  fractured; 
inclusions  when  present  are  arranged  symmetrically  in 
zones  or  radiating  from  the  center ;  Colorless ;  Relief  very 
low;  Large  crystals  are  doubly  refracting;  Interference 
color  first  order  gray,  showing  polysynthetic  twinning; 
p.  415. 
B.  Index  of  refraction  above  that  of  Canada  balsam. 

Spinels.     Isometric  ;  n  =  1.71-1.76. 

In  square  or  six-sided  crystalline  outlines  or  rounded  grains ; 
Cleavage  not  developed ;  Colorless  to  opaque,  according  to  the 
species ;  Relief  very  marked  and  surface  very  rough ;  p.  371. 

Garnet.     Isometric;  n  =  1.75-1.85. 

In  crystalline  outlines,  rounded  grains  or  irregular  masses; 
Cleavage  not  developed ;  Irregularly  fractured  and  cracked ; 
Relief  very  marked  and  surface  rough  ;  Colorless,  to  reddish 
of  various  shades,  also  green,  brown,  to  nearly  opaque ;  p. 
442. 

III.   Transmits  light  anisotropically. 
A.    Uniaxial. 


THE  PRINCIPAL  ROCK-FORMING  MINERALS        611 

1.  Index  of  refraction  less  than  that  of  Canada  balsam. 

a.  Double  refraction  equal  to  or  less  than  that  of  quartz ; 
not  pleochroic.1 

Nephelite.     Hexagonal;  n  =  1.54;  a)  —  €  =  .005. 

In  nearly  square  or  hexagonal  outline,  irregular  masses, 
and  rounded  grains ;  Colorless,  gray,  greenish,  bluish 
or  brown ;  Cleavage  prismatic  and  basal,  though  not 
well  developed  in  sections ;  Relief  very  flat ;  Inter- 
ference color  low  first  order  gray ;  Optically  negative 

(-);  P.  440. 

Tridymite;  Hexagonal;  n  =  1.477;  €  —  o)  =  .002. 

In  hexagonal  plates  and  scaly  aggregates;  Colorless 
and  transparent ;  The  surface  appears  rough  from  the 
low  refraction;  Interference  color  very  low  gray  of 
the  first  order ;  Optically  positive  (+) ;  p.  361. 

Quartz;  Hexagonal;,  n  =  1.547;  €  —  to  =  .009. 

Irregular  grains  and  angular  masses ;  No  cleavage  or 
relief;  Interference  colors  first  order  gray  to  yellow; 
Optically  positive  (+) ;  p.  352. 

2.  Index  of  refraction  greater  than  that  of  Canada  balsam, 
a.   Double  refraction  less  than  that  of  quartz. 

Apatite;  Hexagonal;  n  =  1.635;  co  -  €  =  .004. 

Hexagonal  sections,  prismatic  elongated  sections,  or 
rounded  grains;  Cleavage  not  developed,  elongated 
sections  show  transverse  parting;  Colorless;  Not 
pleochroic ;  Relief  not  marked ;  Interference  colors 
first  order  gray ;  Optically  negative  (— ) ;  p.  508. 

Vesuvianite;   Tetragonal;   n  =  1.72;  (0  —  €  =  .006. 

In  short  square  prisms,  grains,  or  irregular  masses ;  Cleav- 
age not  developed ;  Colorless,  yellow,  green,  brown, 
or  blue ;  Relief  marked,  surface  rough  ;  Slightly  pleo- 
chroic ;  Interference  color  first  order  gray,  often 
anomalously  high;  Optically  negative  (— ) ;  p.  455. 

Corundum;  Hexagonal;  n  =  1.766;  (0  -  €  =  .009. 

Crystalline  outlines,  elongated  parallel  to  c,  grains  or 
irregular  masses  ;  Rhombohedral  parting ;  Colorless, 
red,  or  blue ;  Pleochroic  only  in  highly  colored  sec- 
tions ;  Relief  high ;  Interference  color  first  order 
gray  to  yellow;  Optically  negative  (— ) ;  p.  341. 

1  Chlorite,  which  is  pleochroic,  may  appear  uniaxial ;  p.  352. 


612  MINERALOGY 

6.   Double  refraction  greater  than  that  of  quartz. 

Scapolite;  Tetragonal;  n  =  1.55-1.58;  o>  —  €  =  .013- 
.035. 

Fibrous  aggregates,  rounded  grains  or  rods;  Cleavage 
prismatic;  Colorless;  Not  pleochroic;  Interference 
colors,  bright,  higher  first  and  lower  second  orders; 
Optically  negative  (  — ) ;  p.  453. 

Calcite;  Hexagonal;  n  =  3.57;  o>  -  €  =  .172. 

Irregular  masses  and  grains,  often  showing  twinning 
lamellae;  Cleavage  well  developed,  rhombohedral, 
74°  55';  Relief  varies  greatly  with  the  direction  of 
the  section ;  Colorless  or  pale ;  Not  pleochroic ;  Inter- 
ference colors  pale  of  high  orders ;  Optically  ( — ) ; 
p.  379. 

Tourmaline;  Hexagonal;  n  =  1.65;  o>  —  €  =  .017- 
.034. 

Lath-shaped,  hexagonal  or  trigonal  outlines,  fibrous 
aggregates  or  rounded  grains ;  Cleavage  none ;  Color, 
greenish  to  dark  brown,  or  pale;  Pleochroism  very 
marked,  increasing  with  the  depth  of  color  of  the 
section;  Relief  marked;  Interference  colors,  upper 
first  and  lower  second  orders;  Optically  negative 
(-);  p.  473. 

Zircon;   Tetragonal;   n  =  1.96;  €  -  <o  =  .044-.062. 

In  short  square  prisms  or  rounded  grains ;  Cleavage  not 
well  developed ;  Nearly  always  colorless ;  Relief  very 
marked;  Not  pleochroic;  Interference  colors  bril- 
liant high  order;  Optically  positive  (+);  p.  456. 

Rutile;  Tetragonal;  n  =  1.76;  €  -  to  =  .287. 

Short  square  or  long  acicular  prisms  or  rounded  grains ; 
Cleavage  not  marked;  Colorless,  brown,  or  red; 
Relief  very  strong;  Not  pleochroic;  Interference 
colors  brilliant  high  order,  but  not  recognizable  in 
deeply  colored  sections;  Optically  positive  (+) ; 
p.  349. 

Biotite  may  at  times  appear  uniaxial,  see  p.  615. 
B.   Biaxial 

1.   Index  of  refraction  less  than  that  of  Canada  balsam. 

a.   Double  refraction  that  of  quartz  or  less.     Not  pleochroic. 

Zeolites;  n  =  1.48  -  1.53  ;  Double  refraction  .003-.012. 

In  acicular  or  fibrous  aggregates  filling  cavities  or  veins. 


THE  PRINCIPAL  ROCK-FORMING  MINERALS        613 

Color,  white ;   Interference  colors  first  order  gray  to 
yellow;  p.  478. 

Orthoclase;   Monoclinic ;   n  =  1.52;  y  —  a  =  .006. 

Plates  and  lath-shaped  crystalline  outlines,  grains,  and 
irregular  masses ;  Cleavages,  001  and  010  well  de- 
veloped; Relief  low,  surface  smooth;  Colorless, 
transparent  or  cloudy  through  decomposition ;  Carls- 
bad twinning  common;  Interference  colors  low  grays 
of  the  first  order;  Optically  negative  (  — );  p.  403. 

Microcline;   Triclinic ;   n  =  1.53;  *y  —  a  =  .007. 

Like  orthoclase  but  twinning  very  common,  showing 
the  characteristic  gridiron  structure ;  Optically  nega- 
tive (-);  p.  409. 

Leucite;  Large  crystals  will  appear  doubly  refracting, 
see  p.  610.  Colorless  amphiboles  and  andalusite 
may  not  show  pleochroism.  See  p.  416. 

Kaolinite;  Monoclinic;  n  =  1.54;  "y  —  a  =  .008. 

White  scales,  leafy  or  fibrous  aggregates;  Relief  very 
low ;  Interference  colors  gray  first  order ;  a  secondary 
or  decomposition  product;  Optically  (  — );  p.  501. 

lolite;  Orthorhombic ;  n  =  1.54;  <y  -  a  =  .009. 

Short  prisms,  grains,  and  irregular  masses;  Cleavage 
not  developed ;  Colorless ;  Relief  very  flat,  much  like 
quartz;  Pleochroism  not  marked  in  thin  sections, 
except  in  the  surrounding  halos ;  Interference  colors 
first  order  grays  to  yellow;  Optically  negative  (— ); 
p.  440. 

Enstatite  ;  Orthorhombic;  n=1.66;  -y— a  =.009. 

Crystalline  outlines,  prismatic,  fibrous,  or  in  rounded 
grains;  Inclusions  often  symmetrically  arranged; 
Cleavage  prismatic  at  92°;  Interference  colors  high 
first  order  about  the  same  as  quartz ;  Optically  (+)  ; 
p.  421. 
6.  Double  refraction  greater  than  that  of  quartz. 

Zeolites;  some  of  the  zeolites  possess  a  double  refrac- 
tion as  high  as  .012;  p.  612. 

Gypsum;  Monoclinic;  n  =  1.524;  *y  —  a  =  .010. 

Granular,  tablets,  and  fibrous  aggregates ;  Cleavage  well 
developed  with  abundant  cracks  and  often  twinned ; 
Colorless;  Interference  colors  first  order  yellow  to 
orange;  Optically  positive  (+);  p.  536. 


614  MINERALOGY 

Albite;   triclinic ;   n  =  1.533;   <y  -  a  =  .010. 

Plates,  grains,  lath-shaped  or  irregular;  Twinning 
lamellae  very  common ;  Extinction  inclined ;  Cleav- 
age, 001  and  010,  usually  well  developed;  Relief 
very  flat ;  Interference  colors  first  order  gray  to  yel- 
low; Optically  positive  (+) ;  p.  411. 

Serpentine;    Orthorhombic ;    n  =  1.54;   *y  —  a  =  .013. 

Scales  or  fibrous  aggregates ;  A  secondary  product ; 
Surface  smooth ;  Colorless  to  green ;  Interference 
colors  first  order  gray  to  orange ;  Optically  positive 
(+);  P.  498. 

2.   Index  of  refraction  greater  than  that  of  Canada  balsam. 
a.   Not  pleochroic. 

Plagioclases.    Triclinic;  n=  1.53-1. 58  ;  Y~a  =  .010-.013. 

Plates,  grains,  lath-shaped  or  irregular  in  outline; 
Polysynthetic  twinning  lamellae  very  common; 
Relief  flat,  surface  smooth;  Colorless,  sometimes 
clouded ;  Interference  colors  first  order  grays  to 
orange;  Optically  (±).  For  extinction  and  dis- 
tinction of  the  species  see  p.  411. 

Talc;  n  =  1.572;  «y  -  a  =  .050. 

Scaly  or  fibrous  aggregates ;  A  secondary  product ; 
Relief  marked,  surface  slightly  rough;  Colorless; 
Interference  colors  bright  second  or  third  orders; 
Optically  negative  (  — );  p.  500. 

Muscovite;  Monoclinic ;  n  =  1.587;  V  —  a  =  .038. 

Scales,  flakes,  or  shreds;  Cleavage  .001  characteristic, 
and  well  developed ;  Tabular  sections  show  no  cleavage 
and  have  a  very  low  relief,  while  the  elongated  sections 
showing  cleavage  have  a  marked  relief  and  nearly  par- 
allel extinction ;  Interference  colors  bright  second  order ; 
Axial  angle  large;  Optically  negative  (— );  p.  489. 

Sillimanite;  Orthorhombic;  n  =  1.667;    -y  -  a  =  .022. 

Fibrous  or  needle-like  aggregates,  often  with  a  trans- 
verse parting;  Colorless;  Relief  marked,  surface 
rough  ;  Parallel  extinction  ;  Interference  color  bright 
upper  first  or  lower  second  orders ;  Optically  posi- 
tive (+);  p.  461. 

Olivine;  Orthorhombic;  n  =  1.67;  -y  -  a  =  .035. 

Short  prismatic,  grains  or  irregular  in  outline;  Cleav- 
age 001  and  010  usually  well  developed;  Irregular 


THE  PRINCIPAL  ROCK-FORMING  MINERALS        615 

fractures  common ;  Relief  marked  and  surfaces  rough ; 
Colorless  to  green  or  brown ;  Interference  colors 
bright  second  or  third  orders;  Parallel  extinction; 
Optically  positive  (+);  p.  446. 

Pyroxenes;  Monoclinic ;  n  =  1.68-1.72;  "y  —  a  = 
.022-.029. 

Prismatic  crystals,  grains  or  irregular  outlines;  Cleav- 
age prismatic,  87° ;  Color,  shades  of  green,  yellow,  or 
brown ;  Relief  strongly  marked ;  Interference  colors 
bright  second  order ;  Extinction  from  36°-54° ;  Op- 
tically positive  (+) ;  p.  419. 

Cyanite  ;   Triclinic  ;  n  =  1  723  ;  \  -  a  =  .012. 

In  elongated  crystals,  columnar  or  tablets;  Cleavage 
distinct,  74° ;  Colorless  to  bluish,  when  dark  in  color 
pleochroic ;  Interference  colors,  gray  to  yellow  or  red 
of  the  first  order;  Axial  angle  large;  Optically  nega- 
tive (-).  p.  461. 
b.  Pleochroic. 

Chlorite;  Monoclinic;  n  =  1.576;  -y  -  a  =  .001- 
.013. 

Sheets  or  scaly  aggregates ;  Cleavage  basal  micaceous ; 
Shades  of  green ;  Relief  not  marked ;  Pleochroism 
marked  ;  Interference  colors  gray  or  yellow  of  the  first 
order,  anomalously  indigo  blue  ;  Optically  (± ) ;  p.  497. 

Biotite;  Monoclinic;  n  =  1.58;  Y  —  a  =  .085. 

Flakes,  plates,  or  shreds;  Micaceous  cleavage  well 
developed ;  Tabular  sections  show  no  cleavage  and 
have  a  very  low  relief ;  Extinction  nearly  parallel ; 
Pleochroism  marked  ;  Interference  colors  bright  second 
order ;  2  V  usually  nearly  zero ;  Optically  ( —  ) ;  p.  492. 

Amphiboles;  Monoclinic;  n  =  1.62-1.64;  "y  —  a  = 
.016-.072. 

In  crystalline  outlines,  irregular  masses,  or  rounded 
grains ;  Cleavage  prismatic,  124°  30',  well  developed  ; 
Relief  marked;  Colorless,  green  or  brown;  Pleo- 
chroism very  marked  in  dark  colored  sections, 
much  less  in  colorless  sections,  absorption  being  the 
greatest  parallel  to  the  cleavage  cracks ;  Interference 
colors  bright  second  order  to  high  orders  in  basaltic 
varieties;  Extinction  9°-20° ;  Optically  negative 
(-);  p.  431. 


616  MINERALOGY 

Andalusite;  Orthorhombic ;  n  =  1.64;  -y  —  a  =  .011. 
Prismatic    or    square    crystalline    outlines,    or    grains; 
Cleavage  prismatic,  at  times  well  developed ;    Color- 
less, reddish,  or  spotted  ;  Relief  marked ;  Pleochroism 
strong  in  colored  specimens ;   Interference  colors  first 
order  grays  to  yellow ;    Extinction  parallel ;     Axial 
angle  large;  Optically  negative  (  —  );  p.  459. 
Hypersthene;      Orthorhombic;      n  =  1.67;      y  —  a  — 

.013. 

Prismatic,  fibrous  or  in  grains ;  Cleavage  prismatic,  92°, 
also  parting  (010) ;    Color,  shades  of  brown ;    Relief 
marked ;  Inclusions  of  plates  or  rods  parallel  arranged, 
characteristic ;   Pleochroism  increases  with  the  depth 
of  color;   Interference  colors  gray,  yellow,  to  orange 
of  the  first  order;    Optically  negative  (  — );   p.  421. 
Staurolite;  Orthorhombic;  n  =  1.741 ;  -y  —  a  =  .010. 
Short  prismatic  crystalline  outlines ;  Twins  common ; 
Color,  shades  of  yellow  or  brown  ;  Cleavage  prismatic 
and  010  distinct ;  Relief  marked ;  Pleochroism  chang- 
ing from  reddish  to  yellow;    Large  crystals  contain 
many  inclusions;   Interference  colors  first  order  yel- 
low to  orange ;  Parallel  extinction ;  Axial  angle  large  ; 
Optically  positive  (+) ;  p.  477. 
Epidote ;  Monoclinic  ;  n  =  1.751  ;*\  —  a  =  .032. 
Crystals  elongated  parallel  to  the  orthoaxis,  or  granular  ; 
Cleavage  basal,  well  developed  ;  Color,  shades  of  green, 
brown  to  colorless ;    Relief  well  marked ;    Extinction 
parallel  in  elongated  sections,  otherwise  the  angle  is 
small ;  Pleochroism  increases  with  the  depth  of  color 
of  the  specimen ;    Interference   colors  third  order ; 
Optically  negative  (— ) ;  p.  466. 
Titanite;  Monoclinic;  n  =  1.938;  -y  —  a  =  .145. 
In    wedge-shaped    crystalline    outlines    or     granular ; 
Cleavage  prismatic,  distinct,  seldom  parallel  to   the 
crystalline    outline ;    Brown,    yellow    to    colorless ; 
Relief  very  marked  ;   Interference  from  low  orders  to 
very   high ;     Extinction    angles    not    characteristic  ; 
Pleochroism  increases  with  the  depth  of  color  of  the 
specimen;  Optically  positive  (+);  p.  503. 


CHAPTER  IV 

DETERMINATIVE   TABLE 

PART  I 

MINERALS  WITH  METALLIC,  OR  SUB-METALLIC  LUSTER 

ALL  minerals  included  in  this  section  yield  a  streak  dark  in  color, 
and  when  finely  ground  the  color  of  the  powder  is  also  dark.  If 
coarse  pieces  with  thin  edges  are  held  between  the  eye  and  the  yel- 
low flame  of  the  burner,  they  will  appear  perfectly  dark,  and  the 
thin  edge  will  transmit  no  light.  In  some  species  one  specimen  may 
be  of  sub-metallic  luster,  while  another  specimen  may  be  of  a  non- 
metallic  luster ;  such  minerals  will  be  found  in  both  Part  I  and 
Part  II. 

A  fragment  of  the  mineral  is  heated  on  coal  in  the  0.  F.  and  R.  F. 
alternately  ;  it  yields  a  coat. 

I.  The  coat  is  white,  and  while  being  heated  the  assay  yields  a 
garlic-like  odor  (arsenic).     The  presence  of  arsenic  may  be  proven 
by  the  tests  on  p.  585.    Minerals  on  p.  620. 

II.  The  coat  is  white  and  when  heated  in  the  inner  blue  cone  of 
the  0.  F.  colors  it  yellowish  green  (antimony).     The  presence  of 
antimony  may  be  proven  by  the  tests  on  p.  584.     Minerals  on 
p.  623. 

III.  The  coat  is  white  and  colors  the  flame  bright  green  (tel- 
lurium).    The  presence  of  tellurium  may  be  proven  by  the  tests 
on  p.  587.     Minerals  on  p.  625. 

ABBREVIATIONS  USED  IN  THE  TABLE 

I.    Crystallization  isometric.  F.     Fusibility,  fuses. 

II.    Crystallization  tetragonal.  G.     Specific  gravity. 

III.  Crystallization  hexagonal.  H.     Hardness. 

IV.  Crystallization  orthorhombic.  O.  F.     Oxidizing  flame. 
V.    Crystallization  monoclinic.  p.     Page. 

VI.  Crystallization  triclinic.  R.  F.     Reducing  flame. 

Amorph.  Amorphous.  Soda.     Sodium  carbonate. 

B.  B.     Before  the  blowpipe.  S.  Ph.     Salt  of  phosphorus. 

C.  Color.  Str.     Streak. 
Cl.    Cleavage. 

617 


618  MINERALOGY 

IV.  The  coat  is  white,  with  a  metallic-like  luster,  and  yields  a 
selenium  odor,  also  colors  the  flame  blue.     Test  p.  587,  p.  626. 

V.  The  coat  is  yellow  when  cold,  at  least  near  the  assay,  and 
with  von  Kobell's  flux  shows  lead  or  bismuth.     Test  p.  579,  p.  627. 

VI.  It  yields  sulphur  dioxide  when  the  powder  is  heated  in  the 
O.  F.  on  coal,  or  it  yields  a  strong  sulphur  reaction  on  silver  after 
fusion  with  soda ;  p.  629. 

VII.  Reduced  with  soda  and  borax  and  a  little  powdered  coal, 
it  yields  a  malleable  metal ;  p.  631. 

VIII.  The  powdered    mineral  when    dissolved   in    the   borax 
bead  shows  manganese.     Test  p.  574.     The  mineral  is  infusible; 
p.  632. 

IX.  The  mineral  when  powdered  and  heated  in  the  R.  F.  on  coal 
becomes  magnetic;  p.  632. 

X.  Minerals  not  included  in  the  preceding  groups ;  p.  634. 


PART  II 

NON-METALLIC  MINERALS  OR  MINERALS  WITHOUT  METALLIC  OR 
SUB-METALLIC  LUSTER 

Minerals  included  here  yield  a  streak  and  powder  light  in  color, 
and  will  usually  transmit  light,  at  least  on  the  thin  edges. 

I.  The  mineral  has  a  decided  taste,  soluble,  or  soluble  to  a  large 
extent  in  water.     Hardness   below  3.     No   minerals   containing 
copper  or  arsenic  will  be  found  in  this  section ;  p.  635. 

II.  Easily  and  quickly  volatile  (if  pure)  when  heated  on  coal. 
If  the  mineral  decrepitates,  it  should  be  heated  in  the  closed  tube, 
when  it  volatilizes  and  yields  a  sublimate ;  p.  640. 

III.  Roasted,  and  then  reduced  with  soda,  borax,  and  a  little 
coal  dust  in  R.  F.  on  coal,  yields  a  malleable  button. 

A.  The  button  is  copper  or  contains  copper ;  p.  641. 

B.  The  button  is  silver ;  p.  645. 

C.  The  button  is  tin ;  p.  646. 

D.  The  button  is  lead,  or  the    mineral  fused  with  von 

Kobell's  flux  shows  lead ;  p.  646. 

IV.  Fused  with  soda  and  borax  in  the  R.  F.  on  coal,  it  yields 
an  oxide  coat.     The  arsenic  coat  is  included  in  Section  V ;  p.  649. 

V.  In  R.  F.  on  coal  yields  an  arsenical  odor,  or  the  powdered 
mineral  heated  with  a  few  fragments  of  coal  in  a  closed  tube  yields 
an  arsenic  mirror.     Test  p.  585,  p.  651. 


DETERMINATIVE  TABLE  619 

VI.  The  powdered  mineral  is  dissolved  in  nitric   acid,   or  if 
insoluble  it  is  fused  with  soda,  then  dissolved ;   the  solution  treated 
with  ammonium  molybdate  yields  a  yellow  precipitate  (phosphoric 
acid);  p.  653. 

VII.  The  powdered  mineral  heated  on  coal  in  the  R.  F.  becomes 
magnetic;  p.  657. 

VIII.  After  intense  ignition  in  the  forceps,  it  yields  an  alkaline 
reaction  with  turmeric  paper.     Hardness  below  5 ;  p.  660. 

IX.  Fused  with  Turner's  flux  it  yields  a  green  flame  (boric  acid) ; 
p.  663. 

X.  The  powdered  mineral  when  dissolved  in  the  borax  bead  in 
the  O.  F.  is  violet-red  when  cold  (manganese) ;  p.  665. 

XI.  The  mineral  is  well  powdered  and  fused  with  borax,  dis- 
solved in  concentrated  HC1,  then  boiled  with  tin ;   the  solution  is 
violet  in  color  (titanium) ;  p  ;  667. 

XII.  Treated  as  in  XI,  but  the  borax  bead  is  powdered,  dissolved 
in  dilute  HC1,  and  zinc  added ;  the  solution  becomes  blue  (tung- 
sten) ;   p.  668. 

XIII.  The  mineral  in  powder  is  dissolved  in  S.  Ph. 

A.  The  bead  is  yellow  in  R.  F.  when  cold ;  p.  669. 

B.  The  bead  is  blue  in  both  flames ;  p.  669. 

C.  The  bead  is  green  in  R.  F.  when  cold ;  p.  669. 

XIV.  Minerals  not  included  in  the  preceding  groups.     They  are 
classified   according  to  their  fusibility,  their  solubility  in  acids, 
and  their  hardness. 

A.   Fusibility  below  5. 

1.  Hardness  below  5. 
+.   Yields  water. 

a.  Soluble  in  HC1 ;  p.  670. 

b.  Gelatinizes  in  HC1;  p.  671. 

c.  Not  attacked  by  HC1;  p.  671. 
- .    Yields  little  or  no  water ;  p.  672. 

2.  Hardness  above  5. 
-f.   Yields  water. 

a.  Soluble  in  HC1;  p.  672. 

b.  Gelatinizes  in  HC1 ;  p.  673. 

c.  Insoluble  in  HC1;  p.  674. 
— .  Yields  little  or  no  water. 

a.   Soluble  in  HC1;  p.  674. 


620  MINERALOGY 

b.  Gelatinizes  in  HC1 ;  p.  674. 

c.  Insoluble  in  HC1 ;  p.  675. 
B.   Fusibility  above  5. 

1.  Hardness  below  5. 
+  .   Yields  water. 

a.   Soluble  in  HC1;  p.  677. 
6.   Gelatinizes  in  HC1 ;  p.  678. 
c.  Insoluble  in  HC1 ;  p.  678. 
— .   Yields  little  or  no  water ;  p.  679. 

2.  Hardness  above  5. 
+.    Yields  water. 

a.  Soluble  in  HC1;  p.  679. 

b.  Gelatinizes  in  HC1 ;  p.  679. 

c.  Insoluble  in  HC1 ;  p.  680. 
— .   Yields  little  or  no  water. 

a.  Soluble  in  HC1;  p.  680. 

b.  Gelatinizes  in  HC1;  p.  680. 

c.  Insoluble  in  HC1;  p.  681. 


PART  I 

MINERALS  WITH  METALLIC  OR  SUB-METALLIC  LUSTER 

I.  The  coat  is  white,  and  while  being  heated  the  assay  yields  an 
arsenic  odor.  The  presence  of  arsenic  may  be  proven 
by  the  tests  on  p.  585. 

A.  When  heated  on  coal,  easily  volatile  without  fusion. 

NATIVE  ARSENIC,  As;  C.  Tin-white;   Str.  Gray;  Cl. 

Basal;  H.  3.5;  G.  5.7;  III. 
Fuses  easily  and  volatilizes  entirely. 
Allemontite,  As  and  Sb;  C.Tin-white;   Str.  Gray;  Cl. 

Basal;  H.  3.5;  G.  6.2;  III. 

B.  After  heating  on  coal  a  non-volatile  residue  remains. 

1.  Roasted  and  reduced  with  soda,  yields  silver ;  p.  578. 

PEARCEITE,  Ag9AsS6;  C.  Black;  Str.  Black;  H.  3;  G. 
6.15;  F.  1;  V. 

2.  Roasted  on  coal,  then  moistened  with  HC1  and  heated  in 

the  blue  cone,  yields  an  azure-blue  flame  (copper). 


ARSENIC   MINERALS  621 

a.  Fused  with  soda,  yields  a  sulphur  reaction  on  silver. 
+  .   Residue  in  R.  F.  is  magnetic. 

TENNANTITE,   (Cu.Fe)8As2S7 ;  C.   Blackish  gray ;  Str. 

Black  to  dark  red;  H.  3;  G.  4.6;  F.  1.5;  p.  324. 
Epigenite,    4  Cu2S,  3  FeS,  AsaSs ;     C.    Steel-gray ;     Str. 

Black;  H.  3.5;  IV. 
— .   Residue  in  R.  F.  is  not  magnetic. 

a.   With  von  Kobell's  flux  shows  lead. 

Lengenbachite,    Pb(Ag.Cu)AsS ;     C.     Steel-gray;   Str. 

Brownish,  marks  paper ;  G.  5.85. 
j8.   Shows  no  lead. 
Binnite,    Cu6As4S9;  3  Cu2S.As2S3;    C.  Iron-black;    Str. 

Black;  H.  2.5-3;  G.  4.5;  I. 
Lautite,    CuAsS;    C.   Iron-black;    Str.    Black;     H.    3; 

G.  4.9. 

b.  Yields  no  sulphur  reaction. 

Domeykite,  Cu3As;  C.  Steel-gray;  Str.  Gray;  H. 3-3.5; 

G.7.5;  G.  2;  Massive. 
Algodonite,  Cu6As;  C.  Steel-gray;  Str.  Gray;  H.  4;  G. 

7.6;  F.  2;  Massive. 
Whitneyite,  Cu9As;   C.  and   Str.  Silver-white;   H.  3.5; 

G.  8.5;  F.  2;  Massive. 

3.  Roasted  and  then  oxidized  beside  borax  on  coal,  as  directed 

in  §  a,  p.  574,  it  yields  a  strong  cobalt  reaction. 
a.  Fused  with  soda,  yields  a  sulphur  reaction. 

COBALTITE,   CoAsS;    C.   Tin- white;    Str.  Black;    H. 

5.5;  G.  6-6.2;  F.  2-3;  p.  316. 
GLAUCODOT,  (Co.Fe)  AsS;  C.  Gray;    Str.  Black;  H. 

5;  G.  5.95;  F.  2-3. 
6.    Yields  no  sulphur  reaction. 

SMALTITE,  CoAs2;    C.  Tin-white;    Str.  Black;    H. 

5.5-6;  G.  6.3;  F.  2.5;  p.  315. 
Safflorite,  CoAs2 ;  C.  Tin-white ;  Str.  Black ;  H.  4.5-5  ; 

G.  7;  F.  2.5;  IV;  p.  315. 
Skutterudite,  CoAs3 ;  C.  Tin-white ;  Str.  Black ;  H.  6 ; 

G.  6.75;  F.  2.5;  I;  p.  315. 
The  nickel  minerals  and  arsenopyrite  below  may  at 

times  contain  considerable  cobalt. 

4.  Roasted  and  treated  beside  borax  as  in  3,  it  yields  a  strong 

nickel  reaction,  with  possibly  the  first  three   beads 
colored  with  cobalt. 


622  MINERALOGY 

a.   Yields  a  sulphur  reaction  with  soda.     Shows  antimony ; 

p.  584. 
Corynite,    Ni(As.Sb)S;     C.    Tin-white;     Str.    Black; 

H.  4,5;  G.  6;  F.  2 ;  I. 
Wolfachite,   Ni(As.Sb)S;    C.   Steel-gray;    Str.   Black; 

H.4.5;  G.  6.6;  F.  2;  IV. 
.  b.   Yields  a  sulphur,  but  no  antimony,  reaction. 

GERSDORFFITE,  NiAsS;    C.Tin-white;   Str.  Black; 

H.  5.5;  G.  5.8-6;  F.  2;  I;  p.  316. 
c.    Yields  little  or  no  sulphur  reaction  with  soda. 

NICCOLITE,  NiAs;   Pale  copper  red;    Str.  Brownish; 

H.  5-5.5;  G.7.5;  F.  2;  III;  p.  309. 
Chloanthite,  NiAs2 ;   Tin-white ;    Str.  Black ;    H.  5.5-6 ; 

G.  6.9-7.2 ;  F.  2  ;  I ;  p.  315. 
Rammelsbergite,  NiAs2j   the  same  as  chloanthite  only 

orthorhombic. 

5.  Roasted  and  treated  as  in  3  above,  yields  an  iron  reaction. 

a.  Yields  with  soda  a  sulphur  reaction. 
ARSENOPYRITE,  FeAsS ;    Silver-white;    Str.  Black; 

H.  5.5-6;  G.  6-6.2;  F.  2;  IV;  p.  319. 

b.  Yields  with  soda  no  sulphur  reaction. 

Lollingite,  FeAs2;   Silver-white;   Str.  Black;   H.  5-5.5; 

G.  7.2-7.3;  F.  2;  IV. 
Leucopyrite,  Fe3As4 ;   Practically  the  same  as  lollingite, 

massive. 

6.  With  von  Kobell's  flux  shows  lead. 

Sartorite,   PbS.As2S3 ;    Lead-gray;    Str.   Dark  brown; 

H.  3;  G.  5.4;  F.  1;  IV. 
Dufrenoysite,  2  PbS. As2S3 ;  Blackish  gray ;  Str.  Black  ; 

H.  3;  G.  5.56;  F.I;  IV. 
Guitermanite,  3  PbS.As2S3 ;    Bluish   gray;    Str.  Black; 

H.  3  ;  G.  5.9  ;  F.  1 ;  V. 
Jordanite,  4  PbS.As2S3 ;    Blackish   gray;    Str.   Black; 

H.  3  ;  G.  6.4 ;  F.  1 ;  V. 
Baumhauerite,       4  PbS.3  As2S3 ;        Lead-gray ;        Str. 

Brown;  H.  3;  G.  5.33;  Cl.  Per. ;  V. 

7.  With  von  Kobell's  flux  shows  bismuth. 

Alloclasite,  Co(As.Bi)S;    Steel-gray;    Str.  Black;    H. 

4.5;  G.  6.6;  F.  2;  IV. 
Bismutosmaltite,  Co(As.Bi)3;    Tin-white;    Str.  Black; 

H.  6;  G.  6.92;  F.  2?;  I. 


ANTIMONY  MINERALS  623 

8.    Yields  tests  for  platinum ;  p.  582. 

Sperrylite,   PtAs2 ;    Tin-white;    Str.   Black;    H.   6-7; 

G.  10.6 ;  F.  2 ;  I ;  p.  287. 

II.  The  coat  is  white,  and  colors  the  inner  flame  pale  yellowish 
green  (antimony).  The  presence  of  antimony  may 
be  proven  by  the  tests  ;  p.  584. 

A.  Easily  and  completely  volatile  (when  pure)  on  coal. 

1.  Yields  with  soda  a  strong  sulphur  reaction. 

STIBNITE,  Sb ;  Tin-white ;  Str.  Gray ;  H.  2 ;  G.  4.57  ; 
F.  1;  IV;  p.  295. 

2.  With  soda  in  the  closed  tube  yields  mercury,  p.  579. 

Livingstonite,     HgS.2  Sb2S3 ;     Lead-gray ;     Str.     Gray- 
black  ;  H.  2 ;  G.  4.8  ;  F.  I ;  IV. 

3.  Yields  with  soda  no  sulphur  reaction. 

ANTIMONY,  Sb;   Tin-white ;  Str.  Gray;  H.  3-3.5;  G. 
6.69;  F.  1;  Cl.  Basal;  III. 

B.  Not  entirely  volatile  on  coal. 

1.   With  soda  yields  a  sulphur  reaction. 

a.  Roasted  and  moistened  with  HC1  yields  a  copper  chlo- 

ride flame;  p.  581. 

+.  With  von  KobelPs  flux  shows  lead. 
BOURNONITE,    2  PbS.Cu2S.Sb2S3 ;     Steel-gray;     Str. 

Black;  H.  2.5-3 ;  G.  5.8 ;  F.I;  IV;  p.  321. 
— .  With  von  KobelPs  flux  shows  neither  lead  nor  bis- 
muth. 
Stylotypite,  3  (Cu2.Ag2.Fe)S,  Sb2S3 ;    Iron-black;    H.  3; 

G.  4.8;  F.  1.5;  IV. 
Famatinite,  3  Cu2S.Sb2S3 ;    Gray;    Str.  Black;    H.  3.5; 

G.  4.57;  F.  1-1.5;  IV. 
Chalcostilbite,  Cu2S.Sb2S3 ;  Blackish  gray ;   Str.  Black  ; 

H.  3.5;  C.  basal;  G.  4.9;  F.  1.5;  IV. 
Some  tetrahedrites,  p.  324,  and  polybasites  may  fall  in 

this  group. 

b.  Do  not  yield  with  HC1  a  copper  chloride  flame. 
-f.  With  von  KobelPs  flux  shows  lead. 

a.  Reduced  with  soda  and  cupeled  shows  silver;    §  a, 

p.  578. 
Andorite,  2  PbS.Ag2S.3  Sb2S3 ;    Steel-gray ;   Str.  Black ; 

H.  3-3.3;  G.  5.33;  F.  1;  IV. 
Brongniardite,  PbS.Ag2S.Sb2S3  ;     Black  ;     Str.    Black  ; 

H.  3-3.5  ;  G.  5.95  ;  F.  1 ;  Massive. 


624  MINERALOGY 

Diaphorite,  5  (Pb.Ag2)S.Sb2S3 ;   Steel-gray;  Str.  Black  ; 

H.  2.5-3;  G.  5.9-6;  F.  1;  IV. 
Freieslebenite,   5  (Pb.Ag2)S.2  Sb2S3 ;     Steel-gray ;     Str. 

Black;  H.  2-2.5;  G.  6.3;  F.I;  V. 
P.  Shows  lead,  but  no  silver. 
Zinkenite,  PbS.Sb2S3;    Steel-gray;    Str.  Black;    H.  3- 

3.5;  G.  5.35;  F.I;  IV;  p.  320. 
Plagionite,  5  PbS.4  Sb2S3 ;    Blackish-gray;    Str.  Black; 

H.  2.5;  G.  5.4;  F.I;  V. 
Warrenite,  3  PbS.2  Sb2S3 ;    Blackish-gray;    Str.  Black; 

F.  1 ;      Capillary. 
JAMESONITE,    2  PbS.Sb2S3 ;      Blackish    gray;     Str. 

Black;  H.  2-3;  G.  5.5-6;  F.I;  IV;  p.  320. 
Semseyite,  7  PbS.3  Sb2S3 ;   Gray;  Str.  Black;   G.  5.95; 

F.  1 ;  V,  Tabular. 
BOULANGERITE,    3  PbS.Sb2S3 ;      Bluish     lead-gray; 

Str.  Black;  H.  2.5-3;  G.  5.88;  F.I;  p.  321. 
Meneghinite,  4  PbS.Sb2S3;  Blackish  gray ;  Str.  Black; 

H.  2.5;  C.  pinacoidal;  G.  6.35;  F.  1;  IV. 
Geocronite,     5  PbS.Sb2S3 ;      Lead-gray ;      Str.    Black ; 

H.  2.5 ;  G.  6.4  ;  F.  1 ;  IV. 
Kilbrickenite,   6PbS.Sb2S3?;    Lead-gray;    Str.  Black; 

H.  ?  ;  G.  6.4 ;  Massive. 
Epiboulangerite,    3  PbS.Sb2S3 ;      Blackish    gray ;     Str. 

Black;  H.  ?;  G.  6.31 ;  F.I;  IV? 
•y.   With  von  KobelFs  flux  shows  lead ;  roasted  in  the 

0.  F.,  and  the  infusible  residue  reduced  with  soda, 

yields  tin. 
Cylindrite,  6  PbS,  6  SnS2,  Sb2S3;    Blackish  gray;    Str. 

Black;  H.  2.5-3;  G.  5.42;  F.  1.5; 
Franckeite,  5 PbS,  2  SnS,,  Sb2S3;  Blackish  gray;   Str. 

Black ;  H.  2-2.5 ;  G.  5.55 ;  F.  1 ;  Tabular. 
— .  With  von  Kobell's  flux  shows  no  lead. 
a.  Reduced  with   soda  yields  silver.     Shows  sulphur 

when  fused  with  soda. 
PYRARGYRITE,  3  Ag2S,  Sb2S3 ;  Dark  red  to  black ;  Str. 

Purplish  red;  H.  2-2.5;  G.  5.8;  F.  1;  III;  p.  322. 
STEPHANITE,  Ag5SbS4;    Iron-black;  Str.  Black;  H. 

2-2.5;    G.  6.3;  F.I;  IV;  p.  325. 
POLYBASITE,   9  (Ag.Cu)2S,   Sb2S3;    Iron-black;    Str. 

Black ;  H.  2-3  ;  G.  6.2 ;  F.  1 ;  V ;  p.  326. 


TELLURIUM  MINERALS  625 

Miargyrite,   Ag2S,   Sb2S3;   Iron-black ;.  Str.   Red-black; 

H.  2-2.5 ;  G.  5.2 ;  F.  1 ;  V. 
Polyargyrite,    12  Ag2S,  Sb2S3;    Black;    Str.  Black;    H. 

2.5  ;  G.  6.97  ;  F.  1 ;  V. 
P.    With    von    Kobell's    flux     shows     bismuth,    §    b, 

p.  580. 
Kobellite,  2  PbS.(Bi.Sb)2S3;  Blackish  gray;  Str.  Black; 

H.  2.5-3;  G.  6.3;  F.I. 

2.  Yields  silver,  but  shows  no  sulphur. 

Dyscrasite,  Ag3Sb ;  Silver-gray ;    Str.  Gray ;    H.  3.5-4 ; 
G.  9.75;  F.  1.5; 'IV. 

3.  Yields  a  magnetic  residue  after  treatment  in  the  R.  F. ; 

yields  a  sulphur  reaction  with  soda,  but  contains  no 
copper,  lead,  or  silver. 

BERTHIERITE,    FeS.Sb2S3;    Steel-gray;    Str.  Black; 
H.  2-3  ;  G.  4.2 ;  F.  2 ;  p.  320. 

4.  Well  roasted  and  dissolved  in  borax,  yields  a  nickel  reac- 

tion; p.  574. 
a.   With  von  Kobell's  flux  shows  bismuth. 

Kallilite,   Ni(Sb.Bi)S;   Light  bluish  gray;   Str.  Black; 

G.  7.01 ;  Massive. 
Hauchecornite,     Ni(Bi.Sb)S;       Bronze-yellow;       Str. 

Black ;  H.  5 ;  G.  6.4 ;  II  or  Massive. 
b.   Yields  no  bismuth. 

+  .  Shows  sulphur  with  soda. 

Ullmannite,    NiSbS ;     Silver-gray;     Str.     Black;     Cl. 

Cubic;  H.  5.5;  G.  6.5 ;  F.  1.5;  I;  p.  311. 
— .  Shows  no  sulphur  with  soda. 
Breithauptite,    NiSb ;    Copper-red ;   H.   5.5 ;   G.   7.54 ; 

F.  1.5-2;  III;  p.  309. 
III.    The  coat  is  white,  and  colors  the  inner  blue  cone  of  the  flame 

bright   green.     The   presence   of  tellurium   may  be 

proven  by  the  tests  in  §  b,  p.  587. 

A .  With  von  KobeWs  flux  shows  lead. 

Altaite,  PbTe ;   Tin-white ;   Str.  Gray ;   Cl.  Cubic ;   H. 

3;   G.  8.16;  F.  1.5;  I. 
Nagyagite,   Au2Pbi4Sb3Te7Si7? ;    Gray;    Str.  Black;    H. 

1-1.5;  G.  7.02;  F.  1.5;  IV. 

B.  With  von  Kobell's  flux  shows  bismuth. 

Tetradymite,  Bi2Te3 ;  Tin-white  ;    Str.  Gray  ;  Cl.  Basal ; 

H.  1.5-2;  G.  7.4;  F.  1.5;  III. 

2s 


626  MINERALOGY 

Griinlingite,  Bi4S3Te;    Pale  steel-gray;  Str.  Gray;    G. 

7.8;  F.  1;  H.  1.5-2;  III. 
Tapalpite,     3  Ag2(S.Te).Bi2(S.Te)3 ;     Pale     Steel-gray; 

Str.  Gray ;  G.  7.8 ;  F.  1 ;  Massive. 
C.    With  von  KobeWs  flux  shows  neither  lead  nor  bismuth. 

1.  Easily  fusible  and  volatile. 

TELLURIUM,  Te ;    Tin-white;    Str.  Gray;    H.  2-2.5; 

G.  6.15;  F.  1;  Cl.  Prismatic;  III. 
a.   With  soda  in  the  closed  tube  yields  mercury. 

Coloradoite,  HgTe ;   Iron-black ;   Str.  Black ;   H.  3 ;   G. 

8.63  ;  F.  1 ;  Massive. 

2.  Not  entirely  volatile. 

a.   Roasted  and  reduced  with  soda  yields  gold  and  silver; 

§  a,  p.  578. 

— .  With  soda  in  the  closed  tube  yields  no  mercury. 
Petzite,  (Ag.Au)2Te ;  Iron-black ;  Str.  Gray ;  H.  2.5-3  ; 

G.  8.86;  F.  1.5;  Massive. 
SYLVANITE    (Au.Ag)Te2;    Silver-white;    Str.   Gray; 

H.  1.5-2;  G.  8.10;  F.I;  V. 
Goldschmidtite,  Au6AgTe6 ;    Silver-white;    Str.  grayish 

black;  H.  2;  G.  8.6;  F.I;  V. 
Krennerite,  AuTe2  +  Ag ;   Silver-white ;   Str.  Gray ;  H. 

2.5;  G.  8.35;  F.I;  IV;  Cl.  Basal. 
CALAVERITE,  AuTe2  +  Ag;  Silver-white;  Str.  Gray; 

H.  2.5;  G.  9.04;  F.  1;  VI. 
+  .  With  soda  in  a  closed  tube  yields  mercury. 
Kalgoorlite,  HgAu2Ag6Te6 ;  Iron-black;  G.  8.76;  Mas- 
sive. 
6.    Reduced  with  soda  yields  silver  only. 

HESSITE,  Ag2Te ;  Steel-gray ;    Str.  Gray ;    H.  2.5-3  ; 

G.  8.60 ;  F.  1 ;  I. 

c.    The  mineral  well  roasted  and  dissolved  in  borax. 
+.  The  bead  shows  copper. 
Richardite,  Cu4Te3. 
— .  The  bead  shows  nickel. 
Melonite,    Ni2Te3;     Reddish    white;     Str.  Gray;     Cl. 

Basal;  III. 
IV.   The  coat  is  white  with  a  metallic-like  luster,  also  yields  a 

selenium  odor  and  colors  the  flame  blue ;  §  6,  p.  587. 
A.   Completely  volatile  on  coal. 

1.   With  soda  in  the  closed  tube  yields  mercury. 


LEAD   AND  BISMUTH  MINERALS  627 

TIEMANNITE,  HgSe;   Blackish  gray ;   Str.  Black;   H. 

2.5;  G.  8.2;  I. 
Orofrite,  Hg(S.Se) ;  Blackish  gray ;  Str.  Black ;  H.  2.5 ; 

G.  8.0 ;  Massive. 

B.  With  von  KobeWs  flux  shows  lead. 

1.  Reduced  with  soda,  the  button  cupeled  shows  silver. 

Naumannite  (Ag2.Pb)Se;   Iron-black;    Str.  Black;    H. 
2.5;   G.  8.0;   F.  2;   Cl.  Cubic;   I;  Massive. 

2.  Yields  no  -silver. 

Clausthalite,  PbSe ;  Lead-gray ;  Str.  Black ;  H.  2.5-3  ; 
G.  8 ;   Cl.  Cubic ;   F.  2  ;   I. 

C.  With  von  KobeWs  flux  shows  bismuth. 

Guanajuatite,  Bi2Se3 ;  Bluish  gray ;  Str,  Black ;   H.  2.5- 
3.5;   G.  6.43;   F.  1.5;    IV. 

D.  With  von  KobeWs  flux  shows  neither  lead  nor  bismuth. 

1.  Roasted,    and    reduced    with    soda    shows    copper    and 

silver. 
Eucairite,     CuAgSe ;     Lead-gray;     H.    2.5;     G.    7.5; 

F.  2 ;  I ;  Massive. 

Crookesite,    (Cu.Te.Ag)2Se ;    Lead-gray;     Str.    Black; 
H.  2.5-3 ;   G.  6.9 ;   F.  1 ;   Massive. 

2.  Yields,  when  reduced,  copper  but  no  silver. 

Berzelianite,  Cu2Se;  Silver-white;  Str.  Shining;  H.  2?; 

G.  6.7;   F.  1.5;   Massive. 

Umangite,  Cu3Se2 ;    Cherry-red  ;  Str.  Black ;  H.  3 ;    G. 
5.62;    F.  1.5;   Massive. 

3.  Yields  silver  but  no  copper. 

Aguilarite,  Ag2Se.Ag2S ;     Iron-black;    Str.    Black;    H. 

2.5;  G.  7.6;  F.  1 ;  I. 
V.  The  coat  is  yellow,  at  least  near  the  assay.     With  von  KobelFs 

flux  shows  lead  or  bismuth. 
A.    With  von  KobeWs  flux  shows  only  lead. 

1.  Fused  with  soda  shows  sulphur  on  silver. 

GALENA,  PbS ;    Lead-gray ;    Str.    Grayish  black ;    H. 
2.5;   G.  7.6;   F.  2;   Cl.  Cubic;   I;  p.  298. 

2.  Fused  with  soda  in  O.  F.  shows  manganese ;  §  b,  p.  574. 

Kentrolite,    (Mn^C^PbaCSi.OJs ;   Black;   Str.    Brown; 

H.  5-5.5;    G.  6.19;   F.  2-2.5;   IV. 
Senaite,  (Fe.Pb) O.2  (Ti.Mn)O2;   Black;  Str.  Brownish; 

H.  6 ;  G.  4.78 ;  Inf. ;   III. 

3.  Roasted  in  the  R.  F.  becomes  magnetic. 


628  MINERALOGY 

Melanotekite,  Pb3Fe4Si3Oi5 ;  Brown  to  black ;  Yellowish 

brown;   H.  5-5.5;  G.  5.86;   F.  2.5;   IV. 
4.   Contains  lead  only. 

Lead,  Pb;  Lead-gray;  Str.  Lead-gray;  H.  1.5;  G. 
11.37;  F.  1;  I. 

Plattnerite,  PbO2 ;  Iron-black ;  Str.  Brown ;  H.  5-5.5 ; 
G.8.5;  F.  1.5;  II. 

B.  With  von  Kobell's  flux  shows  lead  and  bismuth. 

1.  With  HC1  a  strong  copper  chloride  flame. 

Aikinite,  3  (Pb,Cu2)S.Bi2S3 ;  Lead-gray;  Str.  Grayish 
black;  H.  2-2.5;  G.  6.7;  F.  1-1.5;  IV. 

2.  Reduced    with     soda    and    cupeled    yields    silver;    §    a, 

p.  578. 

Schirmerite,  3  (Ag2.Pb)S.2Bi2S3 ;  Lead-gray  ;  Str.  Gray- 
ish black ;  G.  6.75 ;  Massive. 

Schapbachite,  PbS.Ag2S.Bi2S3 ;  Lead-gray;  Str.  Gray- 
ish black;  H.  3.5;  G.  6.43. 

3.  When  reduced  yields  neither  copper  nor  silver. 

Galenobismuthite,  PbS.Bi2S3;  Lead-gray;   Str.  Grayish 

black;  H.  3-4;   G.  7.00;  Columnar. 
Chiviatite,    2  PbS.3  Bi2S3 ;     Lead-gray;     Str.    Grayish 

black  ;   G.  6.92 ;   Foliated. 
Rezbanyite,  4  PbS.5Bi2S3 ;     Lead-gray;     Str.    Grayish 

black ;   H.  2.5-3 ;   G.  6.24 ;   Massive. 
Cosalite,  2PbS.Bi2S3 ;    Lead-gray ;  Str.  Grayish  black  ; 

H.  2.5-3;  G.  6.58;   F.  1-1.5;   IV. 
Lillianite,      3  PbS.Bi2S3 ;      Steel-gray ;      Str.     Grayish 

black;   G.  6.1 ;   F.  1-1.5;   Massive. 
Beegerite,    6  PbS.Bi2S3 ;    Gray;    Str.    Grayish    black; 

Cl.  Cubic?;   G.  7.27;  I?. 

C.  With  von  KobeWs  flux  shows  bismuth,  but  no  lead. 

1.   Roasted  and  moistened  with  HC1  yields  a  strong  copper 

chloride  flame. 
Emplectite,    Cu2S.Bi2S3 ;    Grayish    white;    Str.  Black; 

H.  2;   G.  6.40;   F.  1;   IV. 
Wittichenite,      3  Cu2S.Bi2S3 ;       Grayish     white ;      Str. 

Black ;   H.  3.5 ;   G.  6.70 ;    F.  1 ;    IV. 
Klaprotholite,  3  Cu2S.2  Bi2S3 ;    Steel-gray ;    Str.  Black  ; 

H.  2.5;    G.  4.60;    F.  1;    IV. 
Cuprobismutite,    3  Cu2S.4  Bi2S3 ;      Bluish   black;     Str. 

Black;  G.  6.49;  F.  1;  Prismatic. 


SULPHIDE  MINERALS  629 

2.  Yields    no    copper,   but  reduced  with  soda  and   cupeled 

yields  silver ;  p.  578. 

Matildite,  Ag2S.Bi2S3 ;  Gray;  Str.  Gray;  H.  2-3?; 
G.  6.92 ;  Slender  prisms, 

3.  Yields  neither  copper  nor  silver. 

BISMUTH,  Bf;    C.  and  Str.  Silver-white;    H.  2-2.5; 

G.  9.8 ;   F.  1 ;   Cl.  Basal ;   III. 
BISMUTHINITE,  Bi2S3;    Lead-gray ;' H.  2 ;    G.  6.45; 

F.  1 ;   IV ;   p.  296. 

VI.  Yields  sulphur  dioxide  when  powdered  and  heated  in  the  O.  F. 
on  coal,  or  fused  with  soda  yields  a  strong  sulphur  re- 
action on  silver,  some  may  yield  slight  gray  coats. 

A.  In  the  closed  tube  with  soda  yields  mercury. 

CINNABAR,  HgS;    Vermilion;    Str.  Red;    H.  1-2.5; 

G.  8.10;   F.  1.5;   III;   p.  304. 

B.  Well  roasted  then  reduced  with  soda  and  borax  yields  copper 

buttons. 

1.  The  roasted  powder  is  magnetic. 

CHALCOPYRITE,  CuFeS2;  Brass-yellow;  Str.  Green- 
ish; H.  3.5;  G.  4.20;  F.  2 ;  II;  p.  310. 

BORNITE,  Cu3FeS3;  Purplish  bronze;  Str.  Grayish 
black;  H.  3;  G.  5-5.4 ;  F.  2 ;  I;  p.  310. 

Cubanite,  CuFe2S4 ;  Brass-yellow;  Str.  Black;  H.  4; 
G.  4.05;  F.  2;  Cl.  Cubic;  I. 

Stannite,  Cu2S.FeS.SnS2 ;  Steel-gray;  Str.  Black;  H. 
4;  G.  4.4;  F.  1.5;  p.  312. 

2.  The  roasted  mineral  is  not  magnetic. 

CHALCOCITE,    Cu2S ;     Steel-gray;     Str.    Black;     H. 

2.5-3 ;   G.  5.7 ;   F.  2-2.5 ;   IV ;   p.  300. 
COVELLITE,    CuS;    Blue;    Str.    Grayish   black;    H. 

1.5-2;   G.  4.6;  F.  2.5;  III;  p.  306. 

a.  The  nitric  acid  solution  shows  silver;  §  b,  p.  578. 
Stromeyerite,  CuAgS;    C.    and  Str.  Dark  steel-gray; 

H.  2.5-3;   G.  6.22;   F.  1.5;   IV. 

b.  Shows  vanadium,  §  b,  p.  577. 

Sulvanite,  3  CuS.V2S5 ;  Bronze-yellow ;  Massive. 

C.  The  nitric  acid  solution  shows  silver  or  reduced  with  soda  yields 

silver  buttons;  p.  578. 
1.   The  roasted  mineral  is  magnetic. 

Sternbergite,  AgFe2S3 ;  Bronze;  Str.  Black;  H.  1-1.5; 
G.  4.15;  Cl.  Basal;  F.  1.5;  IV. 


630  MINERALOGY 

2.  The  roasted  mineral  is  not  magnetic. 

ARGENTITE,  Ag2S ;    C.  and  Str.  Blackish  gray ;    H. 

2-2.5;   G.  7.2;   F.  1.5;   I;   p.  297. 
Acanthite,    Ag2S ;     Iron-black;     H.    2-2.5;     G.    7.25; 

F.  3.5 ;  IV. 

3.  When  roasted  on  coal  it  yields  a  yellow  co'at,  also  reacts 

for  germanium,  p.  586. 

Argyrodite,  Ag8GeS6;    Black;    Str.  Grayish  black;    H. 
2.5-3;  G.  6.27;  F.  1.5;  I. 

4.  When  reduced  with  soda  yields  tin  buttons,  also  reacts  for 

Germanium,  p.  586. 

Canfieldite,  Ag8(Sn.Ge)S6;  Black;  Str.  Grayish  black ; 
H.  2.5-3;   G.  6.27;    F.  1.5-2;    I. 

D.  With  soda  and  borax  in  R.  F.  on  coal  yields  a  zinc  coat;    §  b, 

p.  573. 

SPHALERITE,    ZnS ;     Brown   to   Black;     Str.    Light 
brown;   H.  3.5-4;   G.  4.05;   F.  5 ;   I;   p.  301. 

E.  Yields  a  green  flame  when  ignited  in  the  forceps. 

MOLYBDENITE,     MoS,     Lead-gray;      Str.     Grayish 
black;  H.  1-1.5;  G.  4.75;  Inf.;  Ill;  p.  296.  . 

F.  The  powdered  mineral  is  well  roasted,  and  dissolved  in  borax  on  wire. 

1.  The  borax  bead  shows  iron;  §  a,  p.  575. 
a.  Naturally  magnetic. 

PYRRHOTITE,  rFenSn  +  1;   Bronze;    Str.  Black;    H.  4; 

G.  4.65 ;  F.  2.5-3 ;  III ;  p.  308. 

Pentlandite,   (Fe.Ni)S;    Yellowish   brown;    Str.  Black; 

H.  3.5-4 ;   G.  5 ;   F.  2 ;   I ;   p.  308. 
Troilite,  FeS;    Bronze;    Str.  Black;    H.  4;    G.  4.75; 

F.  2.5;  Massive, 
6.   Naturally  not  magnetic. 

PYRITE,  FeS2;    Brass-yellow;    Str.  Brownish    black; 

H.  6-6.5 ;   G.  5.03 ;   F.  3 ;   I ;   p.  313. 
MARCASITE,  FeS2;   Pale  yellow;   Str.  Grayish  black ; 

H.  6-6.5;   G.  4.88;   F.  3;   IV;   p.  317. 

2.  The  borax  head  shows  nickel;  §  a,  p.  574. 

MILLERITE,  NiS ;   Brass-yellow ;    Str.  Greenish  black ; 

H.  4.5 ;   G.  4.8  ;   F.  2 ;   III ;   p.  307. 
Beyrichite,  Ni3S4;  Lead-gray;  Str.  Gray;  H.  3-3.5;  G. 

4.70;   F.  2. 
Polydymite,    Ni4S5 ;     Steel-gray ;     Str.    Grayish  black  ; 

H.  4.5;   G.  5.65;   F.  2;   I.   * 


SULPHIDE   MINERALS  631 

3.  The  borax  bead  shows  manganese;   §  a,  p.  574. 

ALABANDITE,   MnS ;    Iron-black;    Str.   Olive-green; 

H.  3.5-4;   G.  3.95;   F.  3 ;   I;  p.  304. 
Hauerite,  MnS2 ;  Brownish  black ;  Str.  Reddish  brown  ; 

H.  4 ;   G.  3.46 ;   F.  3  ;   I. 

4.  The  borax  bead  shows  cobalt;    §  a,  p.  574. 

Linnaeite,    (Co.Ni)3S4;     Pale   steel-gray;     Str.   Black; 

H.  5.5;   G.  4.9;   F.  2;   I. 

VII.    Reduced  with  soda  and  borax  on  coal,  it  yields  a  malleable 
metal.     Or  it  is  a  malleable  metal. 

A.  The  button  is  copper. 

COPPER,    Cu;     Copper-red;     Str.    Copper-red;     H. 

2.5-3;  G.  8.85;   F.  3 ;  I;  p.  288. 
CUPRITE,  Cu2O;    Grayish  red ;    Str.  Red;  H.  3.5-4; 

G.  6.00;   F.  3;   I;   p.  337. 
MELACONITE,  CuO;  Iron-black;  Str.  Grayish  black ; 

H.  3-4;   G.  6.02;  F.  3 ;  V;   p.  340. 
Paramelaconite,  CuO  ;   Purplish  black ;   H.  5 ;   G.  5.83  ; 

F.  3 ;  IV. 

Crednerite,   Cu3Mn409 ;    Iron-black ;    Str.   Black ;    H. 
4.5;   G.  5.00;   F.  5.5;   V. 

B.  The  button  is  silver  or  gold,  or  both. 

SILVER,    Ag;     Silver-white;     Str.    Silver-white;     H. 

2.5-3 ;    G.  10.5 ;    F.  2 ;   I ;   p.  290. 
GOLD,  Au;    C.  and  Str.  Gold-yellow;    H.  2.5-3;    G. 

19.3;   F.  2.5-3;   I;   p.  291. 
Electron,  Au.  Ag ;  C.  and  Str.  Yellowish  white ;  H.  2.5-3 ; 

G.  13-16 ;   F.  2.5 ;   I. 

With  soda  in  the  closed  tube  shows  mercury. 
Amalgam,  Hg  +  Ag ;    C.  -and  Str.    Silver-white ;     H. 
3-3.5;    G.  13.7-14;   I. 

C.  The  button  is  tin. 

CASSITERITE,    SnO2 ;    Brown  to  black ;    Str.  Brown  ; 
H.  7-8  ;   G.  6.95 ;   Inf. ;   II ;   p.  347. 

D.  Not  included  above. 

Iron,  Fe ;  Steel-gray ;  Str.  Steel-gray ;  H.  4-5  ;  G.  7.55 ;  I. 
Platinum,  Pt;    Whitish  steel-gray;    H.  4-4.5;    G.  14- 

17;   F.  Dif.;  I;   p.  287. 

Mercury,  Hg;  Tin- white ;  G.  13.6;  Liquid;  I;  p.  293. 
Zinc,  Zn ;    C.  and  Str.  Grayish  white ;    H.  2 ;    G.  7.0 ; 

F.  1.5;  III. 


632  MINERALOGY 

Tin,  Sn ;    Tin-white ;    Str.  Tin-white ;    H.  2 ;    G.  7.2 ; 

F.  1 ;  IV. 

Palladium,  Pd ;   Steel-gray;   Str.  Gray;    H.  4-4.5;    G. 

11.55;  F.  Dif.;  I. 
Iridosmine,  Ir,  with  Pt,  Os,  Rh  ;  Tin-white ;  Str.  Gray ; 

H.  6-7 ;   G.  19-21 ;   Inf. ;   III. 
Indium,  Ir,  with  Pt ;  Tin-white ;    Str.  Gray ;    H.  6-7 ; 

G.  22.7 ;  F.  Inf. ;  I. 

Awaruite,  FeNi2;    Iron-black;    Str.  Black;    H.  5.5-6; 

G.  8.3 ;  Massive. 
Josephinite,  Fe2Ni5;   Color,  Gray;   Str.  Steel-gray;   H. 

5 ;  G.  6.20 ;  F.  Inf. ;  Mass. 

VIII.  The  powdered  mineral  dissolved  in  borax  on  wire  yields  a 

manganese    reaction,    §  a,    p.    574.      The    minerals 
included  here  are  infusible. 

A.  In  the  closed  tube  yields  water. 

MANGANITE,    Mn2(OH)2O2;    Iron-black;    Str.  Dark 

brown;  H.  41 ;   G.  4.3;   IV;   p.  340. 
1.  With  soda  in  R.  F.  on  coal  yields  a  zinc  oxide  coat;   §  a, 

p.  573. 
Chalcophanite,  (MnZn)Mn2O5.H2O ;  Bluish  black;   Str. 

Dark  brown;   H.  2.5;   G.  4;   III. 
Pyrochroite  may  at  times  be  very  dark  and  yield  a  dark 

streak,  p.  362. 
Psilomelane  and  pyrolusite  below  may  contain  a  little 

water. 

B.  Yields  little  or  no  water. 

PYROLUSITE,   MnO2;    Iron-black;    Str.   Black;    H. 

2-2.5 ;  G.  4.75 ;  Pscudomorphs ;  p.  352. 
PSILOMELANE,  MnO2,  with  MnO;    Iron-black;    Str. 

Brownish ;   H.  5-6 ;   G.  4.3 ;   Massive ;   p.  368. 
BRAUNITE,  MnMn03;    Black;    Str.  Brownish  black ; 

H.  6-6.5 ;    G.  4.8 ;   II. 
HAUSMANNITE,   Mn3O4;    Black;    Str.    Brown;    H. 

5-5.5;   G.  4.8;   II. 
Polianite,  Mn02;    Steel-gray;    Str.  Black;    H.  6-6.5; 

G.  5.00;  II. 
Pyrophanite,  MnTiO3 ;    Deep  red ;    Str.  Ocher-yellow ; 

H.  5;   G.  4.54;   Cl.  Rhom. ;   III. 

IX.  The  powdered  mineral  heated  in  the  R.  F.  on  coal  becomes 

magnetic. 


MINERALS  WHICH  BECOME  MAGNETIC  633 

A.  Fusibility  below  4. 

1.  Gelatinizes  with  HC1. 

ALLANITE,  (Ca.Fe)2(Al.Ce.Fe)2(AlOH)(SiO)4;  Brown 
to  pitch-black ;  Str.  Gray ;  H.  5.5-6 ;  G.3.90;  F.  2.5; 
V;  p.  468. 

ILVAITE,  CaFe2(Fe2.OH)  (Si04)2 ;  Iron-black ;  Str. 
Black;  H.  5.5-6;  G.  4.05;  IV;  p.  472. 

2.  Do  not  gelatinize  with  HC1.     Shows  tungsten,  §  6,  p.  587. 

WOLFRAMITE,    (Fe.Mn)WO4;    Black;    Str.   Black; 

H.5.5;  G.  7.35;  F.  3-3.5;  V;  p.  542. 
Reinite,  FeW04 ;   Blackish  brown ;   Str.  Brown ;   H.  4 ; 

G.  6.64 ;  F.  3-3.5 ;  II. 

3.  Shows  titanium ;  §  b,  p.  570. 

Neptunite,  (Na.K)2(Fe.Mn)Ti(SiO3)4;  Black;'  Str. 
Brown;  H.  5-6 ;  G.  3.23  ;  F.  3-4;  V. 

B.  Fusibility  above  4. 

1.  In  the  closed  tube  yields  water. 

LIMONITE,  2(Fe203).3H20;  Dark  brown;  Str. 
Yellow-ocher ;  H.  5-5.5;  G.  3.80;  p.  363. 

GOTHITE,  Fe2O3.H2O;  Brown  to  black;  Str.  Yellow- 
ocher  ;  H.  5-5.5  ;  G.  4.35  ;  p.  363. 

TURGITE,  2Fe2O3.H2O;  Reddish-black;  Str.  Indian- 
red;  H.  5.5-6;  G.  4.14;  p.  363. 

2.  In  the  closed  tube  yields  no  water. 

a.  Fused  in  a  soda  bead  on  wire  in  the  0.  F.  with  a 

little  niter  shows  green  sodium  manganate;  §  b} 
p.  574. 

FRANKLINITE,  (Fe.Zn.Mn)0.(Fe.Mn)2O3;  Iron-black; 
Str.  Dark  brown ;  H.  6  ;  G.  5.15 ;  I ;  p.  375. 

Jacobsite,  (Mn.Mg)0(Fe.Mn)2O3;  Black;  Str.  Brown- 
ish ;  H.  6 ;  G.  4.75 ;  I. 

Bixbyite,  FeMn03;  Black;  Str.  Black;  H.  6-6.5; 
G.  4.91;  F.  4.5;  I. 

The  powdered  mineral  fused  with  soda  on  coal  yields 
an  antimony  coat. 

Melanostibian,  6  (Fe.Mn)O,  Sb2O3;  Black;  Str.  Cherry- 
red;  H.  4;  G.?  ;  IV? 

b.  Fused  with  soda,  boiled  in  1  cc.  strong  HC1,  and  reduced 

with  tin  shows  titanium ;  §  b,  p.  570. 
ILMENITE,  FeTiO3;    Iron-black;    Str.  Black;     H.  6; 
G.  5.18;  III;  p.  346. 


634  MINERALOGY 

Pseudobrookite;     Fe4(Ti04)3 ;      Brownish  black ;      Str. 

Reddish  brown  ;  H.  6 ;  G.  4.94  ;  IV. 
When  fused  with  soda  on  coal  yields  an  antimony  coat. 
Derbylite,  5  FeTiO3.FeSb2O6;  Pitch-black;  Str.  Brown; 

H.  5;  G.  4.5;  IV. 

c.  The  borax  bead  shows  iron  only. 

MAGNETITE,  Fe3O4 ;    Iron-black ;   Str.  Black ;    H.  6  ; 

G.  5.18;  I;  p.  373. 
HEMATITE,    Fe2O3 ;    Steel-gray    to    iron-black;     Str. 

Indian-red  ;  H.  5.5-6.5  ;  G.  5.20  ;  III ;  p.  343. 
Magnesoferrite,  MgO,  Fe2O3 ;   Iron-black;   Str.  Black; 

H.  6-6.5;  G.  4.6;  I;  p.  371. 

d.  The  borax  bead  is  yellow  after  oxidation  with  niter,  and 

shows  chromium ;  §  6,  p.  569. 
CHROMITE,  FeO,  Cr203,  Iron-black;  Str.  Brown;  H. 

5.5  ;  G.  4.2  ;  I ;  p.  376. 
X.   Minerals  which  are  not  included  in  the  preceding  divisions. 

A.  Compare  chromite,   wolframite,   and  molybdenite  in  preceding 

sections. 
GRAPHITE,  C;  Iron-black;  Str.  Black;  H.  1-1.5;  G. 

2.20  ;  Cl.  Basal ;  III ;  p.  284. 
URANINITE,     UO2,  UO3 ;      Black ;     Brownish  black ; 

H.  5.5 ;  G.  9.35 ;  I ;  p.  525. 

B.  The  HCl  solution  reduced  with  tin  shows  titanium;  §  b,  p.  570. 

RUTILE,  Ti02;  Brownish  black ;  Str.  Yellowish 
brown  ;  H.  6-6.5  ;  G.  4.22  ;  II ;  p.  349. 

Brookite,  TiO2 ;  Brown  to  black ;  Str.  yellowish  ;  H.  6  ; 
G.  3.95;  IV;  p.  351. 

Perovskite,  CaTiO3 ;  Brown  to  black ;  Str.  Grayish  ; 
H.  5.5;  G.  3.95;  I;  p.  347. 

C.  The  HCl  solution  reduced  with  zinc  shows  columbium;  §  a,  p.  570. 

COLUMBITE,  (Fe.Mn)(Cb.Ta)2O6;  Black;  Str.  Dark 
red  to  black ;  H.  6  ;  G.  5.68  ;  IV ;  p.  505. 

Fergusonite,  (Y.Er.Ce)(Cb.Ta)O4 ;  Brownish  black; 
Str.  Dark  brown ;  5.5-6;  G.  5.80  ;  IV;  p.  506. 

Samarskite,  (Fe.Ca.UO2)3(Y.  Er.  Ce)2(Cb.Ta)6O2i ;  Vel- 
vet-black ;  Str.  Reddish  brown ;  H.  5-6  ;  G.  5.70  ;  IV. 

Mossite,  Fe(Cb.Ta)2O6;  Black;  Str.  Black;  H.  6; 
G.  6.45 ;  IV. 

^schynite,  4(Ce.La.Er)  5  (Ti.Th)Cb4026;  Black;  Str. 
Gray ;  H.  5-6  ;  G.  4.93  ;  IV. 


MINERALS  HAVING   TASTE  635 

Polycrase,  Cb,  Ti,  Y,  Er,  Ce,  U,  Fe,  H,  O?;  Black;  Str. 

Grayish  brown ;   H.  6;  G.  6.10. 
Polymignite,  Cb,  Zr,  Ti,  Ca,  Th,  Ce,  Y,  Fe,  0?;  Black; 

Str.  Dark  brown;  H.  6.5;  G.  4.8  ;  IV. 
D.    Reacts  for  tantalum;  §  a,  p.  571 ;  only  slight  reaction  if  any  for 

columbium. 
Tantalite,    FeMnTa2O6 ;    Black;    Str.    Black;    H.    6; 

G.  7.15;  IV. 
Tapiolite,    FeTa2O6;    Black;     Str.    Black;    H.   6;     G. 

7.43;   II. 
Hielmite,  Ta,  Cb,  Sn,  U,  Y,  Ce,  Fe,  Mn,  Ca,  H,  O?; 

Black ;  Str.  Grayish  ;  H.  5  ;  G.  5.8  ;  IV. 
Yttrotantalite,    (Fe.Ca)    (Y.Er.Ce)2(Ta.Cb)6O15  4  H20 ; 

Brown  to  black ;  Str.  Gray;  H.  5-5.5;  G.  5.70;  IV. 


PART  II 

NON-METALLIC  MINERALS,  OR  MINERALS  WITHOUT  METALLIC  OR 
SUB-METALLIC  LUSTER 

I.  Having  taste,  or  those  easily  soluble,  or  soluble  to  a  large  extent 
in  water.  Hardness  below  3.  No  arsenic  or  copper 
minerals  are  placed  in  this  division. 

A .    Dissolved  in  dilute  HCl,  yields  a  white  precipitate  with  barium 

chloride  (sulphates). 
1.   Heated  on  coal  in  the  R.  F.  it  becomes  magnetic. 

a.  Heated  on  wire  it  yields  a  strong  yellow  flame  (sodium). 
Sideronatrite,     Na2(Fe.OH)(S04)2.2  H2O ;     Orange     to 

yellow;    L.  Silky;    H.  2-2.5;    G.  2.35;    F.  2;    IV; 
Fibrous. 

Ferronatrite,     Na,Fe(SO4)3.3  H2O ;      Pale    green;      H. 
2;  G.  2.55;  F.  1.5;  III;  radiated. 

b.  Heated  on  wire  it  yields  a  violet  flame  (potassium). 
JAROSITE,     K(Fe.2  OH)3(SO4)2.2  H2O ;      Yellow"    to 

brown;    L.  Vitreous;    H.  2.5-3.5;    G.  3.2;    F.  4.5; 
III. 

Metavoltaite,      (Ka.Na^.Fe^Fe^Fe.OHMSCXWe  H20 ; 
Yellow;  H.  2.5;  G.  2.53;  F.  4.5 ;  III;  Scales. 

c.  Fused  with  soda  and  niter  in  the  O.  F.  shows  man- 

ganese. 


636  MINERALOGY 

Dietrichite,    (Zn.Fe.Mn)Al(S04)4.22  H2O  ;    Dirty  white 

to  brownish  yellow ;  Silky ;  Fibrous. 
Ilesite,  (Mn.Zn.Fe)SO4.4H2O;  Green  to  white ;  V. 

d.  Shows  magnesia;  §  c,  p.  567. 

Knoxvillite,  (Fe.Mg)[(Fe.Cr.Al.)OH]7(SO4)8.5  H20 ; 

Greenish  yellow ;  IV. 

Botryogen,  (Mg.Fe)  (Fe.OH)  (SO4)2.7  H2O ;  Hyacinth- 
red;  H.  2-2.5;  G.2.09;  F.4.5;  V. 

e.  Shows  aluminium ;  §  b,  p.  568. 

Halotrichite,  FeAl2(SO4)4.24  H20 ;  Yellowish  white; 
Silky;  F.  4.5-5. 

Voltaite,    Fe3(Fe.OH)2(Fe.Al)4(SO4)io.l4  H2O;     Green- 
ish black;  Resinous;  H.  3;  G.  2.79;  F.I. 
/.    Contains  iron  as  the  only  base. 

Reacts  for  ferrous  iron,  but  not  for  ferric.1 

MELANTERITE,  FeSO4.7  H2O ;  Apple-green;  Vitre- 
ous; H.  2;  G.  1.9;  V;  p.  539. 

Contains  both  ferrous  and  ferric  iron.1 

Roemerite,Fe"Fe'"2(SO4)4.12  H20  ;  Light  to  dark  brown; 
H.  3-3.5;  G.  2.15;  F.4.5;  VI. 

Contains  ferric  iron  only.1 

Coquimbite,  Fe2(S04)3.9  H20 ;  White,  yellowish;  Vit- 
reous; H.  2;  G.  2.11;  III. 

Quenstedtite,  Fe2(S04)3.10  H2O;  Reddish  violet;  Vit- 
reous; H.  2.5;  G.  2.11;  V. 

Ihleite,  Fe2(SO4)3. 12  H2O  ;  Orange-yellow ;  Vitreous  ; 
H.  ?;  G.  1.81;  F.4.5;  Botrypidal. 

COPIAPITE,  Fe2(FeOH)2(SO4)5,17H20;  Sulphur-yel- 
low; Pearly;  H.  2.5;  G.2.10;  F.  4.5-5;  V. 

Castanite,  (Fe.OH)SO4.3iH2O ;  Chestnut-brown;  Vit- 
reous; H.  2.5;  G.2.12;  F.4.5;  V. 

Amarantite,  (Fe.OH)SO4.3  H2O ;  Orange  to  brownish 
red ;  Resinous ;  H.  2.5 ;  G.  2.28 ;  F.  4.5 ;  VI. 

Utahite,%3  (FeO)2SO4.4  H2O ;  Orange  yellow;  Silky; 
F.  4.5 ;  III ;  Tabular. 

Fibroferrite,  (Fe.OH)SO4.4iH20 ;  Pale  yellow;  Silky; 
H.  2-2.5;  G.  1.85;  Fibrous. 

1  Ferrous  iron  is  detected  by  adding  potassium  f  erricyanide  to  the  cold  solu- 
tion, when  a  Prussian  blue  precipitate  will  form  ;  no  piecipitate  is  formed  in  a  solu- 
tion of  ferric  salts. 

To  detect  ferric  iron  a  drop  or  two  of  ammonium  sulphocyanate  is  added  to  the  solu- 
tion, when  if  any  ferric  iron  is  present,  the  solution  will  assume  a  deep  blood-red  color. 


MINERALS  HAVING  TASTE  637 

Raimondite,  Fe4(OH)6(SO4)3.4H2O;    Honey  to  ocher- 

yellow;  H.  3-3.5;  G.  3.20;  III. 
Carphosiderite,  Fe6(OH)10(SO4)4.4  H2O ;  Straw-yellow; 

Resinous;  H.  4-4.5 ;  G.  2.5;   III. 

g.   Dissolved  in  the  borax  bead  shows  nickel ;  §  a,  p.  574. 
Morenosite,    NiS04.7  H20 ;    Apple-green  to    greenish 

white,  H.  2 ;  G.  2 ;  Amorph. ;  IV. 

h.   Dissolved  in  the  borax  bead  shows  cobalt;  §  a,  p.  574. 
Bieberite,   CoSO4.7H2O;  Flesh  to  rose-red;  Vitreous; 

G.  1.92;   V;  Incrusted. 
2.   When  heated  in  R.  F.  on  coal  it  does  not  become  magnetic. 

a.  Effervesces  with  HC1. 

HANKSITE,  9  Na^SO^  Na^COa.KCl ;  Colorless  or 
white;  Vitreous;  H.  3-3.5;  G.  2.55;  F.  1.5;  III; 
p.  535. 

b.  Yields  a  strong  sodium  flame   (yellow) ;  through  the 

blue  glass  shows  a  potassium  flame  also;  §  a,  p.  563. 
.+.   With  sodium  hydroxide  shows  ammonia;  p.  565. 
Lecontite,  (Na.NH4.K)2SO4.2  H20 ;    White;    Vitreous; 

H.  2-2.5 ;  G.  ?  ;  F.  1 ;  IV. 
— .   Shows  no  ammonia. 
Aphthitalite,    (K.Na)2S04 ;     Colorless ;     Vitreous ;     H. 

3-3.5;  G.  2.65;  F.  1.5;  III. 

c.  It  yields  a  sodium  flame  only. 

+.   Yields  water  in  the  closed  tube. 

a.   Shows  aluminium  with  cobalt  solution ;  §  a,  p.  568. 

Mendozite,  NaAl(S04)2.12  H2O;  White;  Silky-vitre- 
ous; H.  3;  G.  1.88;  F.  1;  Massive. 

p.   Shows  magnesium  with  cobalt  solution  ;  p.  567. 

Loeweite,  MgSO4.Na2SO4.2i  H20 ;  White,  yellow,  red  ; 
Vitreous;  H.  2.5-3;  G.  2.38;  F.  1.5;  II. 

Bloedite,  MgS04.Na2SO4.4  H2O;  Colorless;  Vitreous, 
H.  2.5;  G.  2.25;  F  1.5;  V. 

•y.    Shows  nitrates,  §  a,  p.  590. 

Darapskite,  Na^SC^.NaNOa.HaO ;  Colorless ;  Vit- 
reous; H.  2-3;  G.  2.20;  F.  3;  V,  Tabular. 

8.   Shows  sodium  as  the  base. 

MIRABILITE,  Na2S04.10  H2O ;  Colorless;  Vitreous;  H. 
1.5-2;  G.  1.48;  F.  1.5;  V;  p.  535. 

— .   Yields  no  water  in  the  closed  tube. 

a.    With  copper  oxide  shows  chlorine ;  p.  580. 


638  MINERALOGY 

Sulphohalite,  3  Na^SO4.2  NaCl ;     Colorless ;     Vitreous ; 

H.  3.5;  G.  2.50;  F.  1.5;  I. 
(3.   Shows  no  chlorine. 
THENARDITE.  Na<>SO4;    Colorless  to  brownish;    H. 

2-3;  G.  2.69;  F.  1.5-2;  IV;  p.  527. 

d.  It  shows  a  potassium  flame  (violet),  p.  563. 

a.   With  cobalt  solution  shows  aluminium,  p.  568. 
Kalinite,  KA1(SO4)2. 12  H20;.  Colorless;   Vitreous;    H. 

2-2.5;  G.  1.75;  F.  1 ;  I;  p.  542. 
p.   With  copper  oxide  shows  chlorine,  §  a,  p.  580. 
KAINITE.  MgSO4.KC1.3  H2O  ;     Colorless ;    H.   2.5-3  ; 

G.  2.13;  F.  1.5-2;  V ;  p.  534. 

"y.   With  cobalt  solution  shows  magnesia;   §  a,  p.  567. 
Picromerite,    MgSO4.K2SO4.6  H2O ;    White;    H.  ?;    G. 

2.15;   F.  1.5-2;   V;   Yields  water. 
Langbeinite,  2  MgS04.K2SO4 ;    Colorless;    H.  3-4;  G. 

2.81;  F.  1.5-2;   I;  Yields  no  water. 
8.   With  sodium  hydroxide  yields  ammonia ;  p.  565. 
Taylorite,    K5(NH4)(SO4)3;     Yellowish   white;     H.    2; 

Massive. 

€.    Shows  calcium ;  §  6,  p.  566. 
Syngenite,  CaK2(SO4)2.H20 ;  Colorless;    H.  2.5;    G. 

2.60;  F.  1.5-2;  V. 

e.  With  cobalt  solution  shows  aluminium  ;  p.  568. 
Alunogen,    A12(SO4)3.18  H2O ;     White ;    H.    1.5-2;     G. 

1.70;  V;  Fibrous. 
Tschermigite,  NH4A1(SO4)2.12  H2O ;    White;    H.  1-2; 

G.  1.50;  I ;  Yields  NH3  with  NaOH. 
/.    With  cobalt  solution  shows  magnesium ;  §  a,  p.  567. 
+  .   With  sodium  hydroxide  shows  ammonia,  p.  565. 
Boussingaultite,    MgSO4.  (NH4)2S04.6  H20  ;    Colorless  ; 

G.  1.7;  V. 

— .   It  yields  no  ammonia. 
EPSOMITE,    MgSO4.7  H2O ;    White;     Vitreous;    H. 

2-2.5;   G.  1.75;   F.  1 ;   IV;   p.  538. 
Kieserite,    MgSO4.H2O ;      White,    gray,     yellow;      H. 

3-3.5;   G.  2.56;   F.  2-3?;   V;   p.  539. 
g.   Entirely  volatile,  boiled  with  NaOH  shows  ammonia. 
Mascagnite,  (NH4)2SO4;   White;    Vitreous;   H.  2-2.5 ; 

G.  1.77;  F.  1;  IV. 
h.   Dissolved  in  borax  on  wire  shows  manganese ;  §  a,  p.  574. 


MINERALS   HAVING   TASTE  639 

Mallardite,  MnS04.7H2O;  Pink  to  white;  V;  Fibrous. 
Szmikite,    MnSO4.H2O ;     White  to   pink;    H.  1.5;    G. 

3.15;  Amorphous. 
Apjohnite,  MnAl2(S04)4. 24H2O ;    White  to  pale  rose; 

Silky;  H.  1.5;  G.  1.78;  V;  Fibrous. 
i.    Fused  with  soda  in  the  R.  F.  on  coal  it  yields  a  zinc 

coat. 
Goslarite,  ZnS04.7  H2O ;   White;    Vitreous;    H.  2-2.5; 

G.  2.00  ;   IV ;  Acicular. 

B.  Effervesces  in  dilute  HCl  (carbonates}. 

1.    Yields  a  strong  sodium  flame  (yellow). 

TRONA,  HNa3(CO3)2.2H2O;  White  to  gray;  Vitre- 
ous; H.  2.5-3;  G.  2.13;  F.  1.5;  V;  p.  401. 

NATRON,  Na2CO3.10H20;  White  to  gray;  Vitreous; 
H.  1-1.5;  G.  1.44;  F.  1 ;  V;  p.  400. 

Thermonatrite,  Na2C03.H2O ;  White,  gray  to  yellow; 
H.  1-1.5;  G.  1.55;  F.  1.5;  IV. 

C.  With  copper  oxide  shows  chlorine;  §  a,  p.  588. 

1.  Yields  a  strong  sodium  flame. 

HALITE,  Nad;    White,  red,  blue ;    H.  2.5;    G.  2.13; 

F.  1.5;  I;  p.  327. 

2.  It  yields  a  strong  potassium  flame. 

SYLVITE,  KC1;    White;    H.  2;    G.  1.98;    F.  1.51;  p. 

328. 

With  cobalt  solution  shows  magnesium,  p.  567. 
CARNALLITE,    MgCl2,KC1.6  H20 ;   White  to  red ;  H. 

1;   G.  1.60;   F.  1-1.5;   IV;   p.  335. 

3.  Yields  a  calcium  flame. 

a.  Shows  magnesium  with  cobalt  solution ;  §  a,  p.  567. 
Tachydrite,  CaMg4Cl6.12  H2O ;    Wax-  to  honey-yellow  ; 

H.  2.5;  F.  1;  III. 

b.  It  contains  no  magnesium. 

Hydrophylite,  CaCl2;  White;  G.  2.2;  F.  1.5;  I. 

D.  Heated  in  a  closed  tube  with  potassium  bisulphate  it  yields 

orange-colored  fumes  (nitrates) . 

1.  It  yields  a  yellow  flame  (sodium). 

SODA  NITER,  NaNO3 ;  White;  Vitreous;    H.  1.5-2; 

G.  2.29;  F.  1.;  Ill;  p.  520. 

2.  It  yields  a  violet  flame  (potassium). 

NITER,  KNO3;  White;  Vitreous;  H.  2;  G.  2.13;  F. 
1;  IV;  p.  521. 


640  MINERALOGY 

3.    It  yields  a  green  flame  (barium). 

Nitrobarite,    Ba(NO3)2;     White;     Vitreous;     H.    2.5; 

G.  3.20;  F.l-1.5;  I. 
E.    With  turmeric  paper  the  HCl  solution  shows  boric  acid;  §  b, 

p.  592. 
1.   In  the  closed  tube  yields  water. 

BORAX,     Na2B403.10  H2O  ;      Vitreous;      H.     2-2.5; 

G.  1.72;  F.  1-1.5;  V;  p.  522. 
Sassolite  B(OH)3;    White;    Pearly;    H.  1 ;    G.  1.48; 

F.  0.5;  VI. 

II.  Easily  and  completely  volatile  (when  pure)  in  a  gentle  O.  F. 
If  the  mineral  decrepitates,  it  should  be  heated  in  a 
closed  tube,  when  it  should  volatilize  and  yield  a 
sublimate. 

A.  The  mineral  burns  with  a  blue  flame,  yielding  sulphur  dioxide. 

SULPHUR,    S;     Pale  yellow ;    Resinous ;    H.  1.5-2.5; 

G.  2.07 ;  F.  1 ;  IV ;  p.  285. 

B.  With  soda  in  R.  F.  on  coal  it  yields  an  arsenic  odor. 

1.  Fused  with  soda  in  R.  F.  it  yields  on  silver  a  sulphur  re- 

action. 

— .   Yields  no  green  flame. 

REALGAR,  AsS ;  Aurora-red;  Resinous;  H.  1.5-2; 
G.  3.55;  F.  1;  V;  p.  294. 

ORPIMENT,  As2S3;  Lemon-yellow;  Pearly  to  resin- 
ous; H.  1.5-2;  G.  3.48;  F.  1;  V;  p.  295. 

-f .   It  yields  a  green  flame  (thallium). 

Lorandite,  T12S.  As2S3  ;  Carmine-red  ;  Adamantine ; 
H.  2-2.5 ;  G.  5.53 ;  F.  1 ;  V. 

2.  It  yields  no  sulphur  reaction  with  soda. 

Arsenolite,    As2O3;     White;     Adamantine;     H.    1.5; 

G.  3.70;  Volatile;  I;  p.  346. 
Claudetite,  As203;    White;    Pearly;    H.  2.5;    G.  4.00; 

Volatile;  V. 

C.  Yields  a  white  coat  which  colors  the  inner  flame  pale  yellowish 

green  (antimony);  p.  584. 

1.  With  soda  it  yields  a  sulphur  reaction. 

KERMESITE,  Sb2S20 ;  Brownish  red  to  maroon; 
H.  1-1.5;  G.4.60;  F.I;  V. 

2.  Yields  no  sulphur  reaction  with  soda. 

Senarmontite,  Sb2O3 ;  White ;  Adamantine ;  H.  2-2.5 ; 
G.  5.25;  F.  1.5;  I. 


COPPER  MINERALS  641 

VALENTINITE,   Sb2O3 ;  White;    Pearly,  adamantine; 
H.  2.5-3;    G.  5.56;   F.  1.5;   IV;   p.  346. 

D.  Fused  with  soda  in  the  closed  tube  it  yields  mercury;  §a,  p.  579. 

1.  Fused  with  soda  it  yields  a  sulphur  reaction  on  silver. 

CINNABAR,     MgS  ;      Red,   vermilion  ;    Adamantine ; 
H.  2-2.5;   G.  8.10;   F.  1.5;   III;  p.  304. 

2.  With  copper  oxide  it  shows  chlorine,  §  a,  p.  588. 

Calomel,    HgCl ;    White;    Adamantine;    H.    1-2;    G. 

6.48;   Volatile;   II. 
Eglestonite,     Hg4Cl2O ;    Yellow    to    black;     H.    2-3; 

G.  8.327 ;  I ;  Darkens  on  exposure. 
Terlinguaite,     Hg2ClO,     Sulphur-yellow,     dark    olive; 

H.  2-3 ;  G.  8.725 ;  V. 

3.  With  copper  oxide  shows  no  chlorine.' 

Montroydite,  HgO ;    Red;   Adamantine;   H.  1.5. 

E.  Reduced  with  soda  on  coal,  it  yields  lead  buttons. 

Contunnite,    PbCl2;     White;     Adamantine;    H.     1-2; 

G.  5.8;   Volatile;  IV. 

III.    Powdered,  roasted,  and  then   reduced  with  soda  and  borax 
in  R.  F.  on  coal,  it  yields  malleable  buttons  or  scales  when 
washed  in  the  mortar. 
A.    The  button  is  copper  or  contains  copper. 

1.  Dissolves  in  hot  dilute  HC1  with  effervescence  (carbonates). 

MALACHITE,  (CuOH)2CO3,  Bright    green;   Vitreous; 

H.  3.5-4;  G.  3.96;  F.  3 ;  V;  p.  397. 
AZURITE,  Cu(Cu.O.H)2(CO3)2 ;  Azure-blue ;  Vitreous ; 

H.  3.5-4;  G.  3.77;  F.  3;  V;  p.  399. 
Aurichalcite,  2  (Zn.Cu)CO3.  3  (Zn.Cu)(OH)2;  Pale 

green  to  blue;  Pearly;  H.  2;  G.  3.6;  F.  Dif. ;  V. 

2.  The  HC1  solution  yields  a  white  precipitate  with  barium 

chloride  (sulphates). 

+  .    Completely  soluble  in  water,  when  pure. 

CHALCANTHITE,    CuSO4.5  H2O  ;     Azure-blue;     Vit- 
reous; H.  2.5;  G.  2.21;  F.  3 ;  VI;  p.  540. 

Pisanite,  (Fe.Cu)SO4.7H2O;   Blue;  Vitreous;    H.  2.5; 

F.  3-4 ;  V ;  Becomes  magnetic. 

Krohnkite,   CuNa2(SO4)2.2  H2O  ;    Azure-blue;    H.  2.5; 

G.  1.98;  F.  1;  V;  Yellow  flame. 

Cyanochroite,    CuK2(SO4)2.6  H2O ;   blue;    Vitreous;   V. 
Hydrocyanite,    CuS04;     Pale  green,    Brownish  yellow ; 

F.  3;  V;  Yields  no  water. 

2T 


642  MINERALOGY 

— .   Not  completely  soluble  in  water. 

a.  Alone  yields  an  azure-blue  flame. 

Spangolite,  (A1.C1)S04.6  Cu(OH)2.3  H20  ;   Dark  green ; 

Cl.  basal;  H.  2-3 ;   G.  3.14;   F.  3 ;   III. 
Connellite,    CuiB(Cl.OH)4SOi6.15  H20  ;      Blue;      H.  3  ; 

G.  3.36  ;   F.  2.5  ;   III.     Prismatic. 

b.  Do  not  yield  an  azure-blue  flame. 
BROCHANTITE,  Cu4(OH)6SO4;    Deep  emerald-green; 

H.  3.5-4;  G.  3.9;  F.  3.5;  IV;  p.  541. 
Stelznerite,  CuSO4.2Cu(OH)2;   Green;    G.  3.88 ;    IV. 
Langite,  CuSO4.3  Cu(OH)2.H2O ;    Blue,  greenish   blue; 

H.  2.5-3;  G.  3.50;  F.  3.5;  IV. 

Herrengrundite,    2(Cu.OH)2S04.Cu(OH)2.3  H20 ;    Em- 
erald-green; H.  2.5;   G.  3.1;   F.  3.5. ;   V. 
Cyanotrichite,    Cu4Al2SOi0.8  H20  ;    Clear  blue ;    Pearly ; 

G.  2.7;  F.  3;  IV. 
Lindackerite,          (Cu.OH)  4Cu2Ni3(SO4)  ( As04)  4.5  H2O ; 

Verdigris-  to  apple-green  ;  H.  2-2.5 ;  G.  2.25  ;  F.  2-3  ; 

IV. 
Dolerophanite,  (Cu20)SO4;  Brown;  F.  3 ;  V;      Yields 

little  or  no  water. 

c.  With  von  KobelFs  flux  shows  lead. 

Arzrunite,  PbS04.PbO,  3CuCl2  Cu(OH)2.H2O  ;  Blue;  F. 

2;  IV. 
Linarite,     [(Pb.Cu)OH]2SO4 ;     Azure-blue;     Cl.    pina- 

coidal;  H.  2.5;   G.  5.45;   F.  1.5;   V. 
Caledonite,     [(Pb.Cu)OH]2SO4;      Bluish     green;     Cl. 

basal;   H.  2.5-3;   G.  6.40;   F.  1.5;   IV. 
3.   The  powdered  mineral  heated  in  the  closed  tube  with  a  few 

small  fragments  of  coal    yields  an  arsenic  mirror; 

§  6,  p.  585. 

a.  The  powdered  mineral  in  R.  F.  becomes  magnetic. 
Chenevixite,   Cu2(Fe.O)2(AsO4)2.3  H2O;   Dark  to  olive- 
green  ;  Dull ;  H.  4.5  ;  G.  3.94 ;  F.  2.5 ;  Massive. 

b.  With  soda  and  borax  in  R.  F.  on  coal  yields  a  zinc  oxide 

coat. 

Veszelyite,7(Cu.Zn)0.(As.P)205.9H20;  Greenish  blue; 
H.  3.5-4;  G.  3.79;  VI. 

c.  With  von  KobelPs  flux  shows  bismuth ;   §  6,  p.  580. 
Mixite,  Cu2(Cu.OH)8Bi(AsO4)5.7H2O;    Pale  green;  H. 

3-4;  G.  3.8;  F.  2;  Capillary. 


COPPER  MINERALS  643 

d.  With  von  Kobell's  flux  shows  lead ;  §  c,  p.  579. 
Bayldonite,  (Pb.Cu)3(AsO4)2(Pb,Cu)(OH)2.H2O ;   Grass- 
to  black-green ;   Resinous;  H.  4.5 ;  G.  5.35;  F.  2-3 ; 
Mammillary. 

e.  Fused  with  soda  in  R.  F.  on  coal  it  yields  a  sulphur  re- 

action; §  6,  p.  587. 

Lindackerite    (Cu.OH)4Cu2Ni3S04(As04)4.5  H2O;   Ver- 
digris- to  apple-green ;  H.  2-2.5 ;  G.  2-2.5 ;    F.  2-3 ; 
IV. 
/.   Yields  a  reaction  for  uranium ;  §  6,  p.  576. 

Zeunerite,  Cu(UO2)2(AsO 4)2-8  H2O;  Emerald-green;  H. 

2-2.5 ;  G.  3.2 ;   F.  2-3 ;  II. 
g.   Contains  calcium,  §  b,  p.  566. 

Conichalcite,   (Cu,  Ca)(Cu.OH)(As.P)O4.i  H2O;  Em- 
erald-green; H.  4.5;  G.  4.12;  F.  2.5;  Massive. 

Tyrolite,  (Cu.Ca)(Cu.OH)4(AsO4)2.7  H2O;  Pale  apple- 
green;  H.  1-1.5;  G.  3.05;  F.  2.5;  IV. 
h.   Contains  aluminium ;  §  6,  p.  568. 

Liroconite,    (Cu.OH)9Al4(OH)6,    (AsO4)5.20  H20;    Sky- 
blue;  H.  2-2.5;  G.  2.9;  F.  3-3.5;  V. 
i.   Not  included  above,  contains  copper  as  the  base. 

Clinoclasite,    (Cu.OH)3AsO4 ;    Dark   to    bluish   green; 
H.  2.5-3  ;  G.  4.36  ;  F.  2.5 ;  V. 

OLIVENITE,  Cu(Cu.OH),  As04;  Blackish  olive-green 
to  brown;  H.  3 ;  G.  4.4;  F.  2.5;  IV;  p.  513. 

Euchroite,  Cu(Cu.OH)AsO4.3  H2O ;  Emerald-green;  H. 
3.5-4 ;  G.  3.39 ;  F.  2.5 ;  IV. 

Chalcophyllite,  (Cu.OH)3AsO4(Cu.OH)2.3i  H2O ;  Grass- 
green;  Cl.  basal;  H.  2;  G.  2.53;  F.  2.5;  III. 

Erinite,    Cu(Cu.OH)4(As04)2 ;    Emerald-green;    Dull; 
H.  4.5  ;  G.  4.04 ;  F.  2.5  ;  Mamillary. 

Cornwallite,      Cu(Cu.OH)4(AsO4)2.3  H20 ;       Emerald- 
green;  H.  4.5;  G.  4.16;  F.  2.5;  Massive. 

Leucochalcite,    Cu(Cu.OH)AsO4.H20 ;    White  to  pale 
green ;   Silky ;   F.  2.5  ;   Capillary. 

Trichalcite,  Cu3(As04)2.5 H2O ;   Verdigris-green;   Silky; 

H.  2.5;  F.  2.5;   Radiated  and  columnar. 
4.    The  powdered  mineral  dissolved  in  nitric  acid  shows  phos- 
phoric acid  with  ammonium  molybdate ;  §  b,  p.  588. 
a.   In  R.  F.  it  becomes  magnetic. 

Chalcosiderite,    Cu(Fe.Al)2(FeO)4(P04)4.8  H2O;     Light 


644  MINERALOGY 

to  dark  green;  Cl.  basal;  H.  2.5 ;  G.  3.1;  F.  4-4.5; 
VI. 

b.  With  von  KobelPs  flux  shows  lead,  p.  579. 
Vauquelinite,   (Pb.Cu)3(P.O4)2.2  (Pb.Cu)Cr04;     Green, 

brown;   Resinous;   H.  2.5-3 ;   G.  5.95;   F.  2?;   V. 

c.  In  S.  Ph.  shows  uranium;  §  a,  p.  576. 
TORBERNITE,    Cu(UO2)2(P04)2.8  H2O ;     Emerald-    to 

apple-green ;    Cl.  basal ;   H.   2-2.5 ;   G.  3.50 ;   F.  3 ; 
II;  p.  519. 

d.  Phosphates  of  copper  only. 

LIBETHENITE,  Cu(Cu.OH)P04;  Dark  to  olive-green; 
H.  4;  G.  3.70;  F.  2.5;  IV;  p.  513. 

PSEUDOMALACHITE,  (Cu.OH)3PO4 ;  Dark  to  emer- 
ald-green; H.  4.5-5;  G.  4-4.4;  Massive. 

Dihydrite,  Cu(Cu.OH)4(PO4)2 ;  Dark  emerald-green; 
H.  4.5-5;  G.  4.20;  F.  2.5;  VorVI?. 

Tagilite,  Cu(Cu.OH)P04.H2O ;  Emerald-green;  H. 
3-4;  G.  4.07;  F.  2.5;  V;  Fibrous. 

5.  Alone  yields  an  azure-blue  flame. 

+.   With  von  Kobell's  flux  shows  no  lead. 

ATACAMITE,  Cu2Cl(OH)3;  Deep  emerald-green;  H. 
3-3.5  ;  G.  3.75  ;  F.  3-4  ;  IV ;  p.  334. 

Footeite,  8  Cu(OH)2.CuCl2.4  H20  ;  Deep  blue  ;   V. 

Nantokite,  CuCl ;  White;  Adamantine;  H.  2-2.5; 
G.  3.93  ;  F.  1.5 ;  I.  Yields  no  water. 

— .   With  von  Kobell's  flux  shows  lead. 

Percylite,  PbCuCl2(OH)2 ;  Indigo-blue;  Brilliant;  Cl. 
cubic  ;  H.  3  ;  G.  5.08  ;  F.  1 ;  I. 

Cumengite,  PbCuCl2(OH)2 ;  Indigo-blue;  Cl.  pyrami- 
dal ;  H.  3 ;  G.  4.71 ;  F.  1 ;  II. 

6.  Decomposed  with  HC1,  leaving  a  residue  of  silica ;  §  d,  p.  592. 

CHRYSOCOLLA,  CuSiO3.2  H2O  ;  Green  to  turquoise- 
blue  ;  H.  2-4  ;  G.  2.12  ;  F.  Difficult ;  Massive,  p.  503. 

DIOPTASE,  H2CuSiO4;  Emerald-green;  Cl.  rhombo- 
hedral ;  H.  5  ;  G.  3.35 ;  F.  Difficult ;  III ;  p.  452. 

7.  Not  included  above. 

CUPRITE,  Cu2O  ;    Ruby-red  ;  Adamantine ;  H.  3.5-4  ; 

G.  6.00;  F.  3;  I;  p.  337. 
a.   Fused  with  soda  it  yields  a  sulphur  reaction. 

COVELLITE,  CuS  ;   Indigo-blue;   Cl.  basal ;   H.  1.5-2; 

G.  4.6;  F.  2.5;    III;   p.  306. 


SILVER  AND   TIN   MINERALS  645 

b.  Heated  in  a  closed  tube  with  potassium  bisulphate  it 

yields  NO2;  §  a,  p.  590. 

Gerhardtite,    Cu2(OH)O3;     Deep    emerald-green;    Cl. 
basal;   H.  2;   G.  3.42;   F.  3;   IV. 

c.  Heated  as  in  b  it  yields  iodine ;  §  a,  p.  589. 

Marshite,  Cul ;   Reddish  brown ;   Resinous;  F.  1.5;  I. 
Cuproiodargyrite,     Agl.CuI ;     Sulphur-yellow;    H.    2; 
Massive. 

d.  It  yields  a  vanadium  reaction  ;  §  6,  p.  577. 
Volborthite,   (Cu.OH)3VO4.6  H2O ;    Cl.  pinacoidal ;    H. 

3-3.5;   G.  3.55;  F.  1.5;  Tabular. 

Calciovolborthite,    (Cu.Ca)(Cu.OH)VO4 ;    Pistachio-  to 
olive-green;   H.  3.5 ;   G.  3.86 ;   F.  1.5-2;   Tabular. 

e.  It  yields  a  tungsten  reaction ;   §  b,  p.  587. 
Cuprotungstite,   CuW04 ;     Pistachio-green ;     Cl.  pina- 
coidal ;  H.  4.5-5  ;  F.  3  ;  Granular. 

/.    It  yields  a  reaction  for  selenium;  §  a,  p.  587. 

Chalcomenite,    CuSe03.2  H2O ;     Blue;     H.   2.5-3;   G. 

3.76;  F.  1.5;  V. 

B.    Treated  as  in  III,  it  yields  silver  buttons  or  it  contains  silver, 
but  no  Cu. 

1.  Alone  in  R.  F.  on  coal  it  yields  an  arsenical  odor. 

PROUSTITE,  3Ag2S.As2S3;  Ruby-red;    Adamantine; 

H.  2-2.5;   G.  5.55;   F.  1  ;  III;  p.  323. 
Xanthoconite,  3  Ag2S.As<>S3 ;   Orange-yellow  to  brown  ; 

Cl.  basal ;   H.  2  ;  G.  5.54  ;   F.  1 ;   V  ;  Tabular. 

2.  Yields  in  R.  F.  on  coal  an  antimony  coat. 

PYRARGYRITE,    3  Ag2S.Sb2S3 ;     Dark-red    to    black; 

H.  2.5;  G.  5.85;  F.  1;  III;  p.  322. 
Pyrostilpnite,     3Ag2S.Sb2S3 ;     Hyacinth-red ;      H.    2 ; 

G.  4.20 ;   F.  1  ;   V ;  Tabular. 

3.  With  copper  oxide  it  shows  chlorine. 

CERARGYRITE,  AgCl ;    Gray  to  colorless;    H.  2-3; 

G.  5.55  ;  F.  1 ;  I ;  p.  330. 
lodobromite,   Ag(Cl.Br.I) ;     Sulphur-yellow  to  green; 

H.  2-3  ;  G.  5.90 ;  F.  1 ;  I. 

4.  Shows  bromine ;  §  d,  p.  589. 

EMBOLITE,  Ag(Br.Cl);    Green  or   yellow;    H.    2-3; 

G.  5.80 ;  F.  1  ;   I ;  p.  330. 
Bromyrite,  AgBr  ;   Green  to  yellow ;    H.  2-3  ;  G.  5.90; 

F.  1 :   I. 


646  MINERALOGY 

5.   Shows  iodine ;  §  c,  p.  589. 

IODYRITE,  Agl ;    Lemon-yellow;    Resinous;   H.  1.5; 

G.  5.65  ;  F.  1 ;  III ;  p.  330. 
Miersite,  Agl ;   G.  5.64 ;   F,  1  ;   I. 

C.  Treated  as  in  III  the  button  is  tin. 

CASSITERITE,    SnO2 ;     Brown    to    black;     H.    6-7; 
G.  6.95;   Inf.;  II;  p.  347. 

1.  Yields  a  boric  acid  flame  with  Turner's  flux;  p.  592. 

Nordenskiodine,  CaSn(B.O3)2  ;  Sulphur-yellow ;  Cl. 
basal ;  H.  5.5-6  ;  G.  4.20 ;  Inf. ;  III. 

2.  Yields  water  in  the  closed  tube. 

Stokesite,  CaSn(Si03)3.H2O ;  Colorless;  H.  6;  G.  3.18; 
IV. 

D.  Treated  as  in  III  the  button  is  lead,  or  with  von  Kobell's  flux 

shows  lead;  §  6,  p.  580. 

1.  The  powdered  mineral  heated  with  a  few  fragments  of 

coal  in  the  closed  tube  yields  an  arsenic  mirror ;  §  b, 
p.  585. 

a.  Becomes  magnetic  in  the  R.  F. 

Carminite,  Pb3Feio(As04)i2 ;  Carmine-red;  Cl,  pris- 
matic; H.  2.5;  G.  4.10;  IV. 

Lossenite,  (Fe.OH)9(As.O4)6.PbSO4.12  H2O ;  Yellow 
to  brownish  red  ;  H.  3-4  ;  F.  2.5  ;  IV. 

Beudantite,  Fe,  Pb,  Cu,  SO4,  (P.As)04?;  Olive-green, 
brown  to  black;  Cl.  basal;  H.  3.5-4.5;  G.  4.15; 
F.  3.5;  III. 

b.  Yields  a  chlorine  reaction;  §  a,  p.  588. 
MIMETITE,  Pb4(Pb.Cl)(AsO4)3 ;  White,  yellow,  brown  ; 

Resinous;  H.  3.5 ;  G.  7.12;  F.  1.5;  III;  p.  51?. 
Ecdemite,  Pb4As2O7.2  PbCl2 ;  Yellow  to  green;  Greasy; 
Cl.  basal;  H.  2.5-3 ;   G.  7.00 ;   F.  1.5?;   IV. 

2.  Shows  antimony,  p.  584,  and  titanium;  §  6,  p.  570. 

Mauzeliite,  (Ca.Pb.Na2)4TiSb4Oi6;  Brown;  H.  6-6.5  ;  I. 

3.  Fused  with  soda  it  yields  a  sulphur  reaction;  §  6,  p.  587. 

a.  Effervesces  with  hot  nitric  acid  (carbonates). 
.Leadhillite,    Pb2(Pb.OH)2(CO3)2SO4 ;     White;    Pearly; 

Cl.  basal;    H.  2.5;    G.  6.45;    F.  1.5;    V. 

b.  Yields  a  strong  sodium  flame  (yellow). 

Caracolite,  Pb(OH)ClNa2SO4 ;  White;  H.  4.5;  Fl. 
5-2;  IV. 

c.  Becomes  magnetic  in  R.  F. 


LEAD   MINERALS  647 

Plumbojarosite,    Pb[Fe(OH)2]6(S04)4 ;     a   dark    brown 
powder;   G.  3.66;    III. 

d.  Lead  sulphates. 

ANGLESITE,  PbS04;   Gray  to  white ;   H.  3 ;   G.  6.35 ; 

IV;  p.  532. 
Lenarkite,  (Pb20)SO4;  Pale  yellow  or  white  ;  Cl.  basal ; 

H.  2-2.5  ;   G.  6.40 ;   F.  2  ;   V. 

e.  Gelatinizes  with  HC1. 

Roeblingite,     5  H2CaSi04.2  (Pb.Ca)SO4 ;     White;     H. 
2.5-3  ;    G.  3.43  ;   F.  3  ;    Granular. 

4.  Effervesces,  with  hot  dilute  nitric  acid  (carborates). 

a.  Yields  a  chlorine  reaction;  §  a,  p.  588. 

Phosgenite,     (PbCl)2CO3;    White;    Adamantine;      Cl. 
basal;   H.  3;   G.  6.2;   F.  1;   II. 

b.  Yields  water  in  the  closed  tubes. 

Hydrocerussite,    Pb(Pb.OH)2(C03)2 ;     White;    Pearly; 
H.  1-2;    G.  6.14;  F.  1-5;    III. 

c.  Yields  no  water. 

CERUSSITE,  PbCO3 ;  White  ;  Adamantine ;  H.  3-3.5  ; 

G.  6.55;    F.  1.5;.  IV;   p.  396. 
Beresonite,    2  PbO.3  PbCr04.PbC03 ;     Dark    red;    Cl. 

perfect ;   G.  6.69. 

5.  Yields  an  antimony  coat  in  R.  F.  on  coal. 

a.  Yields  a  chlorine  reaction;  §  a,  p.  588. 

Nadorite,    PbClSb02 ;     Smoky    to     yellowish    brown; 

Resinous;    H.  3.5-4;   G.  7.0;    F.  1.5;    IV. 
Ochrolite,  Pb4Sb2O7.PbCl2 ;  Sulphur  to  grayish  yellow ; 

F.  1.5?;  IV. 

b.  Yields  no  chlorine.     Yields  water. 

Bindheimite,    Sb2O3.PbO     and    H20 ;     Gray,     yellow, 
brown;    H.  4 ;   G.  4.68 ;    F.  3-4 ;    Amorphous. 

6.  The  hot  nitric  acid  solution  yields  a  white  precipitate  with 

silver  nitrate  (chlorine);  §  b,  p.  588. 
Cotunnite,    PbCl2 ;     White  ;     H.  2  ;     G.  5.80  ;     F.  1 ; 

IV. 
Penfieldite,  2PbCl2.PbO  ;    White  ;    Cl.  basal ;    H.  2.5 ; 

F.  1  ;  III. 
Matlockite,   PbCl2.PbO ;    Pale   yellow    to    white;    Cl. 

basal ;   H.  2.5-3 ;   G.  7.20 ;   F.  1 ;   II. 
Mendipite,   PbCl2.2  PbO ;    Pale   yellow  to  white ;    Cl. 

prismatic;   H.  2.5;   G.  7.10;   F.  1;   IV. 


648  MINERALOGY 

Laurionite,  PbCl(OH);  White;    H.  3-3.5;    F.  1;    IV; 

Yields  water. 
Nasonite,  Pb4(Pb.Cl)2Ca4(Si2.07)3;  White;   H.  4;   F.  2 ; 

G.  5.42;  II? 
In  the  closed  tube  yields  a  sublimate  of  lead  iodide; 

§  d,  p.  589. 

Schwartzenbergite,  Pb(I.Cl)2.2  PbO  ;    Honey-  to  straw- 
yellow  ;   H.  2-2.5;    G.  6.2;    F.  1;    III. 

7.   The  nitric  acid  solution  shows  phosphoric  acid  with  ammo- 
nium molybdate ;  p.  590. 

PYROMORPHITE,  Pb4(Pb.Cl)(P04)3 ;  White,  yellow, 
brown,  green;  H.  3.5-4  ;  G.  6.80  ;   F.  2  ;   III;  p.  511. 
8-    The  mineral  powdered  and  dissolved  in  nitric  acid  shows 
silica;  §  6,  p.  591. 

a.  Becomes  magnetic  in  R.  F. 

Melanotekite,  (Fe4O3)Pb3(Si04)3;  Dark  brown  to  black; 

H.  5-5.5  ;    G.  5.85 ;    F.  2 ;    IV. 
Hancockite,       (Pb.Ca.Mn)2(Al.Fe.Mn.OH)(Al.Fe.Mn)2 

(Si.04)3;    Brownish  red ;    H.  6.5-7;    G.  4.03;  F.  3; 

V  ;   Shows  Mn  with  borax. 

b.  With  Turner's  flux  it  yields  a  green  flame  (boric  acid). 
Hyalotekite,  (Pb.Ca.Ba)4(F.OH)B(Si03)6 ;    White;    H. 

5.5;   G.  3.80;   F.  3?;   Massive. 

c.  In  the  borax  bead  it  shows  manganese. 

Kentrolite,    (Mn403)Pb3(SiO4)3 ;     Black;     H.    5.5;     G. 

6.19  ;   F.  2-2.5  ;   IV. 
Ganomalite,    Pb3Si2O7(Ca.Mn)2Si04 ;     Gray   to   white; 

H.  3;   G.  5.74;   F.  3?;   II. 

d.  Silicate  of  lead  only. 

Barysilite,  Pb3Si207 ;  White  ;    Pearly ;  Cl.  basal ;  H.  3 ; 

G.  6.50;   F.  2.5;   III. 
9.   Not  included  above. 

a.   Shows  molybdenum;  §  a,  p.  586. 

WULFENITE,  PbMoO4 ;    Yellow,  orange,  red,  white; 

H.  4.5-5;   G.  6.05;    F.  2 ;    II;   p.  545. 
,    6,   Shows  vanadium ;  §  6,  p.  577. 

DESCLOIZITE,  Pb(Pb.OH)VO4;    Brown,  red;    Resin- 
ous; H.  3.5;  G.  6.05;  F.  1.5;  IV;  p.  514. 
Brackenbuschite,  Pb,Fe,Mn,(VO4).H2O?;  Dark-brown; 

F.  1.5;  V. 
c.   Shows  vanadium,  §  b,  p.  577,  and  chlorine. 


ZINC   AND   BISMUTH   MINERALS  649 

Vanadinite,    Pb4(Pb.Cl)(VO4)3;   Ruby-red,  brown,  yel- 
low; H.  3;  G.  6.83;  F.  1.5;  III. 

d.  Shows  tungsten ;  §  6,  p.  587. 

STOLZITE,    PbW04;     Green,    yellow,    brown,    red; 

Resinous ;    H.  3  ;   G.  8.00 ;    F.  2.5 ;    II ;   p.  545. 
Raspite,  PbWO4 ;    Wax-brown ;    Resinous ;    Cl.  pina- 

coidal;   H.  2.5-3 ;   G.?;   F.  2.5-3;  V. 

e.  Shows  chromium ;  §  b,  p.  569. 

CROCOITE,  PbCrO4;    Bright  red;  Adamantine;    H. 

2.5-3;    G.  6.00;    F.  1.5;    V;   p.  533. 
Phoenicochroite,  3  PbCrO4.PbO ;   Red;    Resinous;   Cl. 

pinacoidal;    H.  3-3.5 ;   G.  5.75 ;   F.  1.5;   IV. 
/.    Lead  oxides  only. 

Plattnerite,  PbO2 ;    Brown-black;    H.  5-5.5;    G.  8.50; 

F.  1.5;.  II. 

Minium,.' Pb304;    Red;    Dull  to  greasy;    H.  2-3;    G. 

4.60;    F.  1.5;    Pulverulent. 
Massicot,    PbO  ;     Sulphur-   to   reddish  yellow ;     Dull  ; 

H.  2;    G.  8-9;    F.  1.5;   Massive. 
IV.  Fused  with  soda  and  borax  in  the  R.  F.  on  coal,  it  yields  a 

coat. 
A.   It  yields  a  zinc  oxide  coat;  §  6,  p.  573. 

1.  Yields  with  soda  a  sulphur  reaction;  §  6,  p.  587. 

a.  In  R.  F.  becomes  magnetic. 

Danalite,  (Zn.Fe)2(Fe2.S)Be3(Si04)3 ;  Flesh-red  to  gray; 
H.  5.5-6  ;   G.  3.43  ;   Fr  5. 

b.  Dissolves  in  HC1,  yielding  H2S. 

SPHALERITE,    ZnS ;     White,    green,    yellow,    brown, 
black;   H.  3.5-4;   G.  4.10;   F.  Difficult;   I;    p.  301. 

2.  Effervesces  with  HC1,  yielding  C02  (carbonates). 

SMITHSONITE,  ZnC03 ;  White,  brown,  green,  pink; 

H.  5;   G.  4.37;   Inf.;   Ill;   p.  391. 
Hydrozincite,  2  ZnC03.3  Zn(OH)2 ;  White,  gray,  yellow; 

H.  2-2.5  ;   G.  3.69 ;   Compact. 

3.  Gelatinizes  with  HC1. 

a.    Yields  little  or  no  water. 
-    WILLEMITE,   Zn2SiO4;    White,   yellow,   green,   blue; 

H.  5.5;   G.  4.10;   Inf.;   Ill;   p.  451. 
Hardystonite,  Ca2ZnSi207 ;  White  ;  H.  3-4 ;  F.  Difficult  ; 

G.  3.39;  Yields  a  calcium  flame. 
6.    Yields  water. 


650  MINERALOGY 

CALAMINE,  (Zn.OH)2SiO3 ;    White,  yellow,  blue;    H. 

4.5-5;   G.  3.45;    IV;   p.  472. 
Clinohedrite,  H2CaZnSiO5 ;   Amethystine  to  white;    H. 

5-6  ;   G.  3.33  ;   F.  4  ;   IV. 
Leucophoenicite,       (Mn.Ca.Zn)5(Mn.Ca.ZnOH)2(SiO4)3 ; 

Purple-red;   H.  5.5-6 ;   G.  3.84 ;   F.  3 ;   V. 
4.   Not  included  above. 

a.  Easily  soluble  in  HC1. 

ZINCITE,  ZnO;  Deep  red  to  orange-yellow;  Cl. 
basal;  H.  4-4.5 ;  G.  5.55;  Inf.;  Ill;  p.  339. 

b.  The  nitric  acid  solution  shows  phosphoric  acid  with 

ammonium  molybdate. 

Hopeite,  Zn3(PO4)2.4  H2O ;  Grayish  white:  Cl.  pina- 
coidal;  H.  2.5-3;  G.  2.76;  F.  3:  IV. 

c.  Not  easily  soluble  in  HC1. 

GAHNITE,  ZnAl2O4 ;  Dark  green  ;H.  7.5-8;  G.  4.55; 
Inf. ;  I ;  p.  377. 

JEFFERSONITE,  (Ca.Mn)(Mg.Fe.Zn)(Si03)2 ;  Green- 
ish black  to  brown;  Cl.  prismatic:  H.  5-6;  G. 
3.6;  F.  4;  V;  p.  424. 

Disluite,  (Zn.Fe.Mn.Mg)(ALFe)0 ;   Green-black,  green, 

gray;   H.  7.5-8;   G.  4.5;    Inf.;    I. 
B.    With  Von  Kobell's  flux  shows  bismuth ;  p.  580. 

1.  The  powdered  mineral  when  heated  in  the  closed  tube  with 

a  few  fragments  of  coal    yields   an   arsenic   mirror, 
p.  585. 

a.  The  S.  Ph.  bead  in  R.  F.  is  green  (uranium). 
Walpurgite,  Bi]0.(UO2)3(OH)24(As04)4;  Wax-yellow;  Cl. 

pinacoidal;   H.  3.5 ;   G.  5.76;   F.  1.5;   VI. 

b.  The  S.  Ph  is  not  green,  or  shows  no  uranium. 
Atelestite,  (Bi.2  OH)(BiO)2AsO4 ;     Sulphur-yellow;    H. 

3-4;   G.  6.40;    F.  1.5;    V. 

Rhagite,  3  Bi(OH)3.2  BiAs04 ;  Yellow  to  greenish; 
Resinous;  H.  5 ;  G.  6.80;  F.  1.5;  Mammillary. 

2.  Effervesces  with  warm  dilute  HC1  (carbonates). 

a.  Yields  little  or  no  water. 

Bismuthosphaerite,  (Bi.O)2CO3;  White  to  gray ;  Dull; 
H.  3-3.5;  G.  7.42;  F.  1.5;  Massive. 

b.  Yields  water. 

Bismutite,  (BiO)(Bi.2  OH)CO3 ;  Yellowish  to  grayish; 
Dull;  H.  4-4.5;  G.  6.88. 


ARSENIC   MINERALS  651 

3.  Gelatinizes  with  HC1. 

Eulytite.  Bi4(SiO4)3 ;  Brown,  yellow,  colorless ;   Resin- 
ous;   H.  4.5;   G.  6.1;   F.  2 ;   I. 
Agricolite,  Bi4(Si04)3 ;  Yellow,  brown;  H.  3?:  G.  6.00; 

F.  2 ;  V. 

4.  Soluble  in  HC1,  shows  vanadium ;  p.  576. 

Pucherite,  BiVO4;  Reddish  yellow ;    Cl.  basal;    H.  1; 

G.  6.25  ;    F.  2  ;    IV. 

5.  Shows  tellurium. 

Montanite,  (Bi.2  OH)2TeO4 ;  Yellow,  green,  white;  H. 
1.5?  ;  Dull ;  Massive. 

C.  Yields  an  antimony  coat;  §  a,  p.  584. 

1.  The  HC1  solution  reduced  with  tin  showrs  titanium;  §  6, 

p.  570. 

Lewisite,  (Ca.Fe)5Ti2Sb6024 ;  Honey-yellow  to  brown; 
Resinous;  H.  5.5;  G.  4.95;  I. 

2.  In  R.  F.  becomes  magnetic. 

Tripuhyite,  Fe2Sb207 ;  Greenish  yellow  ;  Resinous  ;  G. 
5.82;  F.  4-5?. 

3.  In  R.  F.  not  magnetic. 

Romeite,  CaSb204 ;    Honey-yellow ;    H.  5.5;    G.  4.70; 

II. 
Manganostibnite,  MnioSb205 ;   Black  ;    Compact. 

D.  Yields  a  cadmium  coat;  §  a,  p.  581. 

With  soda  a  sulphur  reaction. 

Greenockite,  CdS ;   Honey-,  lemon-  or  orange-  yellow ; 

H.  3-3.5  ;   G.  4.95  ;   III ;  p.  306. 
V.    In  R.  F.  on  coal  yields  an  arsenical  odor,  or  the  powdered 

mineral  heated  with  a  few  fragments  of  coal  in  a 

closed  tube  yields  an  arsenic  mirror. 

A.  In  the  borax  bead  it  shows  cobalt. 

ERYTHRITE,  Co3(AsO4)2.8  H2O ;  Crimson  to  peach- 
red;  Pearly;  H.  1.5-2.5;  G.  2.95;  F.  2.5;  V;  p. 
517. 

Forbesite,  H(Ni.Co)As04.3J  H20  ;  Grayish  white ; 
Silky;  H.  2.5 ;  G.  3.1 ;  Fibrous. 

B.  With  borax  shows  nickel. 

Annabergite,      Ni3(AsO4)2.8  H20 ;       Apple-green ;      H. 

1.5-2.5;    F.  4;    V;    Capillary. 
Cabrerite,  (Ni.Mg)3(AsO4)2.8H2O  ;  Apple-green ;  Pearly  ; 

H.  2  ;   G.  2.96  ;   F.  4-5  ;   V. 


652  MINERALOGY 

C.  *  The  residue  after  treatment  in  R.  F.  is  magnetic.     The  bead 

shows  iron. 

1.  With  soda  in  R.  F.  it  yields  a  sulphur  reaction. 

Pitticite,  Fe,  (As04),  (SO4).H2O?;  Yellowish  to  red- 
dish brown  ;  H.  2-3  ;  G.  2.35  ;  Massive. 

2.  No  sulphur,  contains  calcium ;  §  b,  p.  566. 

Arseniosiderite,  [Fe4(OH)6](Ca.OH)3(AsO4)3 ;  Yellow- 
ish to  golden  brown;  Silky;  H.  1-2;  G.  3.6; 

F.  3;   Fibrous. 

Mazapilite,      [Fe4(OH)6]Ca8(AsO4)4.3  H2O ;      Black    to 

brownish  red;  H.  4.5;  G.  3.57;  IV. 
3.   Contains  no  sulphur  or  calcium, 

PHARMACOSIDERITE,  Fe(Fe.OH)3(AsO4)3.6  H2O ; 
Green,  yellow,  brown,  red;  H.  2.5;  G.  2.95;  F. 
1.5-2;  I. 

SCORODITE,  FeAs04.2H2O;  Pale  green  to  brown; 
H.  3.5-4;  G.  3.29;  F.  2-2.5;  IV. 

Symplesite,  Fe3(AsO4)2.8  H20 ;  Blue  to  mountain- 
green;  Cl.  Pinacoidal;  H.  2.5;  G.  2.95;  F.  Diffi- 
cult V.  . 

D.  Roasted  and  dissolved  in  borax  on  wire  it  shows  manganese. 

1.  It  dissolves  in  HC1.  liberating  chlorine. 

Synadelphite,  2(Mn.Al)AsO4.5  Mn(OH)2  ;  Brownish 
black  to  black ;  H.  4.5  ;  G.  3.47 ;  F.  2-3  ;  V. 

Flinkite,  MnAsO4.Mn(OH)2 ;  Greenish  brown;  H. 
4-4.5;  G.  3.87;  F.  2-3 ;  IV. 

Hematolite,  (Al.Mn)AsO4,  4Mn(OH)2 ;  brown  to  red; 
Cl.  basal ;  H.  3.5  ;  G.  3.35  ;  F.  Difficult ;  III. 

2.  Soluble  in  HC1  without  liberating  chlorine. 

Brandtite,    Ca2Mn(AsO4)2.2  H20 ;    White;     H.    5-5.5; 

G.  3.67  ;   F.  2.5-3  ;   VI. 

Berzeliite,  (Ca.Mg.Mn)3(As04)2 ;  Sulphur-  to  orange- 
yellow  ;  Resinous ;  H.  5  ;  G.  4.08  ;  F.  3  ;  I. 

Larkinite,  Mn(Mn.OH)  AsO4 ;  Flesh,  rose,  or  yellowish 
red;  Greasy;  H.  4-5 ;  G.  4.18;  F.  2 ;  V. 

Hemafibrite,  Mn3(AsO4)2.3  Mn(OH)2 ;    F.  2?;  IV. 

Allactite,  Mn3(AsO4)2.4  Mn(OH)2  ;  Brownish  red  ;  H. 
4.5;  G.  3.84;  F.  2?;  V. 

E.  The  roasted  mineral  in  the  S.  Ph.  bead  shows  uranium;  p.  576. 

Trogerite,  (U02)3(As04)2.12  H2O ;  Lemon-yellow;  Cl. 
pinacoidal ;  G.  3.3  ;  F.  2.5  ;  V. 


PHOSPHATES  653 

Uranospinite,   Ca(U02)3(AsO4)2.8  H20  ;    Bright    green ; 
Cl.  basal;    H.  2-3;   G.  3.45;    IV. 

F.  Fused  with  potassium  bisulphate  in  the  closed  tube,  it  shows 

fluorine. 

1.  It  yields  a  sodium  flame  (yellow). 

Durangite,   Na(AlF)AsO4;    Light   to  dark  orange-red; 
C.  prismatic  ;   H.  5  ;   G.  4.0 ;   F.  2 ;   V. 

2.  Shows  calcium ;  §  6,  p.  566. 

Tilasite,    Ca(MgF)AsO4;    Gray  to    violet;    Cl.   pina- 

coidal ;   F.  4.5-5  ;   Foliated. 
Svabite,    Ca4(CaF)(As04)3;    Colorless;    Cl.   prismatic; 

G.  3.5;   F.  4.5-5;   III. 

G.  The  concentrated  HCl  solution  yields  a  precipitate  with  a  drop 

of  sulphuric  acid  (calcium). 

Adelite,  Ca(Mg.OH) As04 ;   Gray;   H.  5;   G.  3.76;    V. 
Haidingerite,     HCaAs04.H2O ;     White;     Pearly;     Cl. 

pinacoidal;   H.  1.5-2.5;   G.  2.85;   F.  2.5;   IV. 
Pharmacolite,  HCaAsO4.2  H20  ;  White  or  gray  ;  Pearly ; 

Cl.  pinacoidal ;   H.  2.5 ;   G.  2.47  ;   F.  2-3  ;   V. 
H.  Becomes  violet  with  cobalt  solution. 

Hoernesite,  Mg3(As04)2.8  H20  ;    Snow-white  ;    Pearly ; 

Cl.  pinacoidal ;   H.  1 ;  G.  2.47 ;   F.  2-3 ;   V. 
VI.   The  nitric  acid  solution  shows  phosphoric  acid  with  ammonium 

molybdate,  §  6,  p.  590. 

A.    The  powdered  mineral  treated  with  the  R.  F.  on  coal  yields  a 
magnetic  residue. 

1.  Yields  a  lithium  flame. 

TRIPHYLITE,  Li(Fe.Mn)P04 ;  Light  blue,  green,  gray ; 
resinous  ;  Cl.  basal ;   H.  4.5  ;  G.  3.55  ;  F.  2.5 ;  IV. 

2.  The  borax  bead  shows  manganese. 

a.  Yields  a  strong  yellow  flame  (sodium). 
Natrophilite,      Na(Mn.Fe)P04 ;      Deep     wine-yellow; 

Resinous;    Cl.  basal ;   H.  4.5-5 ;    G.  3.41 ;    F.  2-2.5 ; 

IV. 
Dickinsonite,    (Mn.Fe.Ca.Na2)3(PO4)2.i  H20  ;    Olive-oil 

or  grass-green  ;    Cl.   basal ;    H.   3.5-4  ;   G.  3.34 ;    F. 

2.5;   V. 
Fillowite,  (Mn.Fe.Ca,Na2)3(P04)2.J  H2O  ;  Wax  yellow  to 

brown;    Greasy;    Cl.    basal;    H.   4.5;    G.  3.43;    F. 

2.5-3;   V. 

b.  It  will  not  yield  a  yellow  flame. 


654 


MINERALOGY 

TRIPLITE,  (Fe.Mn)(Mn.F)P04;  Chestnut  to  blackish 

brown;    Resinous;   H.  4.6;    F.  2.5;   V. 
Yields  water. 
Triploidite,   (Mn.Fe)(Mn.OH)PO4 ;    Yellow   to   reddish 

brown;  Cl.  pinacoidal;  H.  4.5-5;  G.  3.70;  F.  3 ;  V. 
Childrenite,       (Fe.Mn)(A1.2  OH)PO4.H2O ;      Yellowish 

brown  to  brownish  black ;  Cl.  pinacoidal ;  H.  4.5-5  ; 

G.  3.20;   F.  4;   IV. 
Perpurite,    2(Mn.Fe)P04.H20 ;     Reddish    purple;     H. 

4-4.5;   G.  3.15;    F.  2. 

3.  The  borax  bead  shows  iron  only. 
a.   With  soda  it  shows  sulphur. 

Diadochite,  2(Fe.OH)SO4.2  FeP04.H2O;  Yellow  or  yel- 
lowish brown ;  Resinous;  H.  3 ;  G.  2.03;  F.  3?;  V. 
6.  The  concentrated  HC1  solution  shows  calcium ;  §  b, 
p.  566. 

Borickite,  Ca(Fe.2  OH)4(P04)2.3  H20 ;  Reddish  brown; 
Waxlike  ;  H.  3.5 ;  G.  2.7  ;  F.  3  ;  Massive. 

Calcioferrite,  Ca3(Fe.OH)3(P04)4.8  H2O ;  Sulphur  to 
greenish  yellow ;  Pearly;  H.  2.5 ;  G.  2.52;  Massive. 

4.  Yields  aluminum  ;  §  b,  p.  568. 

Barrandite,    (Al.Fe)P04.2  H2O ;     Pale   blue,    green,    or 

yellow;   H.  4.5 ;   G.  2.57;   Radiated. 
LAZULITE,    (Mg.Fe)(AlOH)2(PO4)2;    Azure-blue;    Cl. 

prismatic  ;  H.  5-5.5  ;  G.  3.06  ;  F.  Difficult ;  V ;  p.  438. 

5.  Phosphates  of  iron  only. 

VIVIANITE,  Fe3(PO4)2.8H2O;  Blue,  bluish  green, 
white;  Cl.  pinacoidal ;  H.  1.52;  G.  2.63;  F.  2-2.5 ; 
V;  p.  516. 

Ludlamite,  Fe5(Fe.OH)2(P04)4.8H20;  Pale  green  ;  Cl. 
basal;  H.  3-4 ;  G.  3.12;  F.  2.5 ;  V;  Tabular. 

DUFRENITE,  Fe2(OH)3PO4 ;  Dull  olive-  to  black- 
green;  Silky;  H.  3.5-4;  G.  3.31;  F.  2.5;  IV;  p. 
510. 

Beraunite,  (Fe.OH)3(PO4)2.2i  H2O  ;  Reddish  brown  ; 
Cl.  pinacoidal ;  G.  2.95 ;  F.  3 ;  V;  Foliated. 

Phosphosiderite,  2  FePO4.3J  H20 ;  Pale  red  or  red- 
dish violet ;  Cl.  pinacoidal ;  H.  3-4 ;  G.  2.76 ;  F. 
2.5;  IV. 

Strengite,  FePO4.2  H20  ;  Pale  red  or  reddish  violet ;  H. 
3-4 ;  G.  2.87 ;  F.  2.5 ;  IV. 


PHOSPHATES  655 

Koninckite,   FePO4.3  H20 ;    Yellow;    H.  3.5;    G.   2.3; 

F.  2.5 ;  Radiated. 
Cacoxenite,       Fe2  (OH)3P04.4i  H2O ;        Golden-yellow  ; 

Silky;   H.  3-4 ;   G.  3.38 ;   F.  2.5 ;    Radiated. 

B.  The  roasted  residue  is  not  magnetic. 

1 .  The  borax  bead  shows  manganese. 

Fairfieldite,  Ca2Mn(PO4)2.2  H2O ;  White  to  greenish 
white;  Cl.  pinacoidal ;  H.  3.5;  G.  3.15;  F.  4-4;  5; 
VI. 

Reddingite,  Mn3(PO4)2.3  H2O  ;  Pale  rose  to  brown  ;  H. 
3-3.5;  G.  3.10;  F.  2.5-3;  IV. 

2.  The  S.  Ph.  bead  in  R.  F.  is  green  (uranium). 

AUTUNITE,  Ca(UO2)2(PO4)2.8  H2O ;  Lemon,  to  sul- 
phur-yellow; Cl.  basal;  H.  2-2.5 ;  G.  3.12;  F.  3; 
IV ;  Tabular,  p.  520. 

Uranocircite,  Ba(UO2)2(P04)2.8  H20  ;  Yellowish  green  ; 
Cl.  basal;  H.  2-2.5 ;  G.  3.53;  F.  3?;  IV;  Tabular. 

Phosphuranylite,  (UO2)3(PO4)2.6  H2O ;  Deep  lemon- 
yellow;  Pearly;  F.  3-4?;  IV. 

C.  The  concentrated  HCl  solution  yields  a  precipitate  with  sulphuric 

acid  (calcium}. 

1.  Yields  little  or  no  water. 

APATITE,  Ca4(CaF)(P04)3;  Green,  blue,  brown,  white ; 
Cl.  basal;  H.  5 ;  G.  3.15;  G.  5-5.5;  III;  p.  508. 

2.  Yields  water  in  the  closed  tube. 

HERDERITE,  Ca[Be(OH.F)]PO4 ;  White,  pale  green 
or  yellow  ;  H.  5  ;  G.  3.00  ;  F.  4  ;  V. 

Hydro-herderite,  Ca(Be.OH.F)PO4 ;  White,  yellow,  pale 
green  ;  H.  5  ;  G.  2.95  ;  F.  4  ;  V. 

Cirrolite,  (Ca.OH)3Al2(PO4)3;  White,  pale  yellow;  H. 
5-6  ;  G.  3.08  ;  F.  4  ;  Massive. 

Monetite,  HCaPO4 ;  Yellowish  white  ;  Cl.  pinacoidal ; 
H.  3.5;  G.  2.75;  F.  3;  VI. 

Collophanite,  Ca3(P04)2.H20 ;  White,  yellow;  Dull; 
H.  2-2.5 ;  G.  2.70 ;  F.  4.5 ;  Amorphous. 

Tavistokite,  Ca3Al2(OH)6(P04)2 ;  White,  pearly  ;  F.  Diffi- 
cult ;  Acicular. 

Svanbergite,  (SO4)(P04),  Al,  Ca.H2O?;  Yellow,  brown, 
rose-red  ;  Cl.  basal ;  H.  5  ;  G.  3.30  ;  F.  Difficult ;  III. 

Hamlinite,  (Sr.OH)(A1.2  OH)3P2O7;  White  to  yellowish 
white ;  Cl.  basal.  H.  4.5 ;  G.  3.23 ;  F.  4 ;  III. 


656  MINERALOGY 

Martinite,    H2Ca5(PO)4.H2O ;     White,   yellow;   G.  3.9; 

'F.  Difficult ;   III. 
3.    Yields  much  water. 

Isoclasite,     Ca(Ca.OH)P04.2  H2O ;    White ;    Cl.   pina- 

coidal;   H.  1.5;  G.  2.92;   F.  4?  ;   V. 
Brushite,  HCaPO4.2  H20 ;    Colorless,  pale  yellow;    Cl. 
'     pinacoidal ;   H.  2-2.5  ;   G.  2.20  ;   F.  3  ;   V. 
Goyazite,    Ca3Al10P2O23.9  H20 ;     Yellowish    white;     Cl. 

basal ;   H.  5  ;   G.  3.26  ;   F.  Difficult. 

D.    Becomes  blue  with  cobalt  solution  (aluminum) .   Fusibility  above  5. 
Variscite,    A1P04.2H20;     White,    apple-    to    emerald- 
green  ;  Cl.  prismatic  ;  H.  4  ;  F.  Difficult ;  G.  2.4  ;  IV. 
Callainite,    AlP04.2i  H20 ;     Apple-    to    emerald-green ; 

H.  3.5-4;  G.  2.51;  Massive. 
Zepharovichite,     A1P04.3  H20 ;      Greenish    to    grayish 

green ;   H.  5.5 ;   G.  2.37  ;   Compact. 
Minervite,  AlPO4.3i  H2O  ;   White  ;    Massive. 
GIBBSITE,  A1P04.4 H2O  ;    White;    Massive;   Foliated; 

p.  365. 
WAVELLITE,     (A1.0H)3(P04)2.5  H20 ;    White,    yellow, 

green,  brown;   H.  3-4 ;   G.  2.33 ;   IV;   p.  510. 
Augelite,    A12(OH)3PO4 ;     White;     Cl.    prismatic;     H. 

4.5-5  ;   G.  2.70  ;   V. 
Peganite,  Al2(OH)3P04.li  H20 ;    Dark   to  light  green; 

H.  3-3.5;   G.  2.50;   IV. 
Fischerite,  A12(OH)3P04.2J  H20 ;  Grass-  to  olive-green  ; 

H.  5;   G.  2.46;   IV. 
Sphaerite,  A15(OH)9(PO4)2.12  H20 ;    Light  gray  or  blue; 

H.  4  ;   G.  2.53  ;   Globular. 
Evansite,   A13(OH)6PO4.6  H20 ;    White,   pale  yellow  or 

blue;   H.  3.5-4;   G.1.94;   Massive. 
LAZULITE,   (Mg.Fe)(A1.0H)2(P04)2;    Azure-blue;    H. 

5.5 ;   G.  3.06  ;   V ;  p.  438. 
Florencite,   Al3Ce(OH)6(P04)2;    Pale  yellow;    Greasy; 

Cl.  basal ;   G.  3.58 ;   III. 
E.    Not  included  above. 

1.   Heated  in  the  closed  tube  yields  the  odor  of  ammonia. 

Struvite,   NH4MgP04.6 H2O ;     White,    yellow,    brown; 

H.  2;   G.  1.65;   F.  3;   IV. 
Stercorite,     H(NH4)NaP04.4  H2O ;      White,     yellow, 

brown;   H.  2;   G.  1.6;   F.  1 ;    V. 


IRON   MINERALS  657 

2.  With  Turner's  flux   it  yields  a  boric   acid  flame    (bright 

green). 

Liineburgite,  Mg3(PO4)2.B203.8  H20  ;  White;  G. ,  2.05 ; 
Fibrous,  earthy. 

3.  Yields  a  strong  sodium  flame  (yellow). 

Beryllonite,  NaBePO4 ;  White;  Cl.  basal;  H.  5.5-6; 
G.  2.84  ;  F.  3-3.5  ;  IV. 

4.  Yields  a  lithium  flame  (red). 

AMBLYGONITE,  Li(Al.F)P04;  Wrhite,  pale  green, 
blue;  Cl.  basal;  H.  6 ;  G.  3.08  ;  F.  2  ;  VI;  p.  512. 

Montebrasite,  Li[Al(OH.F)]PO4 ;  White,  blue,  pale 
green;  Cl.  basal;  H.  6 ;  G.  3.00;  VI. 

5.  The    HC1    solution    yields    a    precipitate    of    magnesium 

ammonium  phosphate  with  ammonia. 
Wagnerite,  Mg(MgF)P04;  Pale  yellow,  gray,  red;  H. 

5.5  ;   G.  3.06  ;   F.  3.5-4  ;   V. 
Bobierrite,    Mg3(P04)2.8  H20 ;   White;    pinacoidal ;    H. 

2.5;   G.  2.43;   V. 

6.  Phosphates  of  the  rare  earths ;  §  a,  p.  575. 

MONAZITE,  (Ce.La.Di)P04;  Yellowish  to  reddish 
brown;  H.  5-5.5 ;  G.  5.10;  V;  p.  507. 

XENOTIME,  YP04;  Yellow  to  reddish  brown;  Cl. 
prismatic;  H.  4-5 ;  G.  4.53 ;  Inf.;  II. 

Rhabdophanite,  (La.Di.Y.Er)P04.H2O ;  Brown,  pink, 
yellow,  white;  H.3.5;  G.3.95;  F.  Difficult;  Massive. 

Churchite,  Ca3Ceio(PO4)i2.24  H20?;  Smoky-gray,  pink- 
ish; H.  3-3.5;  G.  3.15. 

VII.    The  powdered  mineral  heated  in  R.   F.  on  coal  becomes 
magnetic. 

A.  Effervesces  in  warm  dilute  HCl  (carbonates). 

SIDERITE,  FeC03;  Light  to  dark  brown;  Cl.  rhom- 
bohedral;  H.  3.5-4 ;  G.  3.85;  Inf.;  Ill;  p.  388. 

Ankerite,  Ca(Mg.Fe.Mn)(CO3)2 ;  Gray,  brown;  Cl. 
rhombohedral ;  H.  3.5-4  ;  G.  3.03  ;  Inf. ;  III. 

Breunnerite  (Mg.Fe)C03,  Brown,  gray ;  Cl.  rhom- 
bohedral ;  H.  3.5-4.5 ;  G.  3 ;  Inf. ;  III. 

B.  The  'finely  powdered  mineral  is  soluble  in  boiling  concentrated  HCl, 

leaving  no  residue  of  silica. 
1.   Yields  little  or  no  water. 

Hematite,   Fe203 ; '  Red  to  reddish  black ;     H.   5.5-6  ; 

G.  5.10;   Inf.;    Ill;    p.  343. 
2u 


(358  MINERALOGY 

2.    Yields  water. 

LIMONITE,  Fe403(OH)6;  Yellow,  brown  to  brownish 
black ;  Silky,  dull ;  H.  5-5.5  ;  G.  3.80 ;  Inf. ;  Mas- 
sive ;  p.  363. 

Gothite,  FeO(OH) ;  Yellow,  brown  to  brownish  black ; 
Dull;  H.  5-5;  G.  4.37 ;  Inf.;  IV;  p.  363. 

Xanthosiderite,  Fe20(OH)4;  Golden-yellow  to  brown; 
Silky,  pitchy ;  H.  2.5  ;  Inf. ;  Earthy,  acicular. 

Turgite,  Fe4O5(OH)2,  Red  to  reddish  brown ;  Dull ;  H. 
5-6;  G. '4.14;  Inf.;  Incrusted. 

a.  Fused  with  soda  it  yields  a  sulphur  reaction. 
Planoferrite,  Fe2(OH)4S04.13H2O ;  Yellowish  green ;  H.3. 

Compare  sulphates  of  iron,  p.  635. 

b.  After  the  separation  of  iron  it  shows  magnesium ;  §  c, 

p.  567. 

Pyroaurite,  Fe(OH)3.3  Mg(OH)2.3  H2O  ;  Golden-yellow 
to  silver-white ;  Pearly;  H.  2-3 ;  Inf.;  G.  2.07  ;  III. 

C.  Gelatinizes  with  HCl;  p.  591. 

1.  Yields  little  or  no  water  in  the  closed  tube. 

ALLANITE,  (Ca.Fe)2(Al.Ce.Fe)2(Al.OH)(Si04)3 ;  Brown 
to  pitch-black  ;  Resinous  ;  H.  5.5-6  ;  G.  3.90  ;  F.  2.5  ; 
V ;  p.  468. 

ILVAITE,  CaFe2(Fe.OH)(SiO4)2;  Iron-black;  H.  5.5-6; 
G.  4.05;  F.  2.5;  IV;  p.  472. 

FAYALITE,  Fe2SiO4 ;    Yellow  to  dark  yellowish  green ; 

Resinous ;   H.  6.5  ;   G.  4.32  ;   F.  4  ;   IV  ;   p.  450. 
a.   Fused  with  soda  and  niter  on  wire,  shows  manganese  ; 
§  6,  P-  574. 

Hortonolite,  (Fe.Mg.Mn)2SiO4 ;  Yellow  to  dark  yel- 
lowish green  ;  Resinous  ;  H.  6.5  ;  G.  4.03  ;  F.  4.5  ;  IV. 

Kenebelite,  (Fe.Mn.Mg)2SiO4;  Gray,  brown,  green; 
Greasy;  Cl.  pinacoidal;  H.  6.5;  G.  3.95;  IV. 

2.  Yields  water  in  the  closed  tube. 

Cronstedtite,  H8Fe"4Fe'"4Si302o ;  Black  to  brownish 
black ;  Cl.  basal ;  H.  3.5  ;  G.  3.35  ;  F.  4  ;  III. 

Thuringite;  HisFesCAl.Fe^SieOu ;  Olive  to  pistachio- 
green;  Dull;  H.  2.5;  G.  3.18;  F.  4 ;  Compact. 

D.  Decomposed  in   HCl  without  gelatinizing,  leaving  a  residue  of 

silica;  §  d,  p.  591. 

1.   Fused  with  soda  and  niter  on  wire  it  shows  manganese, 
§  b,  p.  574. 


IRON   MINERALS  659 

Pyrosmalite,     H7(Fe.Cl)(Fe.Mn)4(SiO4)4 ;     Olive-green 

to  brown;   Cl.  basal ;   H.  4-4.5  ;   G.  3.16;   F.  3 ;    III. 
ASTROPHYLLITE,         (K.Na.H)4.(Fe.Mn.Mg.Ca)4  Ti- 

(SiO4)4;    Bronze  to    golden-yellow;    Cl.  pinacoidal  • 

H.  3  ;   G.  3.35  ;   F.  3  ;   IV. 
2.    Shows  no  manganese. 

LEPIDOMELANE,     (K.H)2Fe"2(Fe.Al)2(SiO4)3;    Black 

to  greenish    black;    Cl.  basal;    H.   3 ;    G.   3.1;    F 

4.5-5;   V. 
ANDRADITE,  Ca3Fe2(SiO4)3 ;    Wine,  yellowish,  green, 

brown;   H.  7 ;  G.  3.85 ;   F.  3.5 ;   I;   p.  444. 
Stilpnomelane,  H6(Fe.Mg)2(Fe.Al)2Si5O18 ;    Greenish   to 

yellowish  bronze;  H.  3  ;  G,  2.75;  F.  4.5  ;  Foliated. 
Hisingerite,  H,  Fe,  Fe,  Mg,  Si,  O?;   Black  to  brown- 
ish black;  H.  3;   2.75;   F.  Difficult ;  Amorphous. 
Chloropal,  H6Fe2(SiO4)3.2  H2O ;    White,  olive,  blackish, 

yellowish  green;    H.   2.5-3;    G.  1.87;    F.  Difficult; 

Amorphous. 

E.    Insoluble  or  only  slightly  attacked  by  HCl. 
1.    Yields  little  or  no  water. 

a.   In  S.  Ph.  an  emerald  green  bead  (chromium). 

CHROMITE,    FeCr204;     Brownish  black ;     H.    7.5-8: 

G.  4.45  ;   Inf. ;   I ;   p.  376. 
6.   The  S.  Ph.  bead  reduced  with  tin  on  coal,  then  dissolved 

in  dilute  HCl,  shows  tungsten,  §  6,  p.  587. 
WOLFRAMITE,      (Mn.Fe)WO4 ;      Brown     to    black; 

Cl.  pinacoidal ;  H.  5-5.5  ;  G.  7.35  ;    F.  4  ;   V  ;  p.  542. 

c.  Fuses  with  intumescence  and  colors   the   flame  yellow 

(sodium). 
ARFVEDSONITE,          (Fe.Na2.Ca)4(Si03)4.Fe2(Al.Fe)>- 

Si2Oi2;     Black;     Cl.    prismatic;     H.    6 ;     G.    3.45; 

F.  3.5;  V;  p.  419. 
Crocidolite,     NaFe(Si03)2(Fe.Mg.Ca)SiO3 ;     Lavender 

blue;  H.  6-6.5;  G.  3.43 ;   F.  3.5 ;   Fibrous. 
Riebeckite,     2  NaFe(SiO3)2.(Fe.Ca)SiO3 ;'     Black;     Cl. 

prismatic;   H.  6-6.5 ;  G.3.43;   F.  3 ;   V. 
JEnigmatite,  (Fe.Mn)(Fe.Al).Na,    (Ti.Si)O?;    Black, 

Cl.  prismatic  ;   H.  6  ;   G.  3.75  ;   F.  3  ;   VI. 

d.  Fuses  quietly,  or  with  difficulty. 

ALMANDITE,  Fe3Al2(SiO4)3 ;     Deep  to  brownish  red  ; 
H.  7-7.5;   G.  4.15;   F.  3 ;   I;   p.  444. 


660  MINERALOGY 

Babingtonite,      (Ca.Mn.Fe)Si03.Fe2(SiO3)3;       Greenish 

black  to  black;  H.  5.5-6 ;  G.  3.36  ;  F.  3-3.5;  VI. 
ACMITE,  NaFe(SiOs)2;    Greenish  to  brownish  black; 

Cl.  prismatic  ;  H.  6-6.5 ;  G.  3.50;   V. 
HYPERSTHENE,    (Mg.Fe)Si03;     Greenish    black    to 

bronze-brown;    Cl.   pinacoidal;    H.  5-6;    G.   3.45; 

F.  5;  IV;  p.  421. 
2.    Yields  water. 

BIOTITE,    (K.H)2(Mg.Fe)2(Al.Fe)2(Si04)s ;     Green    to 

greenish    black;      C.    basal;    H.    2.5-3;     G.    2.90; 

F.  5;   V;   p.  492. 
Chloropal,    H6Fe2(SiO4)3.2  H20 ;     Greenish    yellow    to 

pistachio-green;     H.  2-4;     G.    3.87;     F.    Difficult; 

Amorphous. 
Compare  minerals  containing  small  amounts  of  iron  in 

sections  below,  some  of  which  may  at  times  become 

magnetic. 
VIII.   After  intense  ignition  in  the  forceps,  it  yields  an  alkaline 

reaction  with  turmeric  paper,  hardness  below  5. 
.4.    Effervesces  with  hot  dilute   HCl,  and  dissolves,  leaving  little  or 

no  residue  when  pure  (carbonates). 

1.  After  intense  ignition  in  the  forceps  it  yields  a  green  flame 

(barium). 
WITHERITE,  BaC03 ;  White  to  gray;  H.  3.5  ;  G.  4.3  ; 

F.  2.5-3;   IV;   p.  394. 

BARYTOCALCITE,  BaCa(C03)2 ;  White,  gray,  yellow, 
green  ;  Cl.  prismatic  ;  H.  4  ;  G.  3.65  ;  F.  Difficult  ; 
V ;  p.  395. 

Bromlite,  (Ca.Ba)C03 ;    White,  gray,  cream  ;   H.  4^.5; 

G.  3.72 ;   F.  Difficult ;   IV. 

2.  It  yields  a  red  flame  (strontium). 

STRONTIANITE,  SrCO3 ;  White,  gray,  yellow,  green; 
Cl.  prismatic ;  H.  3.5-4 ;  G.  3.70;  IV;  p.  395. 

3.  After  intense  ignition  it  yields  a  carmine  flame  (calcium). 

CALCITE,  CaCO3;    White  and  variously  tinted;    Cl. 

rhombohedral ;  H.  3  ;  G.  2.72  ;  Inf. ;  III ;  p.  379. 
ARAGONITE,  CaCO:5 ;    White   and   variously   tinted; 

Cl.  pinacoidal ;   H.  3.5-4  ;   Inf. ;   IV ;   p.  392. 
DOLOMITE.  CaMg(C03)2 ;  White  and  variously  tinted  ; 
Cl.  rhombohedral ;  H.  3-4 ;  G.  2.85  ;  Inf. ;  III ;  p.  386. 
a.   Yields  a  fluorine  reaction ;  §  a,  p.  589. 


MINERALS   ALKALINE   AFTER  IGNITION  661 

Parisite,    Ce(CeF)(CaF)(CO3)3 ;    Yellowish    brown    to 
brown;  Cl.  basal ;   H.  4.5 ;   Inf;  G.  4.36;   III. 

b.  In  the  closed  tube  yields  water. 

Thaumasite,      CaCO3.CaSiO3CaSO4.15  H20 ;      White; 
H.  3.5;   G.  1.87;   F.  Difficult;   III;  Fibrous. 

c.  In  S.  Ph.   a  green  bead  (uranium). 

Uranothallite,     Ca2U(C03)4.10  H2O;     Yellowish    green; 
H.  2.5-3  ;  IV ;   Tabular. 

4.  Upon  ignition  in  the   forceps  it  yields  an  intense  yellow 

flame  (sodium). 
a.    In  the  closed  tube  it  yields  water, 

GAYLUSSITE,   Na2Ca(CO3)2.5  H2O ;    White  to   gray; 

Cl.  prismatic;  H.  2-3 ;   G.  1.99;    F.  1.5;  V;  p.  401. 
Dawsonite,,  Na(A1.2  OH)CO3 ;     White;     H.    3.5;    G. 

2.40  ;   F.  4.5-5  ;   V  ;   Radiated. 
Pirssonite,  Na2Ca(C03)2.2  H2O  ;     White   to   gray ;    H. 

3-3.5;   G.  2.35;   F.  1.5;   IV. 
6.    In  the  closed  tube  it  yields  no  water. 

The    nitric    acid    solution    yields    a    precipitate    with 

AgNO3  (chlorine). 
Northupite,     MgNa2(CO3)2Cl ;     Colorless      to    brown; 

H.  3.5-4 ;   G.  2.38 ;    F.  1  ;   I. 
Fused  with  soda  it  yields  a  sulphur  reaction. 
Tychite,    MgNa6(C03)4S04;     Colorless;     H.    3.5;     G. 

2.58  ;   F.  1 ;  I. 

5.  It  contains  magnesium  as  the  base ;  §  a,  p.  567. 
a.    It  yields  no  water  in  the  closed  tube. 

MAGNESITE,  MgC03 ;    White,  gray,  yellow,  brown; 

Cl.  rhombohedral ;    H.  3.5-4.5;    G.  3.06;   Inf.;   Ill; 

p.  385. 

Breunnerite,  (Mg.Fe)C03,  Brown,  gray;    Cl.  rhombo- 
hedral ;  3.5-4.5  ;   G.  3  ;  Inf. ;  III ;  p.  385. 
6.    Yields  water. 

Hydromagnesite,    Mg2(Mg.OH)2(CO3)3.3  H20 ;    White; 

H.  3.5;   G.  2.15;  Inf.;   V. 
Lansfordite,     Mg2(Mg.OH)2(CO3)3.21  H2O  ;    White;  H. 

2.5;   G.  1.54;   Cl.  basal;   Inf.;   VI. 
Hydrogioberite,     (Mg.OH)2CO3.2  H20 ;      White ;      Cl . 

prismatic;  G.  2.16;  Inf.;   Compact. 
Nesquehonite,    MgCO3.3  H2O  ;    White ;    Cl.    prismatic  ; 

H.  2.5;  G.  1.84;   Inf.;   IV. 


662  MINERALOGY 

B.  Fused  with  soda  and  coal  dust  in  the  R.  F.  on  coal  it  yields  a 

sulphur  reaction  on  silver;  §  6,  p.  587. 

1.  Yields  a  barium  flame  (green). 

BARITE,  BaSO4;  White,  blue,  yellow,  red;  Cl.  basal 
and  prismatic  ;  H.  3.5  ;  G.  4.5;  F.  4  ;  IV;  p.  528. 

2.  Yields  a  red  flame  (strontium). 

CELESTITE,  SrSO4 ;  White,  blue,  red;  Cl.  basal, 
prismatic  ;  H.  3-3.5  ;  G.  3.97  ;  F.  3.5-4  ;  IV  ;  p.  530. 

3.  It  yields  a  calcium  flame  (carmine),  or  a  calcium  reaction; 

§  a,  p.  566. 

a.  Yields  no  water. 

ANHYDRITE,  CaSO4 ;  White,  blue,  red,  gray ;  Cl.  3 
pinacoidals  ;  H.  3-3.5  ;  G.  2.95  ;  F.  3-3.5  ;  IV  ;  p.  531. 

Glauberite,  CaNa^SO^ ;  White  to  gray ;  Cl.  Basal ; 
H.  2.5-3;  G.  2.75;  F.  1.5-2;  V. 

Oldhamite,  CaS ;  Pale  chestnut-brown;  Cl.  cubic; 
H.  4;.  G.  2.58;  F.  Difficult ;  I. 

b.  Yields  water. 

GYPSUM,  CaS04.2H2O;  White,  gray,  reddish;  Cl. 
pinacoidal ;  H.  2  ;  G.  2.32  ;  F.  3  ;  V  ;  p.  536. 

c.  Yields  a  yellow  flame  (sodium). 

Whattevillite,  CaNa2(SO4)2.4  H2O  ;  White  ;  Silky  ; 
G.  1.81;  F.  1.5-2;  Acicular. 

d.  Yields  a  potassium  flame  through  the  blue  glass. 
Polyhalite,    Ca,MgK2(SO4)4.2  H2O ;    Brick-red    to  yel- 
low;   Cl.  pinacoidal;  H.  2.5-3;  G.  2.77;   F.  2;  V?. 

Syngenite,  CaK2(SO4)2.2  H2O  ;    White  ;   Cl.  pinacoidal ; 

H.  2.5;  G.  2.60;  F.  1.5-2;  V. 
Ettringite,    (Ca.OH)6(SO4)3.2  A1(OH)3.24  H2O  ;    White  ; 

H.  2-2.5;  G.  1.75;   F.  3 ;   III. 

C.  Fused   with    potassium   bisulphate   in    the   closed   tube    yields 

a  flourine  reaction;  §  a,  p.  589. 
1.   Yields  little  or  no  water. 

a.   Will  not  become  blue  with  cobalt  solution. 

FLUORITE,  CaF2 ;  White,  green,  yellow,  violet,  pink  ; 

Cl.  Octahedral ;   H.  4  ;   G.  3.18  ;   F.  3  ;  I ;   p.  331. 
6.   Becomes  blue  with  cobalt  solution. 

CRYOLITE,  Na3AlF6 ;    White  to  brownish  ;    Cl.  pina- 
coidal;  H.  2.5;  G.  2.97;  F.  1.5;  V;  p.  333. 
Chiolite,    5NaF.3AlF3;     Snow-white;    H.   3.5-4;    G. 
2.95;   F.  1.5;   II. 


BORATES  663 

2.    Yields  water. 

Thomsenolite,      NaCaAlF6.H20 ;     White    to     brown; 

Cl.  basal;   H.  2;   G.  2.93;   F.  1.5;    V. 
Pachnolite,  NaCaAlF6.H20 ;    White ;    H.  3 ;    G.    2.98 ; 

F.  1.5;  V. 

Gearksutite,    CaF2Al(F.OH)3.H2O ;   White;    Dull;    H. 

2;   F.  1.5;  Earthy. 
Prosopite,    CaF2.2Al(F.OH)3;      White    to    gray;     Cl. 

prismatic ;   H.  4.5 ;  G.  2.80 ;   F.  Difficult ;   V. 
D.    Not  included  above. 

1.  Becomes  pink  with  cobalt  solution  (magnesia). 
a.   Yields  water. 

BRUCITE,  Mg(OH)2 ;  White,  gray,  green  ;  Pearly ;    Cl. 

basal;  H.  2.5 ;   G.  2.39 ;  Inf.;  Ill;  p.  362. 
TALC,     H2Mg3(Si03)4;      Apple-green,     gray,     white; 

Pearly;  Cl.  basal;  H.I;  G.  2.80;  F.  5 ;  Foliated. 
6.    Yields  no  water. 

Periclase,  MgO ;   White,  gray,  dark  green ;   Cl.  cubic ; 

H.  5.5  ;  G.  3.8  ;  Inf. ;  I. 

2.  Becomes  blue  with  cobalt  solution. 

Hydrotalcite,     Mg3Al(OH)6.3H2O ;      White;      Pearly; 
Cl.  basal ;  H.  2  ;   G.  2.07  ;  Inf. ;  III. 

3.  With  Turner's  flux  it  yields  a  boric  acid  flame  (bright  green). 

ULEXITE,    NaCaB509.8H2O ;    White;     Silky;    H.    1, 

G.  1.55;   F.  1.5;   Fibrous;    p.  524. 

4.  Heated  in  the  closed  tube  yields  iodine. 

Dietzeite,    7  Ca(I03)2.8  CaCrO4 ;     Golden-yellow ;    H. 

3-4  ;  G.  3.70  ;  F.  1.5  ;  V.     Green  in  S.  Ph. 
Lautarite,  Ca(I03)2 ;    Sulphur-yellow  to   colorless ;    Cl. 

prismatic;  N.  3.5-4 ;  G.  4.59 ;  F.  1.5;  V. 
Some  silicates  may  contain  enough  calcite  to  yield  an 

alkaline    reaction    after    ignition;    in    general   their 

hardness  will  be  above  5  and  are  not  placed  in  this 

group ;   compare  the  silicates  beyond. 

IX.   With  Turner's  flux  it  yields  a  bright  green  flame  (boric  acid). 
A.   Soluble  in  HCl 

1.   Yields  little  or  no  water. 

BORACITE,  Mg7Cl2Bi603o ;  White,  gray,  green,  brown; 

H.  7 ;  G.  2.95  ;  F.  3  ;  I ;  p.  522. 
Rhodizite,  K(A1O)2(BO2)3 ;  white ;    H.  8 ;  G.  3.41 ;    F. 

4.5;  I. 


664  MINERALOGY 

• 

Pinakiolite,  3  MgB2O4.Mn"Mn'"2O4  ;    Black  ;    Cl.  pin- 
acoidal ;   H.  6 ;   G.  3.88  ;   F.  5  ;   IV. 
2.   Yields  water. 

a.   In  the  borax  bead  shows  manganese. 

Sussexite,    H(Mn.Mg.Zn)BO3 ;    Gray;     Silky;    H.    3; 

G.  3.12;  F.  2.5;  IV?;  Fibrous. 
6.   The  concentrated  HC1  solution  yields  a  precipitate  with 

H2SO4  (calcium) ;  §  6,  p.  566. 
COLEMANITE;   Ca2B6On.5  H2O  ;     White  ;    Cl.   pina- 

coidal;  H.  4-4.5 ;  G.  2.42 ;   F.  1.5;   V:  p.  524. 
Hydroboracite,    CaMgB6On.6  H2O  ;     White  ;     H.    2 ; 

G.  1.95;   F.  1.5;   Fibrous,  foliated. 
Mr i-    Bechilite,  CaB4O7.4  H2O  ;   Massive. 
c.    Yields  magnesium  reaction  §  a,  p.  567. 

Szaibelyite,  Mg5B4On.li  H2O ;    White   to   yellow;    H. 

3-4;   G.  3.0;   Nodular,  acicular. 
Pinnoite,    MgB2O4.3  H2O ;    Sulphur   to    straw-yellow; 

H.  3-4  ;   G.  3.3  ;   F.  3  ;   II. 

Heintzite,  KMg2BnO19.7  H20  ;   White;    Cl.  pinacoidal ; 
H.  4-5;  G.  2.13;  F.  1 ;  V. 

B.  The  finely  powdered  mineral  gelatinizes  with  HCL 

DATOLITE,  Ca(B.OH)Si04 ;  White,  pale  green,  yellow ; 

H.  5-5.5;   G.  2.95;  F.  2.5;  V;  p.  463. 
Bakerite,  8  CaO,  2  B2O3,  6  SiO2.6  H2O  ;   White  to  pale 

green;  Dull;  H.  4.5 ;  G.  2.13;  F.I;  Massive. 

C.  Insoluble  in  HCl  or  only  slightly  soluble. 

1.  Yields  water. 

Howlite,    H5Ca2B5Si014 ;     White;     H.    3.5;     G.   2.59; 

F.  Difficult ;  Nodular,  fibrous. 

Hambergite,  Be(Be.OH)BO3 ;    Grayish  white ;    Cl.  pin- 
acoidal ;  H.  7.5  ;  G.  2.35  ;  Inf ;   IV. 

2.  Yields  little  or  no  water. 

AXINITE,       H2(Ca.Fe.Mn)4(BO)Al3(SiO4)5 ;        Clove- 
brown,  green,  yellow,  gray  ;  Cl.  pinacoidal ;  H.  6.5-7  ; 

G.  3.29;  F.  2.5-3;  VI;  p.  469. 
TOURMALINE,  complex  borosilicate  ;  Various  colors ; 

H.  7-7.5  ;  G.  3.14  ;  F.  3-5  ;  III ;  p.  473. 
Danburite,  CaB2(SiO4)2 ;    White  to  pale  yellow ;  H.   7  ; 

G.  3.00;  F.  3.5-4;  Nodular,  fibrous. 
Homilite,    (Ca.Fe)3(BO)2(SiO4)2 ;     Brownish    black    to 

black  ;   Resinous  ;   H.  5  ;   G.  3.38  ;   F.  2 ;   V. 


MANGANESE  MINERALS  665 

Cappelenite,  BaY6B6Si3025;  Greenish  brown  ;  H.  6-6.5; 
G.  4.41;  F.  4-5;  III. 

a.  Yields  a  titanium  reaction  §  b,  p.  570. 
Warwickite,    (Mg.Fe)4TiB209 ;  Hair-brown  ;    Dull ;  Cl. 

pinacoidal ;    H.  3-4  ;   G.  3.36  ;   Inf. ;   IV. 

b.  Becomes  blue  with  cobalt  solution. 

Jeremejevite,  A1BO3 ;    White  to  pale  yellow;    H.  6.5; 

G.  3.28;   Inf.;   III. 
3.    Contains  the  rare  earths;  §  a,  p.  572. 

Melanocerite,  Si,  Ta,  B,  Ce,  La,  Di,  Y,  Ca,  Na,  H,  F,  ? ; 

Deep  brown  to  black ;    Inf. ;   Greasy ;    H.   5-6 ;  G. 

4.13;  III. 
Caryocerite,  Si,  Ta,  B,  Th,  Ce,  La,  Di,  Y,  Ca,  Na,  H, 

F;    Nut-brown;    Greasy;    H.  5-6;    G.  4.29;    Inf.; 

III. 

X.    The  finely  powdered  mineral  dissolved  in  borax  on  wire  in  con- 
siderable quantities  in  the  O.F.  is  violet-red    when 

cold  (manganese). 

A .  The  borax  bead  is  powdered,  dissolved  in  1  cc.  of  dilute  HCl  and 

the  solution  is  reduced  with  tin  when  it  becomes  blue; 

§  b,  p.  587. 
HUBNERITE,   MnWO4  ;    Brown  to    brownish  black  ; 

Resinous ;    Cl.    pinacoidal ;    H.  5-5.5 ;    G.  7.2 ;    F. 

4  ;  V  ;  p.  542. 
Mangano-columbite,  MnCb206 ;  Reddish  to  dark  brown  ; 

Resinous;   Cl.  pinacoidal ;    H.  5 ;   G.  6.6;   Inf;  IV. 

B.  The  solution  is  green  (vanadium). 

Ardennite,  I^M^AUVSi^s? ;  Yellow  to  yellowish 
brown;  Resinous;  Cl.  pinacoidal ;  H.  6-7  ;  G.  3.65; 
F.  2-2;  IV. 

C.  The  solution  is  colorless  or  nearly  so. 
1.    Fusibility  below  5. 

or.    Gelatinizes  with  HCl. 

— .    Yields  no  water.  !•• 

TEPHROITE,  Mn2SiO4 ;  Smoky-gray  to  brownish  red ; 

Cl.  pinacoidal ;   H.  5.5-6 ;  G.  4.04;   F.  3.5  ;  IV. 
Glaucochroite,   CaMnSi04 ;    Blue,    green;    H.    6 ;    G, 

3.40;  F.  3.5;  IV. 
The  HNO3  solution  yield  a  precipitate  with  silver  nitrate 

(chlorine). 
Eudialyte,  Si,  Al,  Ca,  Na,  K,    O,    Cl,    (S04)(CO3)?; 


666  MINERALOGY   ! 

Rose  to  brown;    Cl.  basal;    H.  5-5.5;    G.  2.92;    F. 

3 ;  III. 

-K   Yields  water. 
Ganophyallite,    Mn7(AlO)2(SiO3)8.6  H20 ;    Brown;    Cl. 

basal;  H.  4-4.5 ;  G.  2.84;  F.  3 ;  V;  Foliated. 
Dissolves  in  HC1,  liberating  hydrogen  sulphide. 
Helvite,     (Mn.Fe)2(Mn2S)Be3(S:  O4)3 ;    Yellow,   brown, 

green,  red;  H.  6-6.5 ;   G.  3.20;   F.  4-4.5;   I. 

b.  Decomposed  in  HC1  without  gelatinizing, 
-f .   Yields  water. 

Friedelite,  H7(MnCl)Mn4(Si04)4;  Rose-red;   Cl.  basal; 

H.  4.5;  G.  3.07;  F.  4;  III. 
Bementite,  H2MnSi04 ;  Pale  grayish  yellow ;  Cl.  basal ; 

H.  2.5-3  ;  G.  2.98  ;  F.  3.5 ;  Foliated. 
Inesite,  (Mn.Ca)2(Si03)2.H2O ;    Rose-  to  flesh-red;   Cl. 

pinacoidal ;  H.  6 ;  G.  3.03  ;  F.  3  ;  VI. 
— .   Yields  no  water. 
Trimerite,  Be(Mn.Ca.Fe)SiO4 ;   White  to  salmon-pink; 

Cl.  basal;  H.  6-7  ;  G.  3.47  ;  F.  4-5?;  VI. 
Dissolves  in  HC1  yielding  hydrogen  sulphide. 
ALABANDITE,  MnS ;  Iron-  to  greenish-black;  H. 

3.5-4;  G.  3.95;  F.  3 ;  I;  p.  304. 

c.  Insoluble  in  HC1. 
-f.    Yields  water. 

Piedmontite,  Ca,(Al.OH)2(Al.Mn.Fe)2  (Si04)3 ;  Reddish 
brown;  Cl.  basal;  H.  6.5;  G.  3.5;  F.  3;  V; 
p.  467. 

Carpholite,  Mn(A1.2  OH)2(SiO3)2 ;  Straw-  to  wax-yellow  ; 
Silky  ;  H.  5-5.5  ;  G.  2.93  ;  F.  3  ;  V ;  Fibrous. ' 

— .   Yields  no  water. 

Neptunite,  (Na.K) (Fe.Mn)  TiSi4O12;  Black;  Cl.  pris- 
matic ;  H.  5-6 ;  G.  3.23  ;  F.  3.5  ;  V. 

SPESSARTITE,  Mn3Al2(Si04)3 ;  Brownish  to  garnet- 
red;  H.  7-7.5;  G.  4.2;  F.  3  ;  I;  p.  444. 

Partschinite,  (Mn.Fe)3Al2(SiO4)3 ;  Yellowish  red ;  Greasy ; 
H.  6.5-7;  G.4.00;  F.  3 ;  V. 

RHODONITE,  MnSiO3 ;  Rose-red,  pink,  brown;  Cl. 
prismatic;  H.  6-6.5-;  G.  3.63  ;  F.  3.5  ;  VI;  p.  430. 

Schefferite,  (Ca.Mn)(Mg.Fe)(SiO3)2 ;  Yellowish  to  red- 
dish brown;  Cl.  prismatic;  H.  5-6;  G.  3.5;  F.  4; 
V. 


TITANIUM  AND  TUNGSTEN  MINERALS  667 

Richterite,    (Mg.Mn.Ca.Na2)4(Si03)4 ;    Brown,   yellow, 
rose-red;  Cl.  prismatic ;  H.  5.5-6;  G.3.09;  F.  4;  V. 
2.    Fusibility  above  5. 

a.  Effervesces  with  warm  dilute  HC1  (carbonates). 
RHODOCROSITE,  MnC03 ;  Rose  to  dark  red,  brown; 

Cl.  rhombohedral ;  H.  2.5-4.5  ;  G.  3.52 ;  III ;  p.  390. 
Torrensite,      MnSiO3.MnC03.i  H20 ;     Reddish    gray ; 
Compact. 

b.  Do  not  effervesce  in  HC1. 
+  .    Yields  water. 

WAD,  Impure  oxides  of  manganese;  Gray,  brown, 
black;  Dull;  Massive;  Earthy;  p.  368. 

Pyrochroite,  Mn(OH)2;  White  to  bronze;  Pearly;  Cl. 
basal ;  H.  2.5 ;  G.  3.26  ;  III. 

— .    Yields  no  water. 

Manganosite,  MnO ;    Dark  emerald-green;   Cl.  cubic; 

H.  5-6;  G.  5.18;  I. 

XI.  The  mineral,  well  powdered,  is  dissolved  in  considerable 
quantities  in  the  borax  bead  on  wire.  The  bead  is 
then  powdered  and  dissolved  in  1  cc.  strong  HC1  ; 
to  the  hot  solution  powdered  tin  is  added. 

I .  The  solution  is  violet  (titanium) .  //  the  solution  is  poured  off  the 
tin  and  a  few  drops  of  hydrogen  peroxide  added,  it  be- 
comes a  deep  orange  or  yellow  according  to  the  quantity 
of  titanium  present. 

1.  Gelatinizes  with  HC1.     Fusibility  below  5. 

a.  Fuses  quietly. 

Schorlomite,  Ca3(Fe.Ti.Al)2[(Si.Ti)04]3;  Black;  H.  7- 
7.5;  G.  3.88;  F.  4;  I. 

b.  Fuses  with  intumescence. 

Tscheffkinite,  Si,  Ti,  Th,  Ce,  Fe,  Ca,  O?;  Velvet-black; 
H.  5-5.5  ;  G.  4.55  ;  F.  4 ;  Massive. 

Rinkite,  Na2,  Can,  (Ti.F2)4(Si04)i2 ;  Straw-yellow,  yel- 
lowish brown ;  Cl.  pinacoidal ;  H.  5 ;  G.  3.46 ;  V. 

2.  Do  not  gelatinize  with  HC1. 
a.   Fusibility  below  5. 

TITANITE,  CaTiSi03;  Gray,  brown,  green,  yellow, 
black;  Cl.  prismatic ;  H.  5-5.5;  G.  3.5;  F.  4;  V; 
p.  503. 

Guarinite,  CaTiSiO5 ;  Sulphur-  to  honey-yellow;  Cl. 
pinacoidal ;  H.  6 ;  G.  3.5  ;  F.  4 ;  IV. 


668  MINERALOGY 

Keilhauite,  CaTiSi05(Y.Al.Fe)2Si05;    Brownish  black; 

Cl.  prismatic  ;  H.  6.6;  G.  3.65  ;  F.  4-4.5;  V. 
Neptunite,    (Na.K)(Fe.Mn)TiSi4Oi2;    Black;    Cl.   pris- 
matic; H.  5-6;  G.  3.23;  F.  3.5;  V. 
Benitoite,  BaTiSiO5;   Colorless,  blue;  H.6.5;   G.  3.64 ; 

III. 
Mosandrite,  H^Na*,  Ce2,  Ca10[(Ti.Zr)(OH.F)2]4(Si04)i2; 

Reddish   to  greenish    brown;    Resinous;    Cl.    one; 

H.  4  ;  G.  2.96  ;  Compact. 
b.   Fusibility  above  5. 

PEROVSKITE,  CaTi03 ;  Yellow,  orange,  brown,  black ; 

Cl.  cubic  ;  H.  5.5 ;  G.  4.93  ;  I ;  p.  347. 
RUTILE,  Ti02 ;    Yellow,  reddish  brown  to  black ;   Cl. 

prismatic ;  H.  6-6.5  ;  G.  4.22  ;  Int. ;  II ;  p.  349. 
OCTAHEDRITE,   TiO2 ;    Yellow,  brown,  blue,  black; 

Cl.  basal  and  pyramidic  ;  H.  5.5-6  ;  G.  3.88 ;   II ;  p. 

351. 
BROOKITE,    Ti02;     Hair-brown    to    black;     H.    6; 

G.  3.94;  IV;  p.  351. 
Zirkelite,     (Ca.Fe.UO2)  (Zn.Ti)205 ;    Black ;    Resinous ; 

H.  5;  G.  4.71;  I. 
XII.   The  solution  is  blue  (tungsten)  or  (columbium).    Fusibility 

above  5. 

1.  The  solution  first  assumes  a  violet  color  and  then  becomes 

blue. 

^schynite,  Cb,  Ti,  Th,  Ce,  La,  Ca,  Fe,  0?;  Brownish 
black  to  black ;  Resinous ;  H.  6.5 ;  G.  4.93 ;  Inf. ; 
IV. 

Euxenite,  Cb,  Ti,  Y,  Er,  Ce,  U,  Fe,  H,  O,  ?  ;  Brownish 
black  to  black ;  Resinous  ;  H.  6  ;  G.  5.00 ;  IV. 

Polycrase,  Cb,  Ti,  Y,  Er,  Ce,  U,  Fe,  H,  O,  ? ;  Brownish 
black  to  black ;  H.  6 ;  G.  5.00. 

Pyrochlore,  Cb,  Ti,  Ca,  Na,  O,  Th,  Ce,  Fe,  F?;  Brown- 
ish black  to  black ;  H.  5-5.5 ;  G.  4.28 ;  I. 

2.  The  solution  is  blue  without  passing  through  violet. 

SCHEELITE,  CaW04;    White,  yellow,  green,  brown; 

Cl.  pyramidal ;  H.  4.5-5 ;  G.  6.05  ;  II ;  p.  543. 
Tungstite,  WO3;    Yellow,  greenish  yellow;    dull;  IV; 

Earthy. 
Hatchettolite,  Cb,  Ta,  U,  Ca,  O,  H,  Fe  ?  ;    Yellowish 

brown ;  Resinous ;  H.  5 ;  G.  4.85 ;  I. 


TITANIUM   AND   TUNGSTEN  MINERALS  669 

Microlite,  Ta,  Cb,  Ca,  Na,  C,  F,  H  ?  ;  Pale  yellow  to 
brown ;  Resinous  ;  H.  5.5  ;  G.  5.5  ;  I. 

FERGUSONITE,  (Y.Er.Ce)(Cb.Ta)04 ;  Brownish 
black;  Resinous;  H.  5.5-6;  G.  5.80;  II;  Mas- 
sive. 

Sipylite,  (Er.Ce.La.DLH3)CbO4,  ?  ;  Brownish  black; 
Resinous  ;  H.  6 ;  G.  4.9  ;  II ;  Massive. 

Compare,  Columbite  p.  634. 

Wohlerite,  Si,  Zr,  Cb,  Ca,  Na,  O,  ?  ;  Straw  to  brownish 
yellow;  Cl .  pinacoidal ;  H.  5.5-6  ;  G.  3.44;   F.  3-3.5; 
V. 
XIII.    The  mineral  in  powder  is  dissolved  in  the  S.  Ph.  bead. 

A.  The  S.  Ph.  bead  in  R.  F.  is  yellow  (nickel). 

1.  Effervesces  in  hot  dilute  HC1  (carbonates). 

Zaratite,  (Ni.OH)2CO3.Ni(OH)2.4  H2O  ;  Emerald-green; 
H.  3  ;  G.  2.63  ;  F.  Inf. ;  Massive. 

2.  Do  not  effervesce  with  HC1. 

GENTHITE,  H4Ni2Mg2(Si04)3.4  H20 ;  Pale  to  deep- 
green  ;  Dull ;  H.  3-4 ;  G.  2.41 ;  Inf. ;  Amorph. ;  p.  500. 

Morenosite,  NiSO4.7  H2O ;  Apple-green,  greenish 
white ;  Cl.  pinacoidal ;  H.  2 ;  G.  2.00 ;  IV. 

B.  The  S.  Ph.  bead  is  blue  in  both  flames  (cobalt). 
1.   Effervesces  with  HC1. 

Sphaerocobaltite,    CoC03;    Rose-red;    H.  4;     G.  4.07; 

Inf.;  III. 
Remingtonite,  CoC03,  +  H2O  ;  Rose-red ;  Soft ;  Earthy ; 

Yields  water. 

C.  The  S.  Ph.  bead  is  green  in  R.  F.  when  cold  (chromium  or  uranium) . 

1.  Shows  chromium;  §  ft,  p.  569. 

UVAROVITE,  Ca3Cr2(SiO4)3;  Emerald-green;  H.  7.5; 
G.3.42;  Inf.;  I;  p.  445. 

Fuchsite,  H2K(Al.Cr)3(SiO4)3 ;  Emerald-green;  Cl. 
basal ;  H.  2.5 ;  G.  2.86 ;  F.  5 ;  V ;  Micaceous. 

Kammererite,  H8Mg5(Al.Cr)2Si3Oi8 ;  Garnet  to  peach- 
blossom-red  ;  Cl.  micaceous ;  H.  2-2.5 ;  G.  2.75 ;  F. 
5;V. 

2.  Shows  uranium ;  §  b,  p.  576. 

Uranophane,  CaU2Si2On.5  H2O ;  Honey-,  lemon-,  or 
straw-yellow ;  H.  2-3  ;  G.  3.86 ;  Inf. ;  VI. 

Uranophilite,CaU8S2O3i.25H20;  Yellow;  G.  3.75;  Vel- 
vety, incrusted ;  Inf. 


670  MINERALOGY 

a.   Yields  a  reaction  for  vanadium  (§  6,  p.  576),  as  well  as 

for  uranium. 

Carnotite,  K2(UO2)2(VO4)2.3  H2O ;  Yellow;  Dull;  H. 
2.5;  F.  3;  Earthy. 

3.  Shows  vanadium ;  §  b,  p.  576. 

Roscoelite,  H8K2(Mg.Fe)(Al.V)4(SiO3)i2?;  Clove-brown 
to  brown-green;  Pearly;  Cl.  basal;  H.  2?;  G. 
2.93 ;  F.  3. 

4.  Shows  molybdenum ;  §  6,  p.  586. 

a.  The  concentrated  HC1  solution  yields  a  white  precipi- 

tate with  H2SO4. 

Powellite,  CaMoO4 ;  Colorless,  green,  yellow ;  Resin- 
ous; H.  3.5;  G.  4.52;  F.  4;  II. 

b.  Yields  no  precipitate  with  H2S04. 
Belonesite,  MgMoO4 ;  White  ;  F.  4-5 ;  II. 

XIV.   Minerals  not  included  in  the  preceding  groups.     They  are 
classified  according  to  their  fusibility,  solubility  in  acids, 
and  hardness. 
A.   Fusibility  below  5. 
1.   Hardness  below  5. 

-f.   Yields  water, 
a.   Soluble  in  HC1  or  decomposed  with  the  separation  of 

silica. 

«.   Fuses  with  intumescence  or  exfoliates. 
VERMICULITE,   Si,   Al,   Mg,   O,    (H2O),    ?;    Yellow, 
brown,    light   to   dark  green ;     Pearly ;     Cl.    basal ; 
H.  1.5  ;  G.  4-4.5  ;  F.  4.5. 
The  dilute  HC1  solution  yields  a  precipitate  with  H2S04 

(barium). 
HARMOTOME,  (Ba.K2)Al2Si5014.5  H2O  ;    White;    Cl. 

pinacoidal;  H.  4.5.;   G.  2.47;   F.  3 ;  V;  p.  482. 
Brewsterite,      H4(Sr.Ba.Ca)Al2(SiO3)6.3  H20  ;      White, 
yellow,  gray ;  CJ.  pinacoidal ;  H.  5  ;  G.  2.45  ;  F.  3  ;  V. 
Wellsite,  (Ca.K2.Ba) Al2Si3O10.3  H2O ;    White;    H.  4-4.5; 

G.  2.32  ;  F.  3  ;  V. 
The  dilute   HC1   solution    yields  no   precipitate   with 

H2S04. 

HEULANDITE,     H4(Ca.Na2)Al2(SiO3)6.3  H20  ;    White, 
yellow,  red;    Pearly,  Cl.  pinacoidal;    H.  3.5-4;    G. 
2.20;  F.  3;   V;  p.  481. 
STILBITE,  H4(Ca.Na2) Al2(SiO3)6.4  H2O  ;    White,    yel- 


FUSIBILITY  AND  HARDNESS  BELOW  5  671 

low,  brown,  red ;  Pearly;  Cl.   pinacoidal ;  H.  3.5-4; 

G.  2.36;  F.  3;  V;  p.  483. 
Gmelinite,    (Na2.Ca)Al2(Si08)4.6  H2O  ;    White,    yellow, 

flesh-red;    Cl.  prismatic;    H.  4.5;    G.  2.10;    F.  3; 

III ;   Rhombohedral. 
Epistilbite,    H4(Ca.Na2)Al2(Si03)63  H20  ;     White  ;    Cl. 

pinacoidal ;  H.  4-4.5  ;  G.  2.25  ;  V. 
ft.   Fuses  quietly. 
DEWEYLITE,    H4Mg4(Si04)3.4  H2O ;     Yellow,    brown, 

apple-green,  resinous;    H.  2.5-4;    G.  2.40;    F.  4-5; 

Amorphous. 

b.  Gelatinizes  in  HC1. 

a.   Shows  calcium ;  §  6,  p.  566. 

Gyrolite,  H2Ca2(Si03)3.H20  ;  White  ;    H.  3.4  ;    F.  3  ; 

Radiated. 
Okenite,  H2Ca(Si03)2.H2O  ;  White,  cream,  bluish  white ; 

Dull ;  H.  4.5-5  ;  G.  2.28  ;  F.  2.5-3  ;  Fibrous,  compact. 
ft.  Becomes  blue  with  cobalt  solution. 
LAUMONTITE,    (Ca,    (A1.2  OH)2(Si205)2  H20 ;   White 

to  gray;  H.  3.5-4;  G.  2.30;  F.  2.5;  V;  p.  479. 
Gismondite,  (Ca.K2)Al2(SiO3)4.4H2O;   White;    H.4.  5; 

G.  2.26 ;  F.  3 ;  V. 
Levynite,     CaAl(A1.2OH)(Si03)3.4H20  ;    White,    gray 

red;  H.  4.5;  G.  2.13;  F.  2.5;  III. 
y.   The    dilute  HC1  solution    yields  a  precipitate  with 

H2SO4  (barium). 
Edingtonite,        BaAl(A1.2  OH)  (SiO3)3.2  H20  ;       White, 

pink;  Cl.  prismatic;  H.  4.5-5;  G.  2,77;  F.  2.5;  IV. 
8.   Shows  magnesium  with  cobalt  solution ;  p.  567. 
Spadaite,    H2Mg5(Si03)6.3  H2O  ;     Flesh-red;     Pearly; 

H.  2.5  ;  F.  4  ;  Massive. 

c.  Not  soluble  or  decomposed  in  HC1. 

a.   Yields  a  red  flame  with  lithium  flux  (lithium), 

LEPIDOLITE,  LiKAl(OH.F)2Al(Si03)3;  Lilac,  grayish 
white  ;  Pearly ;  Cl.  Basal ;  H.  2.5-4 ;  G.  2.85 ;  2 ;  V ; 
p.  495. 

Cookeite,  Li(A1.2  OH)3(Si03)2  ;  White;  Pearly;  Cl. 
basal ;  H.  2.5  ;  G.  2.65  ;  F.  4 ;  V. 

ft.  Decomposed  with  hot  concentrated  H2S04 ;  all  mica- 
ceous. 

*.   Shows  potassium. 


672  MINERALOGY 

BIOTITE,  (K.H)2(Mg.Fe)2(Al.Fe2)2(SiO4)3;  Green,  yel- 
low, black;  Cl.  basal;  H.  2.5;  G.  2.90;  F.  5;  V; 
p.  492. 

PHLOGOPITE,  (H.K)8(Mg.Fe)8(ALFe)(SiO4)3 ;  Yel- 
lowish brown,  green,  white;  H.  2.5-3;  G.  2.86; 

F.  4.5-5;  V;  p.  494. 
**.   Shows  no  potassium. 

CLINOCHLORE,  H8Mg5Al2Si3Oi8 ;  Green  of  various 
shades;  H.  2.2;  G.  2.72;  F.  5;  V;  p.  497. 

y.   Not  decomposed  with  H2SO4. 

*.  Yields  a  potassium  flame  through  the  blue 
glass ;  Micaceous. 

MUSCOVITE,  H2KAl3(SiO4)3 ;  White,  brown,  green, 
yellow;  H.  2-2.5  ;  G.  2.86 ;  F.  4.5-5;  V;  p.  489. 

Alurgite,  H(K.Mg.OH)2(A1.0H)Al(SiO3)4;    Rose-red  to 
deep  red ;  Pearly ;  H.  3  ;  V. 
Yields  water ;  Not  micaceous. 

HARMOTOME,  (Ba.K2) Al2Si5O14.5  H2O ;  White;  Cl. 
pinacoidai ;  H.  4.5 ;  G.  2.47 ;  F.  3 ;  V ;  p.  482. 

Mordenite,  (K2.Na2.Ca)Al2SiioO24.6f  H20 ;  White, 
yellow,  pink;  Cl.  pinacoidai ;  H.  3-4;  G.  2.15;  F. 
4-5;  V. 

**.   Shows  no  potassium. 

Paragonite,  H2NaAl3(SiO4)3 ;  Yellow  to  grayish  white; 
Pearly ;  H.  2.5-3  ;  G.  2.89  ;  F.  5 ;  V. 

MARGARITE,  H2CaAl4Si2Oi2 ;  Pink,  gray,  white; 
Pearly;  H.  3.5-4;  G.  3.05;  F.  4-4.5;  V;  p.  496. 

— .   Yields  no  water. 

Leucophanite,  Na(BeF)Ca(SiO3)2;    Pale  green,  yellow, 

white ;  Cl.  basal ;  H.  4 ;  G.  2.96  ;  F.  2.5-3 ;  IV. 
2.    Hardness  above  5. 

+.   Yields  water  in  the  closed  tube,  some  only  upon 

intense  ignition, 
a.   Decomposed  or  soluble  in  HC1. 

<*.    Fuses  with  swelling  or -intumescence. 

CHABAZITE,  (Ca.Na2)Al2(SiO3)4.6  H2O  ;  White,  yel- 
low, red  ;  Cl.  rhombohedral ;  H.  5;  G.  2.12;  F.  3; 
III ;  p.  484. 

APOPHYLLITE,  H7KCa4(SiO3)8.4i  H2O ;  White,  yel- 
low, rose,  pale  green ;  H.  5 ;  Cl.  basal ;  Pearly ; 

G.  2.35;  F.  2;  II.  p.  480. 


FUSIBILITY   BELOW,   HARDNESS   ABOVE  5  673 

Brewsterite,      H4(Sr.Ba.Ca)Al2(SiOs)6.3  H20 ;      White, 

yellow,  red ;  Cl.  pinacoidal ;  H.  5  ;  G.  2.45 ;  F.  3  ;  V ; 

Shows  barium. 
Phillipsite,     (Ca.K2.Na2)Al2Si4Oi2.4H20;     White;     Cl. 

pinacoidal ;   H.  5  ;   G.  2.20 ;   F.  3  ;   V. 
Faujasite,  H2(Ca.Na2)Al2(SiO3)5.9  H2O ;  White,  brown; 

Cl.  octahedral;  H.  5;  G.  1.92;   F.  3;  I. 
(3.   Fuses  quietly. 

PECTOLITE,  HNaCa^SiO-Og ;  White,  gray ;    Cl.  pina- 
coidal; H.  5;  G.  2.73;  F.  2.5-3;  V;  p.  427. 
ANALCITE,     NaAl(SiO3)2.H2O ;     White;      H.     5-5.5; 

G.  2.27  ;   F.  3.51 ;-  p.  485. 
Catapleiite,  H4(Na2.Ca)ZrSi3On ;    Yellow,  brown,  gray, 

violet ;   Cl.  prismatic  ;   H.  6 ;   G.  6.28 ;   F.  2.5. ;    III ; 

Shows  Zr. ;  §  a,  p.  571. 
b.   Gelatinizes  with  HC1. 
a.    Effervesces  with  HC1. 
CANCRINITE,         H6(Na2.Ca)4(Al.NaC03)2Al6(Si04)9 ; 

Yellow,  pink,  gray,  white  ;    Cl.    prismatic ;     H.  5-6 ; 

G.  2.45;   F.  2.5;   III;, p.  441. 
Cenosite,  Si,  Y,  Ca,  O,  CO3.H2O,  ?;    Yellowish  brown; 

Greasy ;   Cl.  pinacoidal ;   H.  5-5.5 ;   G.  3.41 ;   IV. 
/?.   The  HC1  solution  yields  a  precipitate  showing  the 

presence  of  aluminium,  but  not  calcium,  §  b,  p.  568. 
NATROLITE,    Na2Al3(AlO)(Si03)3.2H2O ;    White  ;'C1. 

prismatic;  H.  5-5.5 ;  G.  2.25;  F.  2.5;  IV;  p.  486. 
Hydronephelite,   HNa2Al(Si04)3.3  H2O  ;  White  to  dark 

gray;  H.  5-6;  G.  2.30;  F.  2.5;  III. 
y.  Shows  both  aluminium  and  calcium. 
SCOLECITE,  CaAl(A1.2  HO)(SiO3)3.2  H2O ;  White  ; 

Cl.   prismatic;   H.  5-5.5;   G.   2.30;   F.   2.5;   V;  p. 

479. 
MESOLITE,    Na2Ca2Al6Si903o,    8  H2O ;    White,    gray, 

yellow;    Cl.   prismatic;     H.   5 ;    G.   2.29;    F.   2.5; 

V;  p.  478. 
THOMSONITE,     (Ca.Na2)Al2(SiO4)2.2i  H2O ;     White, 

gray;   Cl.  pinacoidal;   H.  5-5.5;   G.   2.35;   F.   2.5; 

IV;  p.  487. 

8.   Shows  calcium  but  no  aluminium;  §  6,  p.  566. 
Okenite,      H2Ca(Si03)2.H2O ;      White,    cream,     bluish 

white ;  Dull ;   H.  4.5-5 ;   F.  3  ;   Fibrous,  compact. 
2x 


674  MINERALOGY 

c.   Insoluble  or  not  decomposed  in  HC1. 

a.    In  the  forceps  yields  a  yellow  flame  (sodium). 
Epididymite,  HNaBeSi3O8 ;    White;    Cl.  basal;    H.  6; 

G.  2.55  ;   F.  2.5-3  ;   IV. 
Eudidymite,  HNaBeSi3O8;    White;    Cl.    basal;  H.  6; 

G.  2.55 ;   F.  2.5-3  ;   V ;  Tabular. 
/?.   Does  not  yield  a  strong  sodium  flame. 
EPIDOTE,    Ca«(Al.OH)(Al.Fc)i(SiO4)ij    Yellowish  to 

blackish  green,  gray;    Cl.  basal;    H.  6-7;    G.  3.40; 

F.  3-4  ;  V  ;  p.  466. 
ZOISITE,  Ca2(Al.OH)Al2(Si04)3;  Grayish  white,  pink, 

green;  Cl.  pinacoidal ;   H.  6-6.5;   G.  3.26;    F.  3-4; 

IV;  p.  465. 
Clinozoisite,     Ca^Al.OH) Al2(SiO4)3 ;     White     to     pale 

pink ;  Cl.  basal ;  H.  6-7  ;  G.  3.37  ;  F.  3-4  ;  V. 
Lawsonite,  Ca(A1.2  OH)(SiO4)3 ;  Grayish  blue  to  white ; 

Cl.  pinacoidal ;   H.  8  ;  G.  3.09  ;   F.  4  ;  IV. 
— .   Yields  no  water  in  the  closed  tube. 

a.  Decomposed  in  HCi  with  separation  of  silica,  but  with- 

out gelatinizing. 
LABRADORITE,  CaAlSi308 ;    White,  gray;    Cl.   basal 

and  pinacoidal ;   H.  6  ;   G.  2.73  ;  F.  4 ;   VI ;  p.  416. 
WERNERITE,  Ca4Al6,  Si6,O25.Na4Al3Si9024Cl.  ;    White, 

gray,  light  green  ;  Cl.  prismatic  ;  H.  5.6  ;  G.  2.68 ;  F. 

3 ;   II ;   p.  453. 

b.  Gelatinizes  with  HCI. 

«.   Yields  a  sulphur  reaction  with  soda  and  a  sodium 

flame  in  the  forceps. 
LAZURITE,  (Na4.Ca)2(Al.Na.Ss)Al2(Si04)s ;  Deep  azure 

to  greenish  blue;    H.  5-5.5;    G.  2.42;    F.  3.5;   I; 

p.  438. 
NOSELITE,    Na4(Al.NaSO4) Al2(SiO4)3 ;     Gray,    green, 

blue,  brown ;  H.  5.5 ;  G.  2.32 ;  F.  3.5-4 ;  I ;  p.  438. 
HAUYNITE,      (Ca.Nas)2(Al.NaSO4)Al2(SiO4)3 ;      Blue, 

green,  yellow,  white;  H.  5.5-6;  G.  2.45;    F.  4;  I; 

p.  438. 
Microsommite,  Si,  Al,  Ca,  Na,  K,  0,  Cl,  (SO4),  (CO,),?; 

White;  Cl.  prismatic;   H.  6 ;  G.  2.44;   F.  3.5;  III. 
£.   Shows  aluminium,  §  &,  p.  568. 

*.   Shows  chlorine ;  §  a,  p.  588.     Yields  a  sodium  flame 
in  the  forceps. 


FUSIBILITY   BELOW,  HARDNESS  ABOVE  5  675 

SODALITE,  Na4(AlCl) Al2(Si04)3 ;  White,  gray,  blue, 
green;  H.  5.5-6;  G.  2.30;  F.  3.5;  I;  p.  438. 

**.   Yields  a  strong  sodium  flame,  but  no  chlorine. 

NEPHELITE,  (Na2.K2.Ca)4Al8Si9034;  White,  gray, 
greenish,  reddish ;  Cl.  prismatic  ;  H.  5.5-6  ;  G.  2.60 ; 
F.  4  ;  III ;  p.  440. 

***.    Yields  calcium ;  §  6,  p.  566. 

ANORTHITE,  CaAl2(SiO4)2 ;  White,  gray;  Cl.  basal 
and  pinacoidal;  H.  6-6.5;  G.  2.75;  F.  4.5;  VI; 
p.  413. 

MELILITE,  Si,  Al,  Fe,  Ca,  Mg,  Na,  0? ;  Green,  yellow, 
brown,  white  ;  Cl.  basal ;  H.  5 ;  G.  3.00  ;  F.  4  ;  II ; 
p.  451. 

Sarcolite,  (Ca.Na^AlsCSiC^)-? ;  Flesh  to  rose-red, 
white ;  H.  6 ;  G.  2.7 ;  F.  3 ;  II. 

y.    Contains  no  aluminium. 

*.   Shows  calcium,  §  6,  p.  566. 

WOLLASTONITE,  CaSIO3;  White,  gray;  Cl.  pina- 
coidal; H.  5-5.5;  G.  2.85;  F.  4;  V;  p.  429. 

**.  Shows  zirconium;  §  a,  p.  571. 

Hiortdahlite,  (Na2.Ca)(Si.Zr)O3? ;  Straw-yellow  to  yel- 
lowish brown;   H.  5.5-6;  G.  3.26;   F.  3;  VI. 
c.   Insoluble  in  HC1. 

«.   Yields  a  red  flame  in  the  forceps  (lithium). 

SPODUMENE,  (Li.Na)Al(SiO3)2;  White,  gray,  pink, 
green;  Cl.  prismatic;  H.  65-7;  G.  3.18;  F.  3.5; 
V;  p.  426. 

PETALITE,  (Li.Na)Al(Si2O5)2 ;  White,  gray,  pink; 
Cl.  basal;  H.  6-6.5;  G.  2.40;  F.  4;  V;  p. 
426. 

/8.   Yields  a  potassium  flame  through  the  blue  glass  when 

•    fused  with  potassium  flux ;  §  a,  p.  563. 

ORTHOCLASE,  KAlSi308 ;  White,  gray,  cream,  flesh- 
red,  green ;  Cl,  basal  and  pinacoidal ;  H.  6 ;  G. 
2.57  ;  F.  5  ;  V ;  p.  403. 

MICROCLINE,  Same,  but  VI,  p.  409. 

Yields  a  little  water  in  the  closed  tube  at  a  high  tem- 
perature. 

Milarite,  HKCa2Al2(Si2O5)6 ;  White  to  pale  green;  H. 
5.5-6  ;  G.  2.55  ;  F.  3  ;  III 

y.   Yields  a  strong  sodium  flame  in  the  forceps. 


676  MINERALOGY 

ALBITE,  NaAlSi3O8 ;    White  to  gray ;     Cl.    basal  and 

pinacoidal;   H.  6 ;   G.  2.62;   F.  4;   VI;   p.  411. 
See  p.  403  for  intermediate  feldspars  which  may  be  of 

various  colors. 
JADEITE,     NaAlSi2O6;      White,     grayish   green;     Cl. 

prismatic;    H.   7;    G.   3.33;    F.   2.5;    V;  p.  426. 
Glaucophane,    Na2Al2(SiO3)4.Mg4(SiO3)4;     Lavender   to 

azure-blue ;    Cl.  prismatic ;  H.  6-6.5 ;  G.  3.11 ;  F.  3-4. 
Epididymite,  HNaBeSi3O8;    White;    Cl.  basal;    H.  6; 

G.  2.55;  F.  2.5-3;  IV. 
Eudidymite,  HNaBeSi3O8 ;    White ;   Pearly ;    Cl.  basal ; 

H.  6 ;  G.  2.55 ;   F.  2.5-3 ;   V ;   Tabular. 
Shows  chlorine,  §  587. 
WERNERITE,     Ca4Al6Si6O25.Na4Al2Si9O24Cl ;      White, 

gray,    light  green;    Cl.   prismatic;    2.5-6;   G.  2.68; 

F.  3;   II;  p.  453. 
Marialite,  Na4Al3Si9O24Cl;  White;   H.  5.5-6;    G.  2.56; 

F.  3-4;  II. 

8.    Yields  test  for  aluminium  and  calcium;  §  6,  p.  566. 
*.   Fuses  quietly. 
ANORTHITE,  CaAl2(SiO4)2 ;  White  to  gray;  Cl.  basal 

and  pinacoidal ;  H.  6  ;  G.  2.75 ;   F.  4.5 ;  VI ;  p.  413. 
GROSSULARITE,    Ca3Al2(SiO4)3 ;      Pale   red,   yellow, 

green,  white;  H.  6.5-7.5;    G.  3.50;    F.  3 ;  I ;  p.  443. 
HORNBLENDE,  Complex  silicate;   Green  to  black ;   CL 

prismatic;   H.  6;   G.  3.15;   F.  3-4;    V;    p.  431. 
AUGITE,  Complex  silicate;    Greenish  black  to  black; 

Cl.  prismatic  ;   H.  57;    G.  3.30;    F.  4-45;  V;  p.  418. 
**.   Fuses  with  intumescence. 
PREHNITE,  H2Ca2Al2(SiO4)3 ;  Apple-green,  gray,  white  ; 

H.  6-6.5;   G.  2.9;   F.  2.5;   IV;   p.  470. 
VESUVIANITE,  Complex  silicate;    H.  6.5;     G.  3.40; 

F.  3;  II;  p.  455. 
EPIDOTE,     Ca2(AlOH)(Al.Fe)2(SiO4)3;     Yellowish    to 

blackish  green,  gray ;    Cl.  basal ;    H.  6-7  ;  p.  466. 
ZOISITE,  Ca2(AlOH)Al2(SiO4)3;  Grayish  white,  green, 

pink;    Cl.  pinacoidal;    H.  6-6.5;    G.  3.26;  F.  3-4 ; 

V;   p.  465. 
Clinozoisite,    Ca2(AlOH)2(Al)(SiO4)3;      White    to    pale 

pink;     Cl.  basal;     H.  6-7 ;     G.  3.37;     F.  3-4;     V; 

p.  465. 


FUSIBILITY  ABOVE,   HARDNESS  BELOW  5  677 

e.    Shows  aluminium  and  magnesium,  but   little  or  no 

calcium;  §  b,  p.  567. 
PYROPE,  MgsA]2(SiO4)3 ;  deep  red ;  H.  6.5-7.5  ;  G.  3.72 ; 

F.  3.5-4;  I;  p.  443. 

£.    Shows  calcium  and  magnesium,  but  little  or  no  alu- 
minium ;  §  6,  p.  567. 
TREMOLITE,  CaMg3(SiO3)4;   White,  gray,  violet;   Cl. 

prismatic;   H.  5-6 ;   G.  3.00;   F.  4;   V;  p.  431. 
DIOPSIDE,   CaMg(SiO8)2;    White  to  pale   green;    H. 

5-6;   G.  3.29;   F.  4;   V;   p.  423. 
PYROXENE,  Ca(Mg.Fe)(Si03)2;  Light  to  dark  green; 

Cl.  prismatic;   H.  5-6;   G.  3.3;   F.  4;   V;  p.  419. 
ACTINOLITE,  Ca(Mg.Fe)3(SiO3)4;    Various  shades  of 

green;    CL  prismatic;    H.  5-6;    G.  3.10;    F.  4;  V; 

p.  431. 
y.   Shows  magnesium  and  iron,  but  little  or  no  calcium, 

§  6,  p.  567. 
ANTHOPHYLLITE,  (Mg.Fe)SiO3;   Gray,  clove-brown 

to  green;    Cl.. prismatic;    H.  5.5-6;    G.  3.10;    IV. 
ENSTATITE,    (Mg.Fe)SiO3;     Gray-brown    to    green; 

Pearly,  bronze-like;   Cl.  pinacoidal ;    H.  5.5-6.5;   G. 

3.20;  F.  5-6;   IV;   p.  421. 
0.    Shows  magnesium,  but  no  silica.     Reacts  for  fluorine ; 

§  a,  p.  589. 
Sellaite,  MgF2 ;  White;  Cl,  basal;    H.  5;    G.  3.06;  F. 

4-5;  II. 

B.   Fusibility  above  5. 
1.    Hardness  below  5. 

+  .   Yields  water, 
a.   Soluble  in  HC1,  or  decomposed  with  the  separation  of 

silica. 

a.   Yields  a  sulphur  reaction  with  soda. 
ALUMINITE,     A12(OH)4SO4.7  H2O  ;     White  ;      Dull  ; 

H.  1-2;   G.  1.66;   V;   p.  542. 
Felsobanyite,  A12(OH)4SO4.2  A1(OH)8.5  H2O ;  White  to 

pearly;  Cl.  perfect;   H.  1  5  ;    G.  2.33  ;  III;  Scaly. 
Alumian,   (A12O)(SO4)2 ;  White;  H.  2-3;  G.  2.7;   III; 

Massive ;   Yields  no  water. 
Yields  a  violet  flame  (potassium). 
Lowigite,     K(A1.2  OH)3(SO4)2.li  H2O  ;     Straw-yellow  ; 

H.  3-4 ;   G.  2.58 ;   Massive. 


678  MINERALOGY 

ft.   Yields  a  fluorine  reaction. 

Fluocerite,  (Ca.La.Di)2OF4.OH ;  Reddish  yellow ;  Cl. 
two  directions;  H.  4;  G.  5.80;  Massive;  III. 

Yttrocerite,  (Y.Er.Ce)F3.5  CaF2.H20 ;  Violet,  gray, 
brown,  white;  Greasy;  H.  2.5-3;  G.  3.45;  Mas- 
sive. 

y.  Ignited  with  cobalt  solution  becomes  flesh-pink 
(magnesium). 

DEWEYLITE,  H4Mg4(Si04)3.4  H2O ;  Yellow  to  apple- 
green  ;  Resinous ; "  H.  3-4 ;  G.  2.40 ;  Amorphous. 

Sepiolite,  H4Mg2Si3O10 ;  White  to  grayish  white ;  Dull ; 
H.  2-2.5;  G.  2.0;  Compact. 

Usually  too  dark  in  color  to  show  pink  with  cobalt 
solution. 

SERPENTINE,   H4(Mg.Fe)3Si209 ;    Olive,   blackish,   or 
yellowish  green,  white;  Greasy;  H.  2.5-6,  G.  2.56; 
Massive;  p.  498. 
6.   Gelatinizes  with  HC1. 

ALLOPHANE,    Al2Si05.H2O;     White,    yellow,    green, 

blue;   H.  3;   G.  1.88;   Amorphous,  p.  502. 
c.   Insoluble  in  HC1. 

a.   Micaceous. 

CLINOCHLORE,  H8Mg5Al2Si3Oi8 ;  Green  of  various 
shades;  H.  2-2.5;  G.  2.75;  V;  p.  497. 

Clintonite,  H3(Mg.Ca)5Al5Si2O8;  Reddish  brown,  cop- 
per-red; Pearly;  H.  4-5 ;  G.  2.87;  V. 

Xanthophyllite,  H8(Mg.Ca)i4Ali6Si5O52;  Light  green ;  H. 
4-5;  G.  3.09;  V. 

Prochlorite,  H4o(Fe.Mg)23Al14Sii209o?;  Green  to  black- 
ish green;  H.  1-2;  G.  2.87;  V. 

j8.  Becomes  blue  with  cobalt  solution, 

PYROPHYLLITE,  H2Al2(Si03)4;  White,  apple-green, 
gray,  brown;  Pearly;  H.  1-2;  G.  2.85;  Foliated, 
compact;  p.  502. 

KAOLINITE,  H4Al2Si209;  White;  Pearly,  dull;  Cl. 
basal;  H.  2-2.5;  G.  2.60;  V;  p.  501. 

GIBBSITE,  AI(OH)3;  White;  Pearly,  dull ;  Cl.  basal; 
H,  2.5-3.5;  G.  2.35;  V;  p.  365. 

BAUXITE,  A12O(OH)4;  White,  gray,  yellow,  red;  Dull, 
earthy ;  G.  2.55  ;  Massive,  clay-like ;  p.  366. 

y.   Yields  a  sulphur  reaction  when  fused  with  soda. 


FUSIBILITY  ABOVE,  HARDNESS  BELOW  5  679 

ALUNITE,  (K.Na)  (A1.2  OH)3(SO4)2 ;  White,  gray ; 
Cl.  basal;  H.  3.5-4;  G.  2.85 ;  III;  p.  541. 

8.  With  potassium  bisulphate  yields  a  fluorine  reac- 
tion; §  a,  p.  589. 

RalstoAite,  (Na2.Mg)F2.3  A1(F.OH)8.2  H20 ;  White  to 
straw-yellow;  H.  4.5;  G.  2.59;  I. 

Prosopite,  Ca(F.OH)2.2Al(F.OH)3;  White,  gray;  H. 
4.5;  G.  2.89;  V. 

Fluellite,  A1F3.H2O ;   White;  H.  3 ;   G.  2.17;  IV. 

e.    With  cobalt  solution  becomes  flesh-pink  (magnesium) . 

TALC,  H2Mg3(SiO3)4;  Apple-green,  gray,  white;  Pearly, 
greasy ;  Cl.  basal ;  H.  1 ;  G.  2.80 ;  Foliated,  com- 
pact; p.  500. 

— .   Yields  no  water. 

a.   Fused  with  potassium  bisulphate  yields  a  fluorine  re- 
action. 

Tysonite,  (Ce.La.Di)F3;  Wax-yellow,  reddish  brown; 
Cl.  basal;  .H.  4.5-5;  G.  6.13;  III. 

Bastnasite,     (Ce.La.Di.F)CO3;      Wax-yellow,     reddish 

brown;   H.  4.5-5;   G.  5.08;   Massive. 
Hardness  above  5. 

+  .    Yields  water, 
a.   Soluble  in  HC1. 

Pollucite,  H2Cs4Al4(Si03)9 ;  White;  H.  5.5-6 ;  G.  2.98; 

I ;  Massive. 
6.    Gelatinizes  with  HC1. 

a.   Fused  with  potassium  bisulphate  shows  fluorine. 

CHONDRODITE,  Mg3[Mg(F.OH)]2(Si04)2 ;  Brownish 
red,  yellow,  white ;  Cl.  basal ;  H.  6-6.5 ;  G.  3.15 ;  V ; 
p.  471. 

Humite,  Mg5[Mg(F.OH)]2(SiO4)3 ;  Brownish  red,  yel- 
low, white;  Cl.  basal;  H.  6-6.5;  G.  3.15;  IV. 

Clinohumite,  Mg7[Mg(F.OH)]2(SiO4)4;  Brownish  red, 
yellow,  white;  Cl.  Basal;  H.  6-6.5;  G.  3.15;  V. 

Prolectite,   Mg[Mg(F.OH)]2SiO4;    Brownish   gray;    V. 

13.   Yields  the  rare  earths ;    p.  572. 

CERITE,  (Ca.Fe)  (CeO)  (Ce2.3  OH)  (Si03)3 ;  Clove- 
brown,  gray,  red;  Dull,  resinous;  H.  5.5;  G.  4.90; 
IV. 

Thorite,  ThSiO4,  Orange-yellow,  brown,  black;  Resi- 
nous ;  Cl.  prismatic ;  H.  5 ;  G.  4.90 ;  II. 


680  MINERALOGY 

c.   Insoluble  in  HC1  or  not  decomposed. 

a.  The  finely  powdered  mineral  when  fused  with  one 
and  one  half  parts  of  soda  yields  a  bead  remaining 
clear  when  cold. 

OPAL,  SiO2  with  water;  White,  colors  of  various 
shades;  H.  5.5-6.5;  G.  2.15;  Amorphous;  p.  369. 

p.   Becomes  blue  with  cobalt  solution. 

DIASPORE,  AIO(OH) ;  White,  gray,  yellowish,  green- 
ish; Cl.  pinacoidal;  H.  6.5-7;  G.  3.40,  IV;  p. 
365 

Zunyite,  [A1.2  (OH.F.Cl)]6Al2(Si.04)3;  White,  gray; 
H.  7;  G.  2.88;  I. 

y.   May  yield  only  a  very  small  amount  of  water. 

STAUROLITE,  (A10)4(Al.OH)Fe(SO4)2 ;  Red-brown  to 
brownish  black;  Cl.  pinacoidal;  H.  7-7.5;  G.  3.65; 
IV;  p.  477. 

IOLITE,  H2(Mg.Fe)4Al8Siio037 ;  Light  or  dark  blue,  white; 
Cl.  pinacoidal ;  H.  7-7.5;  G.  2.61 ;  IV;  p.  440. 

8.  Shows  berlium ;  p  571.  Lavender  with  cobalt  solu- 
tion. 

Bertrandite,  Be2(Be.OH)2Si207 ;  White,  yellow ;  Pearly ; 
Cl.  prismatic;  H.  6-7;  G.  2.60;  IV. 

Euclase,  Be(A1.0H)SiO4 ;  White  to  pale  green;  Pearly; 
Cl.  pinacoidal;  H.  7.5;  G.  3.10;  V. 

c.   Fused  with  soda  yields  a  sulphur  reaction. 

Melanophlogite,  SiO2  SO3H20,etc.  ? ;  White,  light  brown  ; 

H.  6.5-7;  G.  2.02;  I;  Cubes. 
2.   Hardness  above  5. 

— .   Yields  no  water. 

a.  Soluble  or  decomposed  with  HC1. 

a.   Fused  with  potassium  flux,  shows  potassium  through 

the  blue  glass. 

LEUCITE,  KAl(Si03)2;  White,  gray;  H.  5.5-6:  G. 
2.47;  I;  p.  416. 

b.  Gelatinizes  with  HC1. 

CHRYSOLITE,  (MgFe)Si04;  Olive  to  grayish  green, 
brown;  Cl.  pinacoidal;  H.  6.5-7;  G.  3.33;  IV; 
p.  446. 

«.    Contains  no  iron. 

Forsterite,  Mg2SiO4 ;  White,  gray,  yellowish ;  Cl.  pina- 
coidal; H.  6.5-7;  G.  3.24;  IV;  p.  450. 


FUSIBILITY  AND  HARDNESS  ABOVE  5  681 

ft.  The  concentrated  HC1  solution  yields  a  precipitate 
with  H2SO4  (calcium). 

Gehlenite,  (Ca.Mg.Fe)3Al2Si2Oi0 ;  Grayish  green,  brown ; 
H.  5.5-6 ;  G.  2.98  ;  F.  5 ;  II. 

y.   Yields  a  test  for  the  rare  earths. 

GADOLINITE,  FeB2Y2Si2Oio ;  Black,  greenish  black, 
brown ;  H.  6.5-7  ;  G.  4.25 ;  V. 

Thorite,  ThSi04;    Orange-yellow,  brown,  black;    Resi- 
nous;  Cl.  prismatic;   H.  5;   G.  4.90;   II. 
c.   Insoluble  in  HC1. 

a.   Yields  a  fine  blue  with  cobalt  solution. 

CORUNDUM,  A12O3;  All  colors;  Cl.  rhombohedral ; 
H.  9;  G.  4.03;  III;  p.  341. 

CYANITE,  Al(AlO)Si04;  Blue,  gray,  white,  green;  Cl. 
pinacoidal;  H.  5-7 ;  G.  3.62;  VI;  p.  461. 

ANDALUSITE,  Al(AlO)SiO4;  Flesh-red,  reddish  brown, 
olive;  Cl.  prismatic;  H.  7.5;  G.  3.18;  IV;  p.  459. 

SILLIMANITE,  Al(A10)SiO4;  Brown,  gray,  greenish 
gray;  Cl.  pinacoidal;  H.  6-7;  G.  3.23;  IV;  p.  461. 

Dumortierite,  Al2(A10)6(Si04)3;  Deep  blue;  CL  pina- 
coidal ;  H.  7 ;  G.  3.26 ;  IV. 

TOPAZ,  Al(Al(O.F2))SiO4;  White,  yellow,  pink,  bluish, 
greenish;  Cl.  basal;  H.  8 ;  G.  3.53  ;  IV;  p.  458. 

Kornerupine,  Mg(A10)2Si04 ;  White,  yellowish  brown; 
Cl.  prismatic ;  H.  6.5 ;  G.  3.27 ;  IV. 

CHRYSOBERYL,  BeAl2O4;  Yellowish  to  emerald- 
green  ;  Cl.  prismatic ;  H.  8.5 ;  G.  3.67 ;  IV ;  p.  377. 

y.  Do  not  yield  a  fine  blue  with  cobalt  solution,  some 
however  may  give  a  dull  blue. 

*.  Fused  with  1|  parts  of  soda  yields  a  bead  remain- 
ing clear  when  cold. 

QUARTZ,  SiO2;  All  colors;  H.  7;  G.2.65;  III;  p.  352. 

TfclDYMITE,  SiO2;  White;  H.  7;  G.  2.30;  III;  p. 
361. 

CHALCEDONY,  SiO2 ;  Various  colors;  Waxlike;  H. 
7 ;  G.  2.62 ;  Massive ;  p.  360. 

**.  Do  not  yield  a  clear  bead. 

Shows  magnesium  §  c,  p.  567. 

ENSTATITE,  (Mg.Fe)SiO3;  Gray,  brown,  green; 
Pearly,  bronzelike;  Cl.  prismatic;  H.  5.5-6.5;  G, 
3.20;  IV;  p  421. 


682  MINERALOGY 

HYPERSTHENE    (Mg.Fe)SiO3;     Brownish    green    to 

greenish  black ;  Pearly ;  Cl.  pinacoidal ;  H.  5-6 ;  G. 

3.45;   IV;    p.  421. 
ANTHOPHYLLITE,  (Mg.Fe)Si03;  Gray,  clove  brown, 

green;    Pearly;  Cl.  prismatic;   H.  5.5-6;   G.    3.10; 

IV. 
SPINEL,  Mg(AlO)2 ;   Various  colors ;   H.  8 ;   G.  3.5-4 ; 

I;  p.  371.    , 

Yields  a  zirconium  reaction  §  a,  p.  571. 
ZIRCON,  ZrSiO4;  Brown,  white,  gray,  green,  red;  Cl. 

prismatic;   H.  7.5;   G.  4.68;   II;   p.  456. 
Baddeleyite,  ZnO2 ;    White,  yellow,  brown,  black;    Cl. 

basal;  H.  6.5;  G.  5.5;  V. 
Shows  beryllium,  §  a,  p.  571. 
BERYL,  Be3Al2(Si.O3)6.iH20;  White,  green,  yellow, 

blue,  pink;  H.  7-7.5;  G.  2.69;  III;  p.  436. 
PHENACITE,  Be2SiO4 ;  White ;  Cl.  prismatic  ;  H.  7.5-8  ; 

G.  2.96 ;  III ;  p.  452. 
Powdered    and    reduced    with    soda    yields    magnetic 

particles. 

Hercynite,  Fe(A10)2 ;  Black ;   H.  7.5-8  ;   G.  3.93  ;   I. 
Extremely  hard. 
DIAMOND,  C ;  Colorless,  yellow,  red,  blue,  gray,  black  ; 

Cl.  tetrahedral;  H.  10;   G.  3.52;  p.  281. 


INDEX 

Numbers  printed  in  italics  refer  to  Part  I ;    those  in  black-faced  type,   to 
Part  II ;  and  those  in  roman  type,  to  Part  III. 


Abbreviations,  595,  617. 
Acanthite,  301,  630. 
Achroite,  475. 
Acicular  habit,  266. 
Acid  salts,  227. 
Acids,  silicic,  233. 
Acmite,  420,  660. 
Actinolite,  433,  677. 
Acute  bisectrix,  173. 
Adamantine  luster,  279. 
Adamantine  spar,  343. 
Adamite,  513. 
Adelite,  653. 
Adularia,  408. 
^Enigmatite,  435,  659. 
^Eschynite,  634,  668. 
Agate,  360. 

Moss,  360. 
Aggregate,  265. 
Agricolite,  651. 
Aguilarite,  627. 
Aikinite,  628. 
Airy's  spirals.  197. 
Alabandite,  304,  631,  666. 
Alabaster,  537. 
Albite,  411,  604,  614,  676. 
Alexandrite,  378. 
Algodonite,  621. 
Alkalies,  222,  562. 
Alkaline  earths,  565. 
Alkaline  reaction,  565. 
Allactite,  652. 

Allanite,  468,  596,  604,  633,  658. 
Allemontite,  620. 
Alloclasite,  622. 
Allophane,  502,  678. 
Almandite,  444,  659. 
Altaite,  625. 
Alternating  axis,  15. 

symmetry,  15. 
Alum,  542. 
Alumian,  677. 

Aluminite,  233,  337,  542,  677. 
Aluminium,  Tests,  568. 
Alunite,  541,  679. 


Alunogen,  542,  600,  638. 

Alurgite,  672. 

Amalgam,  631. 

Amaranite,  636. 

Amazon  stone,  411. 

Amblygonite,  512,  657. 

Amethyst,  359. 

Ammonium,  tests  for,  565. 

Amorphous,  3,  238. 

Amphiboles,  431,  432,  604,  608,  615. 

Amplitude,  181. 

Amygdaloidal,  271. 

Amygdule,  271. 

Analcite,  485,  603,  610,  673. 

Analogous  pole,  475. 

Analyzer,  177. 

Analysis,  227. 

Anatase  (octahedrite) ,  347,  351. 

Andalusite,  459,  605,  616,  681. 

Andesine,  414. 

Andorite,  623. 

Andradite,  444,  659. 

Angle,  between  faces,  10. 

between  normals,  12. 

between  optic  axes,  205. 

constancy  of,  10. 

critical,  166. 

of  extinction,  185. 
Anglesite,  532,  602,  647. 
Anhydrite,  531,  602,  662. 
Anisotropic,  163. 
Ankerite,  657. 
Annabergite,  316,  517,  651. 
Anomite,  493. 
Anorthite,  413,  604,  676. 
Anthophyllite,  677,  682. 
Antilogous  pole,  475. 
Antimonides,  294. 
Antimony,  293,  623. 

glance,  295. 

oxides,  346. 

tests,  584. 

Apatite,  96,  508,  603,  611,  655. 
Aphthitalite,  .637. 
Apjohnite,  639. 
Aplome,  444. 

Apophyllite,  480,  603,  672. 
683 


684 


INDEX 


Apparatus  blowpipe,  546. 
Apparent  angle,  206. 
Aquamarine,  437. 
Aragonite,  392,  602,  660. 
Ardennite,  665. 
Arfvedsonite,  435,  659. 
Argentite,  397,  596,  630. 
Argyrodite,  630. 
Arsenates,  236. 
Arsenic,  393,  620. 

tests,  585. 

Arsenical  pyrite,  318. 
Arsenic  oxides,  346. 
Arsenides,  232,  294. 
Arseniosiderite,  652. 
Arsenolite,  346,  640. 
Arsenopyrite,  318,  597,  622. 
Arzrunite,  642. 
Asbestos,  433,  500. 
Asbolite,  369. 
Astrophyllite,  659. 
Assay,  578. 
Asymmetric,  132. 
Atacamite,  334,  644. 
Atomic  weights,  223. 
Augelite,  656. 
Augite,  123,  424,  676. 
Aurichalcite,  641. 
Autunite,  520,  655. 
Awaruite,  632. 
Axes,  Crystallographic,  15. 

calculations,  83,  128. 

hexagonal,  85. 

monoclinic,  128. 

orthorhombic,  120. 

projection  of,  35. 
Axial  angles,  16. 

optic,-  205. 
Axial  planes,  16. 
Axial  ratios,  24. 
Axinite,  469,  664. 
Axis,  didigonal,  14.    .- 

digonal,  14. 

dihexagonal,  14. 

ditetragonal,  14. 

hexagonal,  14. 

of  alternating  symmetry,  15. 

optic,  198. 

polar,  55. 

trigonal,  14. 

tetragonal,  14. 

twinning,  138. 

zonal,  22. 
Azurite,  399,  608,  641. 


Babingtonite,  660. 
Baddeleyite,  682. 
Bakerite,  664. 


Balance,  Joly's,  261. 

Westphal,  264. 
Barite,  116,  528,  601,  662. 
Barium,  tests,  565. 
Barkevikite,  435. 
Barrandite,  654. 
Barysilite,  648. 
Barytocalcite,  395,  660. 
Basal  cleavage,  256. 
Basal  pinacoid,  68. 
Basic  salts,  226. 
Base,  68. 
Bastnasite,  679. 
Baumhauerite,  622. 
Bauxite,  366,  602,  678. 
Baveno  twins,  406. 
Bayldonite,  643. 
Bechilite,  664. 
Beegerite,  628. 
Belonesite,  670. 
Bementite,  666. 
Benitoite,  103,  505,  668. 
Beresonite,  647. 
Berthierite,  320,  625. 
Bertrandite,  680. 
Beryl,  89,  436,  605,  682. 
Beryllium,  tests,  571. 
Beryllonite,  657. 
Berzelianite,  627. 
Berzeliit'e,  652. 
Beudantite,  646. 
Bevelment,  22. 
Beyrichite,  307,  630. 
Biaxial,  171. 

Biaxial  interference,  200. 
Bieberite,  637. 
Bindheimite,  647. 
Binnite,  621. 

Biotite,  492,  601,  615,  660,  672. 
Birefringency,  168. 

sign  of,  194. 
Bisectrices,  173. 

dispersion,  207. 
Bismite,  346. 
Bismuth,  293,  629. 

tests,  580. 

Bismuthindte,  296,  629. 
Bismutite,  650. 
Bismutosmaltite,  622. 
Bismutosphserite,  650. 
Bixbyite,  633. 
Black  jack,  301. 
Black  sands,  288. 
Blende,  301. 
Bloedite,  539,  637. 
Bloodstone,  360. 
Blue  earth,  283,  444. 
Blue  vitriol,  540. 
Bobierrite,  657. 
3og  iron  ore,  363. 
Boleite  (percylite),  644. 


INDEX 


685 


Bone  ash,  559. 
Bone  phosphate,  509. 
Bone  turquoise,  517. 
Boracite,  523,  663. 
Borates,  236,  507. 
Borax,  522,  600,  640. 
Borax  bead,  553. 
Borickite,  654. 
Bornite,  310,  599,  629. 
Boron,  tests,  591. 
Bort,  282. 
Botryogen,  636. 
Botryoidal,  273. 
Boulangerite,  321,  624. 
Bournonite,  321,  597,  623. 
Boussingaultite,  638. 
Brachy-axis,  113. 
Brachydome,  115. 
Brachypinacoid,  116. 
Brachyprism,  115. 
Brackenbuskite,  649. 
Brandtite,  652. 
Braunite,  632. 
Brazilian  twins,  146. 
Breithauptite,  309,  625. 
Breunnerite,  657,  661. 
Brewsterite,  479,  670,  673. 
Brittle,  258. 
Brittle  micas,  496. 
'  Brochantite,  541,  642. 
Bromine,  tests,  588. 
Bromlite,  395,  660. 
Bromyrite,  330,  645. 
Brongniardite,  623. 
Bronzite,  421. 
Brookite,  351,  634,  668. 
Brucite,  91,  362,  601,  663. 
Brushite,  656. 
Bunsenite,  340. 
Bytownite,  414. 


Cabrerite,  651. 
Cacoxenite,  655. 
Cadmium,  tests,  581. 
Caesium,  tests,  564. 
Calamine,  118,  472,  603,  650. 
Calaverite,  626. 
Calcioferrite,  654. 
Calciovolborthite,  645. 
Calcite,  91,  380,  602,  612,  660. 

twins,  145,  146,  382. 
Calcium,  tests,  566. 
Calculations  of  axes,  83,  112,  120,  128. 

formula,  231. 

2  V,  205. 
Caledonite,  642. 
Callainite,  656. 
Calomel,  641. 
Cancrinite,  441,  673. 


Canfieldite,  630. 

Cape  ruby,  443. 

Capillary,  habit,  266. 

Cappelenite,  665. 

Caracolite,  646. 

Carat,  282. 

Carbon,  tests,  590. 

Carbonado,  282. 

Carbonates,  233,  370. 

Carbonates,  basic,  227. 

Carbuncle,  444. 

Carlsbad  twins,  148,  405. 

Carminite,  646. 

Carnallite,  335,  639. 

Carnelian,  360. 

Carnotite,  514,  670. 

Carpholite,  666. 

Carposiderite,  637. 

Caryocerite,  665. 

Cassiterite,  347,  596,  598,  631,  646. 

Castanite,  636. 

Catapleiite,  673. 

Cat's-eye,  379. 

Celestite,  530,  662. 

Celsian,  414. 

Cenosite,  673. 

Center  of  symmetry,  16. 

Central,  ditesseral,  47, 

tesseral,  59. 

Centrosymmetric,  130. 
Cerargyrite,  330,  601,  645. 
Cerite,  679. 
Cerium,  tests,  572. 
Cerussite,  396,  602,  647. 
Cervantite,  296. 
Chabazite,  484,  603,  672. 
Chalcanthite,  540,  641. 
Chalcedony,  360,  681. 
Chalcocite,  300,  596,  629. 
Chalcomenite,  645. 
Chalcophanite,  632. 
Chalcophyllite,  643. 
Chalcopyrite,  310,  599,  629. 
Chalcosiderite,  643. 
Chalcostibite,  320,  623. 
Chalcotrichite,  339. 
Chalk,  385. 
Chalybite,  388. 
Charcoal,  551. 

coats  on,  552. 
Chemical  formula,  231. 
Chenevixite,  642. 
Chessylite,  399. 
Chiastolite,  460. 
Childrenite,  654. 
Chile  saltpeter,  520. 
Chiolite,  662. 
Chiviatite,  628. 
Chloanthite,  315,  622. 
Chlor-apatite,  509. 
Chlorides,  327. 


686 


INDEX 


Chlorine,  tests,  588. 
Chlorite,  497,  601,  615. 
Chloropal,  659,  660. 
Chlorophane,  332. 
Chondrodite,  471,  679. 
Chromates,  236. 
Chromic  iron  ore,  376. 
Chromite,  233,  337,  376,  596,  609, 

659. 

Chromium,  tests,  568. 
Chrysoberyl,  377,  681. 
Chrysocolla,  503,  601,  644. 
Chrysolite,  446,  680. 
Chrysoprase,  360. 
Chrysotile,  500. 
Churchite,  657. 
Cinnabar,  304,  598,  629,  641. 
Cinnamon  stone,  443. 
Circular  polarization,  196. 
Cirrolite,  655. 
Citrine,  359. 
Classes  of  crystals,  4?- 
Classification  of  minerals,  222,  232. 
Claudetite,  346,  640. 
Clausthalite,  627. 
Clay,  502. 
Cleavage,  256. 
Cleveite,  525. 
Cliftonite,  284. 
Clinoaxis,  120. 
Clinochlore,  497,  672,  678. 
Clinoclasite,  643. 
Clinodome,  123. 
Clinographic  projections,  33. 
Clinohedrite,  127,  650. 
Clinohumite,  471,  679. 
Clinopinacoid,  123. 
Clinozoisite,  465,  674,  676. 
Clintonite,  496,  497,  678. 
Closed  tube,  557. 
Clove  oil,  215. 
Cobalt  bloom,  517. 
Cobalt  solution,  562. 
Cobalt,  tests,  574. 
Cobaltite,  316,  597,  621. 
Colemanite,  524,  664. 
Collophanite,  655. 
Coloradoite,  626. 
Color,  of  beads,  594. 

of  coats,  593. 

of  flame,  552, 

of  minerals,  273. 

order  of,  183. 
Columbates,  236. 
Columbite,  505,  596,  634. 
Columbiun,  tests,  570. 
Columnar  habit,  265. 
Combinations  of  forms,  22,  53. 
Common  garnet,  442. 
Common  opal,  369. 
Common  salt.  327. 


634, 


Compact  structure,  268. 

Compensation,  189.  ' 

Compensator,  189. 

Composition,  chemical,  231. 
plane,  138. 

Comptonite,  488. 

Conchoidal  fracture,  258. 

Conichalcite,  643. 
Connellite,  642. 
Constancy  of  angles,  10. 
Constitutional  formula,  234. 
Constitution,  water  of,  226. 
Contact  goniometer,  12,  149. 
Contunnite,  641. 
Convergent  light,  191. 
Cookeite,  496,  671. 
Copiapite,  636. 
Copper,  288,  598,  631. 
glance,  300. 
pyrite,  310. 
tests,  580. 
Copperas,  539. 
Coprolites,  5,09. 
Coquimbite,  540,  636. 
Cordierite,  440. 
Cornwallite,  643. 
Corundophilite,  498. 
Corundum,  91,  341,  605,  611,  681 
Corynite,  622. 
Cosalite,  628. 
Cotunnite,  647. 
Covellite,  306,  629,  644. 
Crednerite,  631. 
Critical  angle,  166. 
Crocidolite,  435,  659. 
Crocoite,  533,  649. 
Cronstedtite,  658. 
Crookesite,  627. 
Crossed  dispersion,  209. 
Crossed  nicols,  186. 
Cryolite,  333,  601,  662. 
Crystal,  6. 
axes,  15. 
aggregate,  ^135. 
forms,  21. 
measurement,  153. 
structure,  4. 

Crystalline,  3,  238. 

'rystalline  elements,  24- 

Crystallization,  6,  245. 

Crystals,  1. 

optical  properties,  160. 
physical  properties,  256. 

'ubanite,  629. 

!ube,  51. 

Cubic  system,  Jfl- 
"'ullinan  diamond,  283. 

•umengite,  644. 

Cuprite,  337,  598,  631,  644. 

:uprobismutite,  628. 

•uproiodargyrite,  645. 


INDEX 


687 


Cuprotungstite,  544,  645. 
Curvature  of  face,  268. 
Curve  of  hardness,  259. 
Cyanite,  461,  604,  615,  681. 
Cyanochroite,  641. 
Cyanotrichite,  642. 
Cyclic  twins,  142. 
Cylindrite,  624. 


D 


Danalite,  649. 
Danburite,  664. 
Darapskite,  637. 
Datolite,  463,  603,  664. 
Dawsonite,  661. 
Decrepitation,  552.   ' 
Demantoid,  444. 
Dendritic,  269. 
Density,  260. 
Derbylite,  634. 
Derivation  of  forms,  52. 
Descloizite,  514,  648. 
Description  of  minerals,  281—545. 
Descriptive  terms,  265. 
Determination  tables,  595. 
Deweylite,  671,  678. 
Diadochite,  654. 
Diallage,  424. 
Diamond,  281,  682. 
Diaphorite,  624. 
Diaspore,  365,  680. 
Diatomaceous  earth,  371. 
Dichroism,  189, 
Dichroite,  440. 
Dichroscope,  190. 
Dickinsonite,  653. 
Didigonal  axis,  14- 

equatorial,  113. 

polar,  117. 
Dietrichite,  636. 
Dietzeite,  663. 
Digonal  axis,  14- 

equatorial,  121. 

holoaxial,  118. 

polar,  124. 
Dihexagonal  alternating,  89. 

axis,  14- 

equatorial,  85. 

hemipyramid,  92. 

polar,  92. 

pyramid,  86. 

prism,  87. 
Dihydrite,  644. 
Dimetasilicic  acid,  234. 
Dimorphism,  220. 
Diopside,  423,  677. 
Dioptase,  99,  452,  644. 
Diorthosilicate,  233. 
Dioxides,  347. 


Diploid,  59. 

Directional  properties,  3. 
Disluite,  650. 
Dispersion,  of  light,  166. 

of  optic  axes,  206. 

of  the  bisectrix,  207. 

crossed,  208. 

horizontal,  208. 

inclined,  208. 
Disthene,  462. 
Distortion,  28. 
Ditesseral  central,  47- 

polar,  64. 
Ditetragonal  alternating,  69. 

axes,  14. 

equatorial,  65. 

hemipyramid,  72. 

polar,  71. 

prism,  67. 

pyramid,  66. 
Ditrigonal  axis,  14. 

equatorial,  101. 

hemipyramid,  103. 

polar,  103. 

prism,  102. 

pyramid,  101. 
Dodecahedron,  pentagonal,  60. 

rhombic,  52. 
Dog-tooth  spar,  381. 
Dolerophane,  642. 
Dolomite,  99,  387,  602,  660. 
Dome,  113,  115. 
Domeykite,  621. 
Double  refraction,  168. 
Drawing  of  crystals,  81. 
Drusy,  268. 
Ductility,  258. 
Dufrenite,  510,  515,  654. 
Dufrenoysite,  622. 
Dull  luster,  279. 
Dumortierite,  681. 
Durangite,  653. 
Dyscrasite,  625. 
Dysluite,  377. 


Ecdemite,  646. 
Eclogite,  445. 
Edingtonite,  479,  671. 
Effloresce,  225. 
Eglestonite,  641. 
Eisenrosen,  344. 
Elaeolite,  440. 
Elastic,  257. 
Electron,  291,  631. 
Elements,  220,  232,  281. 

table  of,  223. 
Ellipsoid,  184. 


688 


INDEX 


Ellipsoid,  Fletcher,  185. 

of  revolution,  184- 
Embolite,  330,  645. 
Emerald,  436. 
Emery,  343. 
Emplectite,  320,  628. 
Enantiomorphous,  27. 
Enargite,  326. 
Endlichite,  511. 

Enstatite,  421,  604,  613,  677,  681. 
Epiboulangerite,  624. 
Epididymite,  674,*676. 
Epidote,  464,  466,  605,  616,  674,  676. 
Epigenite,  621. 
Epistilbite,  479,  671. 
Epsomite,  119,  538,  600,  638. 
Equatorial,  126. 
didigonal,  113. 
digonal,  121. 
dihexagonal,  85. 
ditetragonal,  65. 
ditrigonal,  101. 
hexagonal,  93. 
tetragonal,  72. 
trigonal,  105. 
Erbium,  tests,  572. 
Erinite,  643. 
Erythrite,  517,  651. 
Etch  figures,  62,  78. 
Ettringite,  662. 
Eucairite,  627. 
Euchroite,  643. 
Euclase,  680. 
Eucryptite,  441. 
Eudialyite,  665. 
Eudidymite,  674,  676. 
Eulytite,  651. . 
Eutectics,  241. 
Euxenite,  668. 
Evansite,  518,  656. 
Excelsior,  diamond,  283. 
Expansion,  3. 
Extinction,  186. 
angles,  185,  198. 
inclined,  198. 
parallel,  187. 
straight,  187. 
Extraordinary  ray,  169. 


Faces,  similar,  11. 

vicinal,  267. 
Fairfieldite,  655. 
False  topaz,  359. 
Famatinite,  623. 
Faujasite,  673. 
Fayalite,  450,  658. 
Feldsite,  408. 
Feldspar  group,  403,  404. 

lime,  413. 


Feldspar   soda,  411. 

potash,  403. 
Feldspars,  monoclinic,  403. 

triclinic,  411. 
Feldspathoids,  439. 
Felsobanyite,  677. 
Ferberite,  542. 
Fergusonite,  506,  634,  669. 
Ferronatrite,  635. 
Ferrites,  233,  337. 
Fibroferrite,  636. 
Fibrolite,  461. 
Fibrous,  266. 
Fillowite,  653. 
First  median  line,  173. 
First  order  colors,  184. 
Fischerite,  656. 
Flame,  548. 
Flame  coloration,  553. 
Fletcher  ellipsoid,  185. 
Flinkite,  652. 
Flint,  361. 
Florencite,  656. 
Flos  ferri,  393. 
Fluellite,  679. 
Fluocerite,  678. 
Fluor-apatite,  509. 
Fluorescence,  280. 
Fluorine,  tests,  589. 
Fluorite,  331,  603,  609,  662. 
Fluorspar,  331,  609,  662. 
Foliated,  269. 
Fontainebleau,  380. 
Footeite,  644. 
Forbesite,  651. 
Forms,  21. 

derivation,  25. 

enantiomorphic,  27. 

fundamental,  23. 

gyroidal,  26. 

hemihedral,  25. 

holohedral,  24. 

holosymmetric,  23. 

polar,  55.. 

tetartohedral,  27. 
Formula,  empirical,  227. 

general,  231. 

molecular,  2. 

structural,  227. 
Forsterite,  450,  680. 
Fowlerite,  430. 
Fracture,  256,  258. 
Franckeite,  624. 
Franklinite,  375,  596,  633. 
Freezing  point,  1. 
Freibergite,  325. 
Freieslebenite,  624. 
Friable,  258. 
Friedelite,  666. 
Fuchsite,  490. 
Fusibility,  555. 


INDEX 


689 


G 


Gadolinite,  681. 
Gageite,  340. 
Gahnite,  377,  650. 
Galena,  298,  627. 
Galenite,  298,  597,  627. 
Galenobismuthite,  320,  628. 
Gallium,  tests,  577. 
Ganomalite,  648. 
Ganophyllite,  666. 
Garnet,  442,  605,  610. 

composition,  231. 

group,  442. 

precious,  443. 
Garnierite,  500,  608. 
Gaylussite,  401,  661. 
Gearksutite,  663. 
Gehlenite,  681. 
Gelatinization,  591. 
Geniculate  twins,  141. 
Genthite,  500,  669. 
Geocronite,  624. 
Geodes,  270. 
Gerhardtite,  645. 
Germanium,  tests,  586. 
Gersdorffite,  316,  597,  622. 
Geyserite,  370. 
Ghost,  276. 

Gibbsite,  365,  656,  678. 
Gismondite,  479,  671. 
Glass,  610. 
Glassware,  556. 
Glassy,  279. 
Glauberite,  528,  662. 
Glaucochroite,  665. 
Glaucodote,  319,  621. 
Glaucophane,  676. 
Gmelinite,  479,  671. 
Goethite,  658. 
Gold,  291,  582,  631. 
Goldschmidtite,  626. 
Goniometer,  11. 

contact,  12,  149. 

reflecting,  12,  150. 

signal,  152. 

two-circle,  150. 

use  of,  149. 
Goslarite,  539,  639. 
Gossan,  253. 
Gothite,  363,  633. 
Goyazite,  656. 
Granular  structure,  268. 
Graphite,  284,  596,  597,  609,  634. 
Gravity,  specific,  260. 
Greasy  luster,  279. 
Greenockite,  93,  306,  651. 
Grossularite,  443,  676. 
Ground  water,  251. 
Griinlingite,  626. 
Guanajuatite,  627. 

2r 


Guarinite,  667. 
Guitermanite,  622. 
Gummite,  526. 
Gypsum,  536,  601,  613,  662. 
Gyrolite,  671. 

H 

Habit,  29. 

Hackly  fracture,  258. 
Haidingerite,  653. 
Halite,  79,  327,  600,  639. 
Haloids,  233,  327. 
Halotrichite,  636. 
Hamlinite,  655. 
Hancockite,  648. 
Hanksite,  89,  535,  637. 
Hardness,  258,  259. 
Hardystonite,  649. 
Hambergite,  664. 
Harmotome,  482,  670,  672. 
Hatchettolite,  668. 
Hauchecornite,  625. 
Hauerite,  631. 
Hausmannite,  632. 
Hatiyne,  438,  674. 
Haiiynite,  438,  674. 
Heat  conductivity,  3. 

of  fusion,  239. 
Heating,  in  closed  tube,  557. 

in  open  tube,  558. 

on  charcoal,  551. 
Heavy  liquids,  263,  264. 
Heavy  spar,  528. 
Hedenbergite,  424. 
Heintzite,  664. 
Helvite,  666. 
Hemafibrite,  652. 
Hematite,  91,  343,  596,  598,  607,  609,  634, 

657. 

Hematolite,  652. 
Hemidomes,  117. 
Hemihedrism,  diagonal-faced,  26. 

gyroidal,  26. 

parallel-faced,  26. 
Hemimorphism,  25. 
Hemimorphite,  118,  472. 
Hemiorthodome,  127. 

pyramid,  117. 

prism,  126. 
Hercynite,  371,  682. 
Herderite,  655. 
Herrengrundite,  642. 
Hessite,  626. 
Hessonite,  443. 
Heulandite,  481,  603,  670. 
Hexagonal  alternating,  97. 

axes,  85. 

axial  ratio,  112. 

axis,  14. 

equatorial,  93. 

hemipyramid,  1st  order,  92. 


690 


INDEX 


Hexagonal  hemipyramid,  2d  order, 
hemipyramid,  3d  order,  100. 
holoaxial,  95. 
holosymmetric,  85. 
polar,  99. 

prism,  1st  order,  88. 
2d  order,  88. 
3d  order,  94. 
pyramid  1st  order,  86. 
2d  order,  87. 
3d  order,  94. 
system,  17,  84. 
trapezohedron,  96 
Hexahedron,  61. 
Hexagonite,  433. 
Hexoctahedron,  48. 
Hextetrahedron,  55. 
Hiddenite,  427. 
Hielmite,  635. 
Hiortdahlite,  675. 
Hisingerite,  659. 
Hoernesite,  653. 
Holoaxial,  61. 
digonal,  118. 
hexagonal,  95. 
tesseral,  61. 
tetragonal,  75. 
trigonal,  108. 
Holohedral,  24. 
Holosymmetric,  24- 
cubic,  47. 
hexagonal,  85. 
monoclinic,  121. 
orthorhombic,  113. 
tetragonal,  65. 
triclinic,  ISO. 
Homilite,  664. 
Hope  diamond,  282. 
Hopeite,  650. 

Horizontal  dispersion,  208. 
Hornblende,  433,  676. 
Horn  silver,  330. 
Horseflesh  ore,  310. 
Hortonolite,  658. 
Howlite,  664. 
Hubnerite,  542,  665. 
Humite,  471,  679. 
Hyacinth,  458. 
Hyalite,  370. 
Hyalotekite,  648. 
Hydrargillite,  365. 
Hydroboracite,  664. 
Hydrocerussite,  647. 
Hydrochloric  acid,  560. 
Hydrocyanite,  641. 
Hydrofluoric  acid.  233. 
Hydrogen  sulphide,  232. 
Hydrogiobertite,  661. 
Hydroherderite,  655. 
Hydromagnesite,  362,  661. 
Hydronephelite,  479,  673. 


Hydrophilite,  639. 
Hydrotalcite,  663. 
Hydroxides,  362. 
Hydrozincite,  649. 
Hypersthene,  421,  604,  616,  682. 


Ice,  337. 

Iceland  spar,  384. 

Icositetrahedron,  61. 

Idocrase,  456. 

Ihleite,  636. 

Ilesite,  636. 

Ilmenite,  346,  609,  633. 

Ilvaite,  472,  633,  658. 

Inclined  dispersion,  208. 

Inclusions,  9. 

Index  of  refraction,  165,  217. 

determination  of,  209. 
Indicatrix,  185,  197. 
Indices,  18. 

determination  of,  156. 

law  of  rational,  20. 

of  refraction,  165,  209. 
Indicolite,  475. 
Indium,  tests,  577. 
Inesite,  666. 
Intercepts,  18. 
Interference,  181. 

colors,  183. 
Interference  figure,  biaxial,  200. 

uniaxial,  191. 

Intergrowths  of  minerals,  243. 
Interpenetration  twins,  141* 
Intumescence,  555. 
Iodine,  589. 
lodobromite,  645. 
lodyrite,  330,  646. 
lolite,  440,  613,  680. 
Iridescence,  183,  274. 
Iridium,  632. 
Iridium,  tests,  582. 
Iridosmine,  632. 
Iron,  631. 
Iron  cap,  253. 
Iron,  tests  for,  575. 

native,  293. 

specular,  344. 

titaniferous,  346. 
Irregularity  of  crystals,  29. 
Isoclasite,  656. 
Isometric  system,  16,  47. 
Isomorphism,  228. 
Isomorphous  groups,  229. 
Isomorphous  mixture,  229. 
Isotropic  crystals,  163. 
Itacolumite,  282. 


Jacobsite,  371,  633. 
Jacynth,  458. 


INDEX 


691 


Jade,  426. 

Jadeite,  420,  426,  676. 
Jamesonite,  320,  624. 
Jargon,  458. 
Jarosite,  635. 
Jasper,  361. 
Jeffersonite,  424,  650. 
Jeremejevite,  665. 
Jordanite,  622. 
Josephinite,  632. 
Joly's  balance,  261. 

K 

Kainite,  534,  638. 
Kalgoorlite,  626. 
Kalinite,  542,  600,  638. 
Kaliophilite,  441. 
Kallilite,  625. 
Kammererite,  669. 
Kaolin,  501. 

(Kaolinite,  501,  601,  613,  678. 
Keilhauite,  668. 
Kentrolite,  627,  648. 
Kermesite,  640. 
Kieserite,  335,  539,  638. 
Kilbrickenite,  624. 
Klaprotholite,  628. 
Knebelite,  451,  658. 
Knoxvillite,  636. 
Kobellite,  625. 
Koh-i-noor  diamond,  282. 
Koninckite,  655. 
Kornerupine,  681. 
Krennerite,  626, 
Krohnkite,  641. 
Kunzite,  427. 


Labradorite,  416,  604,  674. 
Lamellar,  269. 
Lampadite,  369. 
Langbeinite,  638. 
Langite,  642. 
Lanthanum,  tests,  572. 
Lapis-lazuli,  439,  608< 
Larkinite,  652. 
Laumontite,  479,  671. 
Laurionite,  647. 
Laurite,  287. 
Lautarite,  663. 
Lautite,  621. 

Law,  of  rational  indices,  19. 
Lawsonite,  674. 
Lazulite,  515,  654,  656. 
Lazurite,  438,  674. 
Lead,  293,  628. 

coat,  580. 

oxide,  341. 

tests  for,  579. 

use  of  test,  559. 
Leadhillite,  646. 


Lecontite,  637. 
Left-handed  crystal,  27. 
Lenarkite,  647. 
Lengenbachite,  621. 
Lepidolite,  495,  601,  671. 
Lepidomelane,  659. 
Lettering,  33. 

Leucite,  416,  604,  613,  680. 
Leucochalcite,  643. 
Leucophanite,  672. 
Leucophcenicite,  650. 
Leucopyrite,  622. 
Leucoxene,  347. 
Levynite,  671. 
Libethenite,  513,  644. 
Light,  160. 

interference,  181. 

convergent,  192. 

polarized,  175. 
Light  waves,  161. 

refraction  of,  164- 
Lillianite,  628. 
Limestone,  385. 

Limonite,  363,  596,  606,  607,  633,  658. 
Linarite,  642. 
Lindackerite,  642. 
Linnaeite,  631. 
Lintonite,  488. 
Liroconite,  643. 
Lithia  mica,  495. 
Lithium,  tests,  564. 

flux,  559. 

Livingstonite,  623. 
Lodestone,  374. 
Lollingite,  622. 
Lorandite,  640. 
Lossenite,  646. 
Loweite,  539,  637. 
Lowigite,  677. 
Ludlamite,  654. 
Liineburgite,  657. 
Luster,  273,  278. 
Lydia  stone,  361. 

M 

Macro-axis,  113. 
Macrodome,  115. 
Macropinacoid,  116. 
Macroprism,  115. 
Macropyramid,  114- 
Magma,  243. 
Magnesia  micas,  497. 
Magnesium,  tests,  567. 
Magnesite,  386,  602,  661. 
Magnesoferrite,  634. 
Magnetism,  552. 
Magnetite,  373,  596,  609. 
Malachite,  398,  608,  641. 
Malacon,  458. 
Mallardite,  639. 
Malleability,  258. 


692 


INDEX 


Mamillary,  273. 
Manebach  twins,  406. 
Manganese,  tests,  574. 
Manganite,  367,  596,  632. 
Mangano-columbite,  665. 
Manganoferrite,  371. 
Manganosite,  340,  667. 
Manganostibnite,  651. 
Marble,  385. 

Marcasite,  317,  599,  630. 
Margarite,  496,  672. 
Marialite,  453,  676. 
Marshite,  645. 
Martinite,  656. 
Martite,  345. 
Mascagnite,  638. 
Massicot,  341,  649. 
Massive,  268. 
Matildite,  629. 
Matrass,  557. 
Mauzeliite,  646. 
Mazapilite,  652. 
Mean  refractive  index,  171. 
Measurement  of  crystals,  149. 
Meerschaum,  501. 
Meionite,  453. 
Melaconite,  340,  596,  631. 
Melanite,  444. 
Melanocerite,  665. 
Melanophlogite,  680. 
Melanostibian,  633. 
Melanotekite,  628,  648. 
Melanterite,  539,  600,  636. 
Melilite,  675. 
Meionite,  626. 
Melting  point,  238. 
Menaccanite,  346,  596. 
Mendeleef's  table,  223. 
Mendozite,  637. 
Meneghinite,  624. 
Mercury,  293,  631. 
Mercury,  tests,  579. 
Mesitite,  229,  388. 
Mesolite,  673. 
Metacinnabar,  305. 
Metallic  luster,  274. 
Metallic  mirror,  557. 
Metamorphism,  248. 
Metasilicates,  233. 
Metasilicic  acid,  233. 
Metastable,  243.     ' 
Metavoltaite,  635. 
Methylene  iodide,  211,  263. 
Miargyrite,  625. 
Mica,  488. 

brittle,  496. 

group,  488. 

percussion  figure,  489. 

plate,  194. 

1st  class,  493. 

2d  class,  493. 


Micaceous,  257. 

cleavage,  489. 

hematite,  343. 

Microcline,  409,  604,  613,  675. 
Microlite,  669. 
Microperthite,  408. 
Microsommite,  674. 
Miersite,  646. 
Milarite,  675. 
Millerian  indices,  18. 
Millerite,  307,  599,  630. 
Mimetite,  95,  512,  646. 
Mineral,  219. 

groups,  232. 
Mineralizers,  246. 
Minerals,  color  of,  273. 

index  of  refraction,  165. 

luster  of,  274,  278. 

rock-forming,  609. 

specific  gravity,  260. 

table  of,  617. 
Minervite,  656. 
Minium,  649. 
Mirabilite,  535,  600,  637. 
Mispickel,  318. 
Mixite,  642. 
Mizzonite,  453. 
Models,  28. 

Moh's  scale  of  hardness,  259. 
Molecular,  volume,  228. 
Molybdates,  236. 
Molybdenite,  296,  596,  597,  630. 
Molybdenum,  tests,  586. 
Monazite,  507,  603,  657. 
Monetite,  655. 

Monobromonaphthalene,  215. 
Monochromatic  light,  183. 
Monoclinic  crystals,  120. 

axial  ratio,  128. 

dispersion,  207. 

extinction,  198. 

pyroxenes,  423. 

system,  17,  120. 

twins,  148. 
Monoxides,  337. 
Montanite,  651. 
Montebrasite,  657. 
Monticellite,  449. 
Montroydite,  641. 
Mordenite,  479,  672. 
Morenosite,  539,  637,  669. 
Morganite,  437. 
Mosandrite,  668, 
Moss  agate,  361. 
Mossite,  634. 
Muscovite,  488,  489,  601,  614,  672. 


N 


Nadorite,  647. 
Nagyagite,  625. 


INDEX 


693 


Nailhead  spar,  379. 
Nantokite,  644. 
Nasonite,  648. 
Native  elements,  281. 
Natrolite,  486,  603,  673. 
Natron,  400,  639. 
Natrophilite,  653. 
Naumannite,  627. 
Negative  birefringence,  169. 

crystals,  169,  170. 

forms,  27,  79. 
Nemalite,  362. 
Neodynium,  572. 
Nepheline,  440,  604,  675. 
Nephelite,  100,  440,  604,  611,  675. 
Neptunite,  505,  633,  666,  668. 
Nesquehonite,  661. 
Niccolite,  309,  599,  622. 
Nickel,  tests,  574. 
Nicols  prism,  179. 
Niobium  (columbium),  570. 
Niter,  521,  600,  639. 
Nitrates,  507. 

tests,  590. 
Nitric  acid,  560. 
Nitrobarite,  640. 
Nitrocalcite,  521. 
Nodular,  269. 
Non-metallic  luster,  274. 
Nordenskioldine,  646. 
Normal,  43. 

salt,  227. 
Northupite,  661. 
Nosean,  438. 
Noselite,  438,  674. 
Nugget,  269. 

O 

Obtuse  bisectrix,  173. 
Ocher,  365. 
Ochrolite,  647. 
Octahedrite,  351,  668. 
Octahedron,  52. 

hex-,  49. 

tetragonal  tris-,  50. 

trigonal  tris-,  50. 
Oldhamite,  662. 
Oligoclase,  415. 
Olivenite,  513,  643 
Olivine,  446,  605,  614,  680. 
Odors,  552,  557. 
Okenite,  671,  673. 
Onyx,  360. 
Oolitic,  271. 

Opal,  369,  604,  609,  680. 
Opaque,  275. 
Open  tubes,  558. 
Optic  axes,  172,  198. 

axial  angle,  17$. 

measurement  of,  205. 


Optical  characters,  174. 

orientation,  174,  175. 

properties,  160,  174,  175. 

sign,  169,  194,  204. 
Ordinary  ray,  169. 
Organic  compounds,  591. 
Oriental  amethyst,  342. 

emerald,  342. 

topaz,  342. 

Origin  of  minerals,  237. 
Orloff  diamond,  282. 
Orochlorite,  497. 
Orofrite,  627. 
Orpiment,  295,  606,  640. 
Orthoaxis,  120. 

Orthoclase,  128,  404,  604,  613,  675. 
Orthodome,  128. 
Orthopinacoid,  123. 
Orthoprism,  123. 
Orthopyramids,  122. 
Orthorhombic  system,  17,  113. 
Orthosalts,  233. 
Orthosilicates,  233,  438. 
Orthosilicic  acid,  233. 
Orthorhombic  amphiboles,  431. 

axial  ratio,  120. 

crystals,  113. 

dispersion,  207. 

hemihedral,  118. 

hemipyramids,  117. 

holoaxial,  118. 

holohedral,  113. 

holosymmetric,  113. 

prisms,  115. 

pryoxene,  419. 

Oscillatory  combinations,  137. 
Osmium,  tests,  582. 
Ottrelite,  497. 
Oxidation,  251. 
Oxides,  233,  337. 
Oxidizing  flame,  549. 


Pachnolite,  334,  663. 
Palladium,  632. 

tests,  582. 
Pandermite,  525. 
Paragonite,  672. 
Parallel-faced  hemihedrons, 
Parallel  growths,  134. 
Paramelaconite,  631. 
Parameters,  17,  156. 

topic,  228. 

Parametral  face,  23. 
Parisite,  661. 
Parting,  257. 
Partschinite,  666. 
Pearceite,  620. 
Pearly  luster,  279. 


694 


INDEX 


Pectolite,  437,  603,  673. 

Peganite,  518,  656. 

Penfieldite,  647. 

Pentagonal  dodecahedron,  60. 
didodecahedron,  61. 

Pentlandite,  308,  630. 

Percussion  figure,  489. 

Percylite,  644. 

Periclase,  340,  663. 

Pericline,  twins,  410. 

Peridote,  446. 

Periodic  table,  323. 

Perovskite,  505,  634,  668. 

Perpurite,  654. 

Perthite,  408. 

Petalite,  675. 

Petzite,  626. 

Phantoms,  376. 

Pharmacosiderite,  652. 

Phase,  338. 

Phenacite,  99,  453,  682. 

Phenocrysts,  216. 

Phillipsite,  479,  673. 

Phlogopite,  494,  601,  672. 

Phoenicochroite,  649. 

Phosgenite,  647. 

Phosphates,  336,  507. 

Phosphorescence,  379. 

Phosphorus,  tests,  590. 

Phosphosiderite,  510,  654. 

Phosphuranylite,  655. 

Physical  properties,  356. 
Picotite,  373. 
Picromerite,  539,  638. 
Piedmontite,  467,  666. 
Pinacoid,  68. 
basal,  116,  123. 
brachy-,  116. 
cleavage,  356. 
clino-,  123. 
macro-,  116. 
ortho-,  123. 
Pinakiolite,  664. 
Pinnoite,  664. 
Pirssonite,  661. 
Pisanite,  540,  641. 
Pisolitic,  371. 
Pitchblende,  535. 
Pitt  diamond,  383. 
Pitticite,  652. 
Placer  mining,  391. 
Plagioclase,  411,  614. 
Plagiohedral,  26. 
Plagionite,  624. 
Plane,  axial,  173. 
basal,  68. 
composition,  138. 
diametral,  16. 
of  polarization,  176. 
Plane  of  symmetry,  13. 
of  vibration,  176. 


Plane,  parametral,  23. 

twinning,  138. 
Planoferrite,  658. 
Plaster  of  Paris,  537. 
Platinum,  387,  631. 

forceps,  554. 

tests,  582. 

wire,  553. 

Plattnerite,  628,  649. 
Pleochroism,  189. 
Plumbago,  384. 
Plumbojarosite,  647. 
Pneumatolysis,  348. 
Point-system,  5. 
Polar,  55. 

axis,  55. 

digonal,  124- 

didigonal,  117. 

dihexagonal,  92. 

ditesseral,  64. 

ditetragonal,  71. 

ditrigonal,  103. 

hexagonal,  99. 

tesseral,  62. 

tetragonal,  78. 

trigonal,  110. 
Polarization,  of  light,  175. 

circular,  176,  196. 
Polarizer,  177. 
Pole  of  face,  43. 
Polianite,  632. 
Pollucite,  679. 
Polyadelphite,  444. 
Polyargyrite,  336,  625. 
Polybasite,  336,  624. 
Polycrase,  635,  668. 
Polydymite,  307,  630. 
Polyhalite,  662. 
Polylithionite,  496. 
Polymignite,  635. 
Polymorphism,  319. 
Polysynthetic  twins,  142. 
Positive  birefringence,  169,  170. 
Positive  forms,  27. 
Potash  alum,  543. 
Potash  feldspar,  403. 
Potash  mica,  489. 
Potassium  mercuric  iodide,  363. 
Potassium,  tests,  563. 
Powdery,  369. 
Powellite,  544,  670. 
Praseodymium,  tests,  572. 
Precious  garnet,  443. 

opal,  369. 

Prehnite,  470,  604,  676. 
Pressure  figure,  490. 
3riceite,  535. 
3rimary  optic' axes,  172. 
3rimary  minerals,  344. 
rimitive  circle,  43. 
Prism,  113. 


INDEX 


695 


Prism,  brachy,  lid. 
dihexagonal,  87. 
ditetragonal,  67. 
ditrigonal,  102. 

macro,  115. 

nicol,  179. 

ortho,  123. 

tetragonal,  67. 

trigonal,  108. 

1st  order  tetragonal,  67. 

2d  order  hexagonal,  88. 

3d  order,  hexagonal,  94- 

2d  order  tetragonal,  68. 
Prismatic  cleavage,  256. 
Prismatic  habit,  265. 
Prochlorite,  678. 
Projection,  31. 

axes,  35. 

clinographic,  31,  33. 

orthographic,  31. 

stereographic,  42. 
Prolectite,  679. 
Prosopite,  663,  679. 
Proustite,  323,  598,  607,  645. 
Pseudobrookite,  634. 
Pseudomalachite,  644. 
Pseudomorph,  361,  363. 
Pseudosymmetry,  141. 
Psilomelane,  368,  596,  632. 
Ptiolite,  479. 
Pucherite,  651. 
Pycnometer,  263. 
Pyramid,  113. 

dihexagonal,  86. 

ditrigonal,  101. 

herni-,  71. 

hexagonal,  86. 

monoclinic,  122. 

tetragonal,  66. 

trigonal,  102. 

Pyrargyrite,  322,  596,  607,  624,  645. 
Pyrite,  313,  599,  609,  630. 

arsenical,  318. 

class,  313. 

copper,  310. 

group,  313. 

magnetic,  308. 

twins,  141. 
Pyritohedron,  60. 
Pyroaurite,  658. 
Pyrochlore,  668. 
Pyrochroite,  362,  632,  667. 
Pyroelectric,  475. 
Pyrolusite,  352,  596,  597,  632. 
Pyromorphite,  95,  511,  602,  606,  648. 
Pyrope,  443,  677. 
Pyrophane,  347. 
Pyrophanite,  632. 
Pyrophillite,  502,  678. 
Pyrosmaltite,  659. 
Pyrostilpnite,  645. 


Pyroxene,  604,  608,  615,  677. 

group,  419,  420. 

monoclinic,  423. 

orthorhombic,  419. 
Pyrrhotite,  308,  599,  609,  630. 


Q 


Quartz,  110,  352,  605,  611,  681. 

interference  figure,  196. 

rotary  polarization,  197. 

smoky,  359. 

wedge,  182,  184,  187,  196. 
Quartzite,  356. 
Quenstedtite,  636. 
Quicksilver,  293. 


R 


Radiated,  269. 
Radicles,  224. 
Radium,  514,  567. 
Raimondite,  637. 
Ralstonite,  679. 
Rammelsbergite,  316,  622. 
Ranite,  479. 
Raspite,  649. 
Rational  indices,  19. 
Ray,  extraordinary.  169. 

ordinary,  169. 
Realgar,  294,  606,  640. 
Reagents,  559,  560. 
Reddingite,  655. 
Reducing  flame,  550. 
Reduction  with  soda,  579. 
Reentrant  angle,  134. 
Reflected  light,  163. 
Reflection  goniometer,  12,  150. 
Reflection  twins,  140. 
Refraction,  164. 

double,  168. 

indices  of,  165. 

mean  index  of,  171.       ( 
Refractometer,  212,  213. 
Regent  diamond,  282. 
Reinite,  633. 
Remingtonite,  669. 
Rensslerite,  500. 
Repeated  twinning,  142. 
Replacement,  22. 
Resinous  luster,  279. 
Reticulated,  269. 
Rhabdophanite,  657. 
Rhagite,  650. 
Rhodium,  582. 
Rhodizite,  663. 
Rhodochrosite,  391,  602,  667. 
Rhodonite,  132,  430,  604. 
Rhombic  dodecahedron,  52. 
Rhombohedral  carbonates,  379. 


696 


INDEX 


Rhombohedron,  90. 
Richardite,  626. 
Hichterite,  667. 
Kirbeckite,  435,  659. 
Right-handed  crystals,  27,  197. 
Rinkite,  667. 
Ripidolite,  497. 
Roasting,  552. 
Rock  crystal,  358. 
Rock  salt,  327. 
Rock  sections,  216. 
Roeblingite,  647. 
Romerite,  540,  636. 
Roepperite,  451. 
Rontgen  ray,  280. 
Roscoelite,  490,  670. 
Rotation  twins,  137. 
Rotatory  polarization,  196. 
Rubellite,  475. 
Rubidium,  tests,  564. 
Ruby,  342. 

silver,  322,  323. 

spinel,  371. 
Ruthenium,  582. 
Rutile,  349,  597,  598,  605,  612,  634,  668 

S 

Safflorite,  315,  621. 
Sagenite,  350. 
Salt,  240. 
Saltpeter,  521. 
Salts,  224. 

acid,  226. 

basic,  226. 

hydrated,  227. 

normal,  226. 
Samarium,  572. ' 
Sand,  black,  288. 

green, 

Sandstone,  356. 
Sanidine,  408. 
Sapphire,  342. 
Sarcolite,  675. 
Sartorite,  320,  622. 
Sassolite,  523,  640. 
Satin  spar,  537. 
Scale  of  colors,  184. 

of  fusibility,  555. 

of  hardness,  260. 
Scalenohedron,  90. 

,  tetragonal,  60,  70. 
Scandium,  572. 
Scapolite,  453,  612. 
Schapbachite,  628. 
Scheelite,  543,  603,  668. 
Schefferite,  424,  666. 
Schiller,  422. 
Schirmerite,  628. 
Schorl,  474. 
Schorlomite,  444,  667. 


Schwartzenbergite,  648. 

Schwartzite,  325. 

Scleromater,  259. 

Scolecite,  479,  673. 

Scorodite,  652. 

Second  order  of  colors,  184' 

Secondary  enrichment,  253. 

Secondary  minerals,  244. 

Secondary  optic  axes,  172. 

Sectility,  258. 

Selenite,  537. 

Selenium,  537,  587. 

Sellaite,  677. 

Senaite,  627. 

Senarmontite,  346,  640. 

Sensitive  plate,  199. 

Sepiolite,  501,  601,  678. 

Sericite,  491. 

Serpentine,  498,  601,  614,  678. 

Sesquioxides,  341. 

Siderite,  91,  388,  602,  606,  657. 

Sideronatrite,  635. 

Sign  of  birefringence,  169,  173. 

Silicates,  233,  403. 

classification  of,  233. 
Silicic  acids,  233. 

salts  of,  233. 
Silicon,  tests,  591. 
Silicified  wood,  361. 
Silky  luster,  279. 
Sillimanite,  461,  614,  681. 
Silver,  290,  578,  597,  631. 

glance,  297. 
Similar  faces,  11. 
Sipylite,  669. 
Skutterudite,  315,  621. 
Smaltite,  315,  598,  621. 
Smarskite,  634. 
Smithsonite,  392,  603,  649. 
Smoky  quartz,  359. 
Snow,  337. 
Soapstone,  500. 
Soda,  558. 

Sodalite,  438,  604,  610,  675. 
Sodalite  group,  438. 
Soda  niter,  520,  600,  639.  . 
Sodium,  tests  for,  564. 
Solutions,  248. 
Space-lattice,  4. 
Spadaite,  671. 
Spangolite,  642. 
Spathic  iron  ore,  388. 
Specific  gravity,  261. 

heat,  239. 

Specular  iron  ore,  344. 
Sperrylite,  287,  623. 
Spessartite,  444,  666. 
Sphaerite,  518,  656. 
Sphaerocobaltite,  393,  669. 
Sphalerite,  301,  596,  602,  606,  607,  630, 
649. 


INDEX 


697 


Sphene,  503,  616. 
Sphenoid,  70. 

orthorhombic,  118. 
Spinel,  371,  605,  610,  682. 

group,  371. 

twins,  143. 
Splintery,  358. 
Spodumene,  426,  605,  675. 
Stalactite,  273. 
Stalagmite,  273. 
Stannite,  312,  629. 
Star  of  the  south  diamond,  282. 
Stassfurtite,  522. 
Staurolite,  477,  605,  616,  680. 
Steatite,  500. 
Stelznerite,  642. 
Stephanite,  325,  597. 
Stercorite,  656. 
Stereographic  projection,  42. 
Sternbergite,  629. 
Stibnite,  295,  597,  623. 
Stilbite,  483,  602,  670. 
Stilpnomelane,  498,  659. 
Stokesite,  646. 
Stolzite,  545,  649. 
Straight  extinction,  187. 
Streak,  274. 
Stream  tin,  349. 
Strengite,  510,  654. 
Striations,  137,  266. 
Stromeyerite,  301,  629. 
Strontianite,  396,  602,  660. 
Structural  formula,  227. 
Struvite,  118,  656. 
Stylotypite,  623. 
Subconchoidal  fracture,  258. 
Sublimates,  7,  557. 
Sulpharsenates,  323. 
Sulphates,  236. 
Sulphides,  232,  294. 
Sulpho  acids,  232,  320. 
Sulphohalite,  638. 
Sulphur,  285,  587,  601,  606,  640. 
Sulvanite,  629. 
Supplementary  forms,  27. 

twins,  140. 

Surfaces  of  crystals,  266. 
Sussexite,  664. 
Svabite,  653. 
Svanbergite,  655. 
Swallow-tail  twins,  536. 
Sylvanite,  626. 
Sylvite,  328,  600,  639. 
Symmetry,  13. 

alternating,  15. 

axes  of,  13,  15. 

center  of,  13. 

planes  of,  13. 

pseudo-,  146. 
Synadelphite,  652. 
Syngenite,  638,  662. 


System,  cubic,  47. 

hexagonal,  84. 

monoclinic,  120.  • 

orthorhombic,  123. 

tetragonal,  65. 

triclinic,  129. 

two-component,  240. 
Szaibelyite,  664. 


Table,  for  blowpipe  determination,  617. 

of  rock-forming  minerals,  609. 

for  determination  of  common  miner- 
als, 596.  . 

of  coats  on  coal,  593. 

for  fusibility,  555. 
Tabular  habit,  265. 
Tachydrite,  639. 
Tagilite,  644. 

Talc,  500,  601,  614,  663,  679. 
Tantalates,  236. 
Tantalite,  506,  635. 
Tantalum,  571. 
Tapalpite,  626. 
Tapiolite,  635. 
Tarnish,  275. 
Taste,  618. 
Tavistokite,  655. 
Taylorite,  638. 
Tellurates,  236. 
Tellurium,  587,  626. 
Tenacity,  258. 
Tennantite,  324,  621. 
Tenorite,  340. 
Tephroite,  451,  665. 
Terlinguaite,  641. 
Tesseral  central,  59. 

holoaxial,  61. 

polar,  62. 

Tetartohedral  forms,  27. 
Tetradymite,  625. 
Tetragonal  alternating,  76. 

axial  ratio,  82. 

axis,  14- 

equatorial,  72. 

hemihedral,  72. 

holoaxial,  75, 

holohedral,  65. 

holosymmetric,  65. 

polar,  78. 

prism,  67. 

1st  order,  67. 
2d  order,  68. 
3d  order,  74. 

pyramid,  66. 
1st  order,  66. 
2d  order,  67. 
3d  order,  73. 

scalenohedron,  69. 


698 


INDEX 


Tetragonal  system,  16,  65. 
trapezohedron,  75. 

trisoctahedron,  50. 

tristetrahedron,  57. 
Tetrahedrite,  324,  596,  597. 
Tetrahedron,  67. 

tetragonal  tris-,  57. 

trigonal  tris-,  56. 

hex-,  65. 

Tetrahexahedron,  49. 
Thallium,  tests,  577. 
Thaumasite,  661. 
Thenardite,  527,  638. 
Thermonatrite,  639. 
Thin  sections  colors,  216. 
Third  order  of  colors,  184- 
Thirty-two  types  of  crystals,  6,  47. 
Thomsenolite,  663. 
Thomsonite,  487,  673. 
Thorite,  457,  679,  681. 
Thorium,  tests,  572. 
Thoulet's  solution,  263. 
Thulite,  465. 
Thuringite,  658. 
Tiemannite,  627. 
Tiger-eye,  435. 
Tilasite,  653. 
Tile  ore,  338. 
Tin,  584,  632. 
Tincal,  523. 
Tinstone,  348. 
Titanates,  233. 
Titanic  iron  ore,  346. 
Titanite,  503,  603,  616,  667. 
Titanium,  tests,  569. 

oxides,  351. 
Topaz,  458,  605,  681. 

false,  359. 

oriental,  342. 
Topazolite,  444. 
Topic  parameters,  228. 
Torbernite,  519,  644. 
Torrensite,  667. 
Total  reflection,  167. 

reflectometer,  211. 
Touchstone,  361. 
Tough,  257,  258. 
Tourmaline,  105,  473,  605,  612,  064. 

sections  of,  177. 

tongs,  178. 
Translucency,  275. 
Transparent,  275. 
Trapezohedron,  hexagonal,  96. 

tetragonal,  75. 

trigonal,  109. 
Tremolite,  431,  677. 
Trichalcite,  643. 
Triclinic  system,  17,  129. 
Tridymite,  361,  611,  681. 
Trigonal  axis,  14, 

equatorial,  105. 


Trigonal  holoaxial,  108. 
polar,  110. 
prisms,  103,  106. 
pyramids,  102,  106. 
trapezohedron,  109. 

trisoctahedron,  50. 

tristetrahedron,  56. 
Trimerite,  666. 
Trimorphism,  220. 
Triphylite,  653. 
Triplite,  654. 
Triploidite,  654. 
Tripolite,  371. 
Tripuhyite,  651. 
Trisilicates,  234. 
Trisilicic  acid,  234. 
Tritomite, 
Trogerite,  652. 
Troilite,  308,  630. 
Trona,  401,  600,  639. 
Troostite,  452. 
Truncation,  22. 
Tscheffkinite,  667. 
Tschermigite,  638. 
Tungstates,  236. 
Tungsten,  587. 
Tungstite,  668. 
Turgite,  363,  633,  658. 
Turingite,  498. 
Turkey  fat,  307,  392. 
Turner's  flux,  559. 
Turquoise,  3,  510,  518,  604. 
Twin  axis,  138. 

Brazilian,  146. 

crystals,  138. 

interpenetrating,  141. 

lamellae,  142. 

plane,  138. 

reflection,  140. 

rotation,  138. 

spinel,  143. 

striae,  142. 

supplementary,  140. 
Twinning,  138. 

lamellae,  142, 

polysynthetic,  142. 
Twins,  137. 
Tychite,  661. 
Tyrolite,  643. 
Tysonite,  679. 

U 

Ulexite,  524,  601,  663. 
Ullmannite,  316,  625. 
Umangite,  627. 
Uniaxial  crystals,  170,  184. 
interference  figure,  191. 
Unit  form,  23. 
Uralite,  435. 
Uralitization,  426. 


INDEX 


699 


Uranates,  236,  507. 
Uraninite,  525,  596,  608,  634. 
Uranium,  tests,  576. 
Uranocircite,  655. 
Uranophane,  669. 
Uranophilite,  669. 
Uranospinite,  653. 
Uranothallite,  661. 
Utahite,  636. 
Uvarovite,  445,  669. 


Valentinite,  346,  641. 
Vanadates,  236. 
Vanadinite,  95,  512,  648. 
Vanadium,  tests,  576. 
Variscite,  510,  519,  656. 
Vauquelite,  644. 
Verdi  antique,  499. 
Vermiculite,  670. 
Vesuvianite,  455,  605,  611,  676. 
Veszelyite,  642. 
Vibrations  plane,  176. 
Vicinal  faces,  267. 
Viluite,  455. 
Vitreous  luster,  279. 
Vivianite,  510,  516,  608,  654. 
Volatilization,  552. 
Volborthite,  645. 
Voltaite,  636. 

W 

Wad,  368,  667. 
Wagnerite,  657. 
Walpurgite,  650. 
Warwickite,  665. 
Water,  225,  240,  337,  557. 

of  constitution,  226. 

of  crystallization,  225. 
Wave  direction,  161. 

front,  161. 

length,  162. 

surface,  169. 

Wavellite,  510,  518,  602,  656. 
Waves  of  light,  161. 
Waxy  luster,  279. 
Wellsite,  479,  670. 
Wernerite,  453,  603,  674,  676. 
Westphal  balance,  264. 


Whattevillite,  662. 
Wheel  ore,  321. 
Whitneyite,  621. 
Willemite,  99,  451,  603,  649. 
Witherite,  395,  602,  660. 
Wittichenite,  628. 
Wohlerite,  669. 
Wolfachite,  622. 
Wolfram  (Tungsten),  586. 
Wolframite,  542,  596,  633,  656 
Wolframum,  tests,  587. 
Wollastonite,  429,  603,  675. 
Wood  opal,  370. 
Wulfenite,  78,  545,  602,  648. 
Wurtzite,  93,  303. 


Xanthoconite,  645. 
Xanthophillite,  678. 
Xanthosiderite,  658. 
Xenotime,  657. 


Ytterbium,  572. 
Yttrium,  tests,  572. 
Yttrocerite,  678. 
Yttrotantalite,  635. 


Zaratite,  669. 
Zeolites,  478,  479,  613. 
Zepharovichite,  656. 
Zeunerite,  520,  643. 
Zinc,  631. 

tests,  573. 
Zinc  blende,  301. 
Zincite,  339,  606,  650. 
Zinkenite,  320. 
Zinnwaldite,  488,  496. 
Zircon,  456,  605,  612,  682. 
Zirconium,  tests,  571. 
Zirkelite,  668. 
Zoisite,  465,  674,  676. 
Zone,  22. 

axis,  22. 

circle,  !$• 

law,  22. 
Zunyite,  680. 


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,  Economic   Geology 

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BY  WILLIAM   B.  SCOTT 

Illustrated,  doth,  i2mo,  §2.60  net 


An  introduction  to  the  science  of  geology  for  both  students  who 
desire  to  pursue  the  subject  exhaustively  and  those  who  wish  merely 
to  obtain  an  outline  of  the  methods  and  principal  results  of  the  science. 
This  is  not  one  of  the  text-books  which  always  pronounces  a  definite 
and  final  opinion.  The  author  holds  that  in  no  science  are  there 
more  open  questions  than  in  geology  ;  in  none  are  changes  of  view 
more  frequent  ;  and  in  none  is  it  more  important  to  emphasize  the 
distinction  between  fact  and  inference.  The  student  is  here  en- 
couraged to  weigh  evidence  and  balance  possibilities  and  to  suspend 
judgment  when  the  testimony  is  insufficient  to  justify  decision.  The 
author  is  an  advocate  of  the  new  geology,  and  his  book  presents  all 
the  latest  advances  in  the  science.  The  book  is  very  fully  illustrated, 
many  of  the  plates  being  from  photographs  taken  by  the  United  States 
Geological  Survey. 


"  I  have*  looked  the  book  through  with  increasing  pleasure.  The 
latest  advances  in  American  geology  have  been  taken  advantage  of, 
so  that  the  book  is  up  to  date.  American  instructors  have  been  waiting 
a  long  time  for  a  book  which  could  be  used  satisfactorily  as  a  guide 
in  an  opening  course  in  geology.  Professor  Scott's  book  seems  to 
me  to  be  admirably  adapted  for  this  purpose."  —  Professor  C.  R.  VAN 
HISE,  University  of  Wisconsin. 


"Professor  Scott's  Geology  seems  to  me  excellently  fitted  for  my 
beginners  at  Smith  College,  and  I  shall  try  it  there  next  year.  It 
is  a  fine  book."  —  Professor  B.  K.  EMERSON,  Amherst  College. 


THE   MACMILLAN   COMPANY 

Publishers  64-66  Fifth  Avenue  New  York 


"  THIS  BOOK  TAKES  HIGH  RANK  AMONG  THE  FEW  GREAT  BOOKS  UPON  GLACIERS.' 


Characteristics  of 
Existing  Glaciers 


BY   WILLIAM    HERBERT    HOBBS 

Professor  of  Geology,  University  of  Michigan 

Illustrated,  cloth,  8vo,  $3.25  net ;  by  mail, 

"  The  author  has  done  good  service  to  the  glaciologist  and  glacial  geologist 
in  bringing  together  his  concise  description  and  classification  of  existing 
glaciers  and  ice-sheets  in  the  present  convenient  form.  Especially  in  the 
parts  devoted  to  Arctic  and  Antarctic  ice  he  has  made  an  exhaustive  digest 
of  the  scattered  literature,  and  has  presented  a  copiously  illustrated  summary 
of  the  available  information  respecting  the  distribution  and  character  of 
the  ice  of  these  regions.  To  the  end  of  each  chapter  he  appends  a  full  list 
of  his  authorities,  so  that  the  book  is  in  every  respect  a  most  useful  work 
of  reference.  .  .  .  Every  geographer  and  geologist  interested  in  ice  will 
appreciate  these  clear  descriptions  and  excellent  illustrations  of  the  earth's 
great  glaciers  —  they  make  up  into  a  most  presentable  book."  —  Nature. 


BY   THE    SAME   AUTHOR 

Earth   Features  and 
their  Meaning 

Profusely  Illustrated,  8w,  $j.oo  net 

"The  book  is  an  excellent  reference  volume  for  students  who  are  interested 
in  a  simple  outline  of  geology.  The  volume  has  been  tested  in  class  work  and 
should  prove  its  worth."  —  Bulletin  of  American  Geographical  Society. 


THE    MACMILLAN  COMPANY 

Publishers  64-66  Fifth  Avenue  New  York 


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